Structure Of Atom - Class 9 Science (Compulsory Test) - Class 9 MCQ

Introduction:
The first use of quantum theory to explain the structure of the atom was made by Niels Bohr. This breakthrough in atomic theory laid the foundation for our understanding of the behavior of electrons and the structure of atoms.
Explanation:
Here is a detailed explanation of how Niels Bohr used quantum theory to explain the structure of the atom:
1. Background:
- Prior to Bohr's work, the prevailing model of the atom was the Thomson model, which depicted the atom as a positively charged sphere with electrons scattered throughout.
- However, this model failed to explain certain experimental observations, such as the stability of atoms and the discrete nature of atomic spectra.
2. Quantum Theory:
- Quantum theory, developed by Max Planck and others, proposed that energy is quantized and exists in discrete packets called quanta.
- Bohr applied this concept of quantization to the behavior of electrons in atoms.
3. Bohr's Model:
- Bohr proposed a new model of the atom, known as the Bohr model or the planetary model.
- According to this model, electrons orbit the nucleus in specific energy levels or shells.
- The energy levels are quantized, meaning that electrons can only occupy certain energy states.
- When an electron jumps from one energy level to another, it either absorbs or emits energy in discrete packets, corresponding to the difference in energy between the levels.
4. Balmer Series:
- One of the key successes of Bohr's model was its ability to explain the observed spectral lines of hydrogen.
- Bohr derived an equation, known as the Balmer formula, which accurately predicted the wavelengths of the spectral lines.
- This formula was based on the quantization of energy levels in the hydrogen atom.
5. Contributions and Impact:
- Bohr's model of the atom revolutionized our understanding of atomic structure and paved the way for further developments in quantum mechanics.
- His work earned him the Nobel Prize in Physics in 1922.
- Bohr's ideas laid the foundation for subsequent theories, such as quantum mechanics and quantum electrodynamics, which continue to be widely used in modern physics.
Conclusion:
Niels Bohr was the first to use quantum theory to explain the structure of the atom. His model, which incorporated the concept of quantized energy levels, provided a comprehensive explanation for the behavior of electrons in atoms. Bohr's contributions to atomic theory have had a lasting impact on the field of physics.

Plum-Pudding Model Explained
The Plum-Pudding Model, also known as the Thomson Model, was proposed by J.J. Thomson in 1904 as an alternative to the Rutherford Model. This model was later replaced by the Bohr Model, which provided a more accurate description of the atom's structure.
Explanation of the Plum-Pudding Model:
- The model suggests that an atom is composed of a positively charged diffuse mass (the "pudding") with negatively charged electrons (the "plums") embedded within it.
- According to Thomson, the positive charge is uniformly distributed throughout the atom, and the negatively charged electrons are scattered within this positive sphere.
- The model was based on Thomson's discovery of the electron and his experiments using cathode rays.
Comparison with other models:
- The Rutherford Model, also known as the Nuclear Model, was proposed by Ernest Rutherford in 1911. It suggests that the atom has a small, dense, positively charged nucleus at the center, with electrons orbiting around it. This model was a significant departure from the Plum-Pudding Model.
- The Bohr Model, proposed by Niels Bohr in 1913, improved upon the Rutherford Model by incorporating the concept of quantized energy levels for electrons. It introduced the idea that electrons occupy specific orbits or energy levels around the nucleus.
Conclusion:
- The Plum-Pudding Model, or Thomson Model, was an early attempt to describe the structure of the atom.
- It suggested that the atom consists of a positive "pudding" with embedded negative "plums."
- However, this model was later replaced by more accurate models, such as the Rutherford Model and the Bohr Model.

Nucleus consists of:
- Protons: Positively charged particles found in the nucleus of an atom.
- Neutrons: Particles with no charge found in the nucleus of an atom.
Explanation:
- The nucleus is the central part of an atom, which contains most of its mass.
- It is made up of protons and neutrons, collectively known as nucleons.
- Protons have a positive charge, while neutrons have no charge.
- Electrons, which have a negative charge, are found in orbit around the nucleus.
- The number of protons in an atom determines its atomic number, and the number of neutrons and protons together determine its atomic mass.
- In option B, it states that the nucleus consists of protons and neutrons, which is correct.
- Options A, C, and D are incorrect because they do not include both protons and neutrons.
Therefore, the correct answer is B: Proton and neutron.

The number of valence electrons in an atom is equal to the number of electrons in its outermost energy level.
In the case of sodium (Na), the atomic number is 11, which means it has 11 electrons in total.
The electron configuration of sodium is 2-8-1, indicating that it has three energy levels.
To determine the number of valence electrons in sodium:
1. Look at the last energy level, which is the third energy level in the case of sodium.
2. Count the number of electrons in that energy level, which is 1 in the case of sodium.
Therefore, the number of valence electrons in sodium is 1.
Answer: A

Mass number of an element:
The mass number of an element is the total number of protons and neutrons in the nucleus of an atom of that element. It is represented by the symbol A.
Explanation:
The correct answer is option C: Number of neutrons and protons in the nucleus.
- The mass number of an element is determined by the number of subatomic particles in the nucleus.
- The nucleus of an atom contains protons and neutrons, while the electrons are present in the electron cloud surrounding the nucleus.
- The number of protons in the nucleus is called the atomic number and is represented by the symbol Z.
- The number of neutrons can vary in different isotopes of the same element, but the sum of protons and neutrons remains constant for a specific isotope.
- Therefore, the mass number is calculated by adding the number of protons (Z) and the number of neutrons (N) in the nucleus of an atom.
- Mathematically, mass number (A) = Z (number of protons) + N (number of neutrons).
In summary, the mass number of an element represents the total number of protons and neutrons in the nucleus of an atom.

Dalton's Atomic Theory
According to Dalton's atomic theory, atoms are the basic building blocks of matter, and they have several fundamental properties. The theory was proposed by John Dalton in the early 19th century and laid the foundation for the modern understanding of atomic structure.
Key Points of Dalton's Atomic Theory:
1. Atoms are indivisible: According to Dalton, atoms are indivisible and cannot be further divided into smaller particles. This concept was later revised with the discovery of subatomic particles like electrons, protons, and neutrons.
2. Atoms of different elements have different properties: Dalton proposed that each element is composed of unique atoms that have specific properties. This idea forms the basis of the periodic table, where elements are arranged based on their atomic number and properties.
3. Atoms combine in whole number ratios: Dalton observed that atoms combine with each other to form compounds in simple, whole number ratios. This principle is known as the law of definite proportions and is still applicable in chemical reactions today.
4. Atoms are rearranged in chemical reactions: Dalton's theory states that atoms are neither created nor destroyed in a chemical reaction. Instead, they are rearranged to form new compounds. This concept is known as the law of conservation of mass.
5. Atoms of the same element are identical: Dalton assumed that all atoms of the same element are identical in terms of their mass, size, and chemical properties. However, this assumption was later revised with the discovery of isotopes, which are atoms of the same element with different numbers of neutrons.
Overall, Dalton's atomic theory provided a framework for understanding the behavior and properties of atoms, although some of its assumptions have been modified with subsequent scientific discoveries.

Neutron is a fundamental particle
- A neutron is a subatomic particle that is found in the nucleus of an atom.
- It has a mass similar to that of a proton but carries no electrical charge.
- Neutrons are classified as fundamental particles, which means they are not made up of smaller particles.
- They play a crucial role in determining the stability and properties of atomic nuclei.
Characteristics of a neutron
- No charge: Neutrons are electrically neutral, meaning they have no positive or negative charge.
- Unit mass: Neutrons have a mass of approximately 1 atomic mass unit (AMU), which is similar to the mass of a proton.
- Magnetic properties: Neutrons have a magnetic moment, which means they can interact with magnetic fields.
- Decay: Neutrons are unstable when they are outside the nucleus and can undergo decay processes.
Options and their correctness
A: 1 unit charge and 1 unit mass
- Incorrect: Neutrons have no charge, so they cannot have a unit charge.
B: No charge and 1 unit mass
- Correct: Neutrons have no charge and a mass of approximately 1 AMU.
C: Have no charge and mass
- Incorrect: This option is incomplete and does not provide the correct information about the mass of a neutron.
D: Have -1 unit charge and 1 unit mass
- Incorrect: Neutrons have no charge, so they cannot have a negative charge.
Therefore, the correct answer is B: No charge and 1 unit mass. Neutrons are electrically neutral and have a mass of approximately 1 AMU.

Answer:
Universal Particle:
The universal particle refers to a particle that is present in all matter and is not composed of smaller particles.
Options:
A. Electron: An electron is a subatomic particle that is found outside the atomic nucleus. It carries a negative charge and is considered to be a fundamental particle.
B. Neutron: A neutron is a subatomic particle that is found in the atomic nucleus. It carries no charge and is considered to be a fundamental particle.
C. Proton: A proton is a subatomic particle that is found in the atomic nucleus. It carries a positive charge and is considered to be a fundamental particle.
D. All of these: This option states that all the given particles (electron, neutron, and proton) are regarded as universal particles.
Explanation:
To determine the correct answer, we need to understand the properties of the given particles.
- Electrons, neutrons, and protons are all subatomic particles.
- Electrons are negatively charged and are found outside the atomic nucleus.
- Neutrons carry no charge and are found in the atomic nucleus.
- Protons carry a positive charge and are found in the atomic nucleus.
From the given options, the correct answer is A: Electron. This is because electrons are not composed of smaller particles and are found in all matter, making them universal particles. Neutrons and protons, on the other hand, are not present in all matter as they are only found in the atomic nucleus.
Therefore, the electron is regarded as a universal particle.

Rutherford's alpha-particle experiment and its discovery:
Introduction:
Rutherford's alpha-particle experiment, also known as the gold foil experiment, was conducted by Ernest Rutherford in 1911. This experiment played a crucial role in understanding the structure of the atom and led to the discovery of the nucleus.
Experimental setup:
1. Rutherford directed a beam of α (alpha) particles towards a thin gold foil.
2. The α particles were obtained from a radioactive source, which emitted positively charged particles.
3. The gold foil was chosen due to its high malleability and the ability to be made into extremely thin layers.
Expected Observations:
Based on the prevailing model of the atom, called the Thomson's plum pudding model, Rutherford expected that the α particles would pass through the foil with slight deflections.
Actual Observations:
1. Most of the α particles passed through the gold foil without any deflection, just as expected.
2. However, a small fraction of the α particles experienced large-angle deflections and even bounced back in the direction from which they came.
Interpretation of results:
1. Based on the unexpected observations, Rutherford proposed a new model of the atom.
2. He concluded that atoms have a tiny, dense, and positively charged core at the center, which he called the nucleus.
3. The nucleus occupies a very small volume compared to the overall size of the atom.
4. The rest of the atom is mostly empty space, with negatively charged electrons revolving around the nucleus in specific energy levels.
Discovery resulting from the experiment:
The key discovery resulting from Rutherford's alpha-particle experiment was the existence of the nucleus.
Answer:
The correct answer is C: Nucleus.
In summary, Rutherford's alpha-particle experiment led to the discovery of the nucleus as the central, dense, and positively charged core of an atom. This experiment revolutionized our understanding of atomic structure and laid the foundation for the development of the modern atomic model.

Question Analysis:
The question asks which species does not contain a neutron. Neutrons are subatomic particles that have no charge and are found in the nucleus of an atom. We need to identify the species that does not have a neutron.
Options Analysis:
A:

H

- Hydrogen (H) is the first element on the periodic table.
- The atomic number of hydrogen is 1, which means it has 1 proton.
- Hydrogen typically has 0 neutrons, but it can have 1 neutron in its deuterium isotope.
- Therefore, hydrogen can have a neutron, but it is not always present.
B:

Li ²

- Lithium (Li) is the third element on the periodic table.
- The atomic number of lithium is 3, which means it has 3 protons.
- The superscript 2 in Li² indicates the atomic mass or the number of protons and neutrons combined.
- Therefore, lithium-2 (Li²) has 2 neutrons.
C:

C

- Carbon (C) is the sixth element on the periodic table.
- The atomic number of carbon is 6, which means it has 6 protons.
- Carbon typically has 6 neutrons in its most common isotope, carbon-12 (C-12).
D:

0

- The number 0 represents an abstract concept and does not refer to a specific element or species.
Answer Explanation:
The species that does not contain a neutron is option A, hydrogen (H). Although hydrogen can have a neutron in its deuterium isotope, the question does not specify the specific isotope. In its most common form, hydrogen does not have a neutron.
Key Points:
- Hydrogen, in its most common form, does not have a neutron.
- Lithium-2 (Li²) has 2 neutrons.
- Carbon-12 (C-12) has 6 neutrons.
- The option "0" does not refer to an element or species.

Discovery of Anode Rays
- The discovery of anode rays is credited to the German scientist Eugen Goldstein in the late 19th century.
- Goldstein observed a glow in a cathode ray tube when a perforated anode was introduced.
- He called these rays "Kanalstrahlen" or canal rays.
- Goldstein's experiments were a continuation of the earlier work done by J.J. Thomson on cathode rays.
- Cathode rays are streams of electrons that are emitted from the cathode in a vacuum tube.
- Goldstein's experiments with cathode rays led him to discover that there were also positively charged rays moving in the opposite direction, which he called anode rays.
- Anode rays are formed when residual gas molecules in the cathode ray tube are ionized by the high-speed electrons from the cathode.
- These positively charged ions are attracted to the negatively charged anode, creating a glow or fluorescence.
- Goldstein's discovery of anode rays provided further evidence for the existence of charged particles and contributed to the development of the understanding of atomic structure and the nature of electricity.
Therefore, the correct answer is B: Goldstein.

To determine the number of electrons in an atom, we need to know the electronic configuration of the atom. The electronic configuration is a representation of the arrangement of electrons in different energy levels or shells.
In this case, we are given that both the K and L shells of the atom are full.
The K shell can hold a maximum of 2 electrons, and the L shell can hold a maximum of 8 electrons.
Since both shells are full, we can determine the number of electrons in the atom by adding the maximum number of electrons in each shell:
- K shell: 2 electrons
- L shell: 8 electrons
Therefore, the total number of electrons in the atom is 2 + 8 = 10.
Hence, the correct answer is option B: 10.

Discovery of the Neutron
Introduction:
The neutron, an elementary particle without an electric charge, was discovered through experimental research in the early 1930s. This discovery was crucial for the understanding of nuclear physics and the development of atomic energy.
James Chadwick:
James Chadwick, a British physicist, is credited with the discovery of the neutron. Here's how he made this groundbreaking discovery:
Experiments and Observations:
1. Chadwick conducted experiments using alpha particles, which are positively charged particles, and beryllium, a light element.
2. He observed that when alpha particles bombarded the beryllium, they emitted a new type of radiation that was not affected by electric or magnetic fields.
3. This radiation had similar properties to gamma rays but was less penetrating.
4. Chadwick concluded that this radiation consisted of neutral particles with a mass similar to that of a proton.
5. These neutral particles were named neutrons, derived from the Latin word "neutrum," meaning "neither."
Significance:
Chadwick's discovery of the neutron had significant implications for nuclear physics and atomic energy:
- The neutron was found to be a key component of the atomic nucleus, along with protons.
- The understanding of neutron interactions with atomic nuclei led to the development of nuclear fission and the atomic bomb.
- Neutrons also play a crucial role in nuclear reactors, where they initiate and sustain nuclear chain reactions.
In conclusion, James Chadwick's experiments and observations led to the discovery of the neutron, a significant advancement in our understanding of atomic physics and its applications in nuclear energy.

Explanation:
The electronic configuration of an element represents the arrangement of electrons in its atomic orbitals. For calcium (Ca), the atomic number is 20, which means it has 20 electrons.
The correct electronic configuration of calcium is:
2, 8, 8, 2
Here's how we determine the electronic configuration:
1. The first energy level (n=1) can hold a maximum of 2 electrons.
2. The second energy level (n=2) can hold a maximum of 8 electrons.
3. The third energy level (n=3) can also hold a maximum of 8 electrons.
4. The fourth energy level (n=4) can hold a maximum of 2 electrons.
Therefore, the electronic configuration of calcium is 2, 8, 8, 2, which can also be written as 2, 8, 18, 2 in the shorthand notation.
Summary:
The correct electronic configuration of calcium is 2, 8, 8, 2.

Answer:
Who said this?
- The statement was made by Rutherford.
Explanation:
- Electrons, which are negatively charged particles, are present in the extra nuclear portion of an atom.
- These electrons do not remain stationary but revolve around the nucleus of the atom.
- They follow a circular path around the nucleus.
- This statement was proposed by Ernest Rutherford, a British physicist, in his atomic model known as the Rutherford model.
- Rutherford's model was proposed in 1911 and it described the structure of an atom where electrons orbit around a positively charged nucleus.
- This model was a significant advancement in the understanding of atomic structure and laid the foundation for further developments in atomic physics.
Key Points:
- Electrons in the extra nuclear portion of an atom are not stationary.
- They revolve around the nucleus at high speed.
- They follow a circular path.
- This concept was proposed by Rutherford in his atomic model.

To determine the number of neutrons in an atom, we need to subtract the atomic number from the mass number.
Given:
Atomic number (Z) = 30
Mass number = 70
Step 1: Determine the number of protons
The atomic number (Z) represents the number of protons in an atom. In this case, the atomic number of zinc is 30, so there are 30 protons.
Step 2: Determine the number of neutrons
The mass number represents the total number of protons and neutrons in an atom. To find the number of neutrons, we subtract the atomic number from the mass number.
Number of neutrons = Mass number - Atomic number
Number of neutrons = 70 - 30
Number of neutrons = 40
Therefore, the total number of neutrons in zinc with a mass number of 70 is 40.
Answer: A. 40

Incorrect Statement: A: Proton is a universal particle.
Explanation:
The incorrect statement is A: Proton is a universal particle.
Reasoning:
Here is the reasoning for why the other statements are correct:
1. Statement B: Neutron is slightly heavier than proton.
- This statement is correct. Neutron has a slightly greater mass compared to a proton.
2. Statement C: Neutrons were known at the time of Rutherford performed the scattering experiment.
- This statement is correct. Neutrons were known at the time of Rutherford's scattering experiment, which took place in the early 20th century.
3. Statement D: Most of the space in an atom is empty.
- This statement is correct. According to the Bohr model of the atom, electrons orbit the nucleus in specific energy levels, and most of the space within an atom is empty.
Therefore, the incorrect statement is A: Proton is a universal particle. Protons are not considered universal particles. They are subatomic particles found within the nucleus of atoms.

Explanation:
The deflection of a few particles on hitting a thin foil of gold indicates that the nucleus is heavy and small. Here's the detailed explanation:
1. Deflection of particles:
- When alpha particles are directed towards a thin gold foil, some of them get deflected from their original path.
- This deflection suggests that there is a concentrated positive charge in the gold foil, which repels the positively charged alpha particles.
2. Nucleus in heavy:
- The deflection of alpha particles indicates that there is a massive and positively charged nucleus present in the gold foil.
- The heavy nucleus is responsible for the strong repulsion experienced by the alpha particles.
3. Nucleus is small:
- The fact that only a few alpha particles get deflected suggests that the size of the nucleus is relatively small compared to the overall size of the gold foil.
- If the nucleus were spread out over a larger area, the alpha particles would not experience significant deflection.
4. Both (1) and (2):
- The deflection of alpha particles indicates both the heaviness and smallness of the nucleus.
- The heavy nucleus provides the necessary positive charge for repulsion, while the small size allows for concentrated charge distribution.
Therefore, the correct answer is C: Both (1) and (2), indicating that the nucleus in the gold foil is both heavy and small.

Answer:
The correct answer is A. The L shell is closer to the nucleus of an atom compared to the M shell. Here is a detailed explanation:
1. Electron Shells:
- Electrons in an atom are arranged in energy levels called electron shells.
- The shells are labeled with letters starting from K, L, M, N, and so on.
2. Shell Distance from Nucleus:
- The distance between an electron shell and the nucleus decreases as the shell's letter designation increases.
- In other words, the shells closer to the nucleus have lower letter designations.
3. The L and M Shells:
- The L shell is the second shell, and it is closer to the nucleus compared to the M shell, which is the third shell.
- The L shell can hold a maximum of 8 electrons, while the M shell can hold a maximum of 18 electrons.
4. Electrons and Energy Levels:
- Electrons occupy the lowest available energy level before filling higher energy levels.
- The L shell is filled before the M shell, indicating that it is closer to the nucleus.
5. Electron Configuration:
- The electron configuration of an atom represents the arrangement of electrons in its shells.
- For example, the electron configuration of carbon (atomic number 6) is 1s² 2s² 2p².
- This means that carbon has 2 electrons in the first shell (1s), 2 electrons in the second shell (2s), and 2 electrons in the second shell's subshell (2p).
Conclusion:
- In summary, the L shell is closer to the nucleus compared to the M shell in an atom.
- Therefore, the correct answer is A.

The correct match of the given terms with their respective scientists is as follows:
Electron: J.J. Thomson
Proton: Ernest Rutherford
Neutron: James Chadwick
Atomic nucleus: E. Goldstein
Explanation:
- J.J. Thomson: He discovered the electron using cathode ray experiments. He proposed the "plum pudding" model of the atom, in which electrons were embedded in a positively charged sphere.
- Ernest Rutherford: He conducted the famous gold foil experiment, which led to the discovery of the atomic nucleus. He proposed the nuclear model of the atom, in which a small, dense, positively charged nucleus is surrounded by orbiting electrons.
- James Chadwick: He discovered the neutron, a subatomic particle with no charge. His experiments provided evidence for the existence of neutrons in the atomic nucleus.
- E. Goldstein: He discovered the canal rays, which were positively charged particles. His experiments led to the identification of the atomic nucleus as the source of positive charge in an atom.
Therefore, the correct match is option D: 1-D, 2-A, 3-C, 4-B.

To match the given statements with the correct response, we need to understand the contributions and theories proposed by different scientists. Let's analyze each statement and match them accordingly:
Statement 1: Canal rays consist of positively charged particles protons
- This statement refers to the discovery of protons and canal rays.
- J.J. Thomson discovered canal rays, but he later realized that they were actually a stream of positively charged particles called protons.
- So, the correct response for this statement is (B) J.J. Thomson.
Statement 2: Electrons are distributed in shells
- This statement describes the arrangement of electrons in an atom.
- The concept of electrons arranged in specific energy levels or shells was proposed by Neil Bohr.
- So, the correct response for this statement is (D) Neil Bohr.
Statement 3: Centre of an atom is dense
- This statement refers to the composition of an atom and its dense central region.
- The idea of the center of an atom being dense and containing most of its mass was proposed by Ernest Rutherford.
- So, the correct response for this statement is (A) Rutherford.
Statement 4: Atom is indivisible
- This statement describes the concept of indivisibility of an atom.
- The indivisibility of an atom was proposed by John Dalton, who stated that atoms are the fundamental particles of matter and cannot be divided further.
- So, the correct response for this statement is (C) J. Dalton.
Now, let's match the statements with the correct responses:
Match:
- Statement 1: (B) J.J. Thomson
- Statement 2: (D) Neil Bohr
- Statement 3: (A) Rutherford
- Statement 4: (C) J. Dalton
Therefore, the correct match for the given statements is option B: 1-B, 2-D, 3-A, 4-C.

Atomic number of any element
The atomic number of an element refers to the number of protons present in the nucleus of an atom. It is a fundamental property of an element and helps identify its placement in the periodic table.
Explanation:
The atomic number of an element is determined by the number of protons in its nucleus. Protons are positively charged particles and are one of the three main subatomic particles along with neutrons and electrons.
Here is a detailed explanation of the options given in the question:
A: Number of protons
- The atomic number of any element is equal to the number of protons it has. This is because each proton carries a unit positive charge, and the atomic number represents the total positive charge in the nucleus.
- The number of protons also determines the identity of the element. For example, hydrogen has an atomic number of 1 because it has one proton, while helium has an atomic number of 2 because it has two protons.
B: Number of neutrons in the nucleus
- Neutrons are neutral particles found in the nucleus of an atom. They do not contribute to the atomic number of an element.
- The number of neutrons can vary for a given element, resulting in different isotopes. Isotopes of an element have the same atomic number but different mass numbers due to varying numbers of neutrons.
C: Nucleus weight
- The weight of the nucleus is determined by the combined mass of protons and neutrons. It is not directly related to the atomic number.
D: Nuclear charge
- The nuclear charge refers to the total positive charge in the nucleus of an atom. While the atomic number represents the nuclear charge, it is more commonly referred to as the atomic number itself.
In conclusion, the atomic number of any element represents the number of protons in its nucleus. It is a fundamental property that helps identify and classify elements in the periodic table.

Explanation:
The lightest particle is the electron. Here is a detailed explanation:
1. Introduction:
The question asks for the lightest particle among the given options. We need to determine which particle has the smallest mass.
2. Neutron:
- A neutron is a subatomic particle found in the nucleus of an atom.
- It has a mass approximately equal to that of a proton.
- Neutrons are neutral particles, meaning they have no charge.
3. Electron:
- An electron is a subatomic particle that orbits the nucleus of an atom.
- It has a much smaller mass compared to a neutron or proton.
- Electrons have a negative charge.
4. Proton:
- A proton is a subatomic particle found in the nucleus of an atom.
- It has a mass approximately equal to that of a neutron.
- Protons have a positive charge.
5. Comparison:
- Comparing the masses of the three particles, we find that the electron has the smallest mass.
- The mass of an electron is about 1/1836 times the mass of a proton or neutron.
6. Conclusion:
Therefore, the lightest particle among the options given is the electron (option B).

To determine the identity of the atom with a mass number of 14 and 8 neutrons, we need to understand what the mass number represents and how it relates to the composition of an atom.
Mass Number:
- The mass number of an atom is the sum of the number of protons and neutrons in its nucleus.
- It is denoted by the letter 'A'.
In this case, the atom has a mass number of 14 and 8 neutrons. This means:
- Mass number (A) = Number of protons (Z) + Number of neutrons (N)
- 14 = Z + 8
Isotopes:
- Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
- Isotopes have the same atomic number (Z) but different mass numbers (A).
Analysis:
- The atom in question has a mass number of 14 and 8 neutrons.
- We need to find an element that has a mass number of 14 and determine if it can have 8 neutrons.

- The only element with a mass number of 14 is carbon, which has 6 protons and 8 neutrons.
- Therefore, the atom in question is an isotope of carbon.
Answer:
- The atom is an isotope of carbon (Option C).

To match the given terms with their correct responses, we need to understand the definitions and characteristics of each term. Let's break down each option:
(1) Valency: The combining capacity of an atom
(2) Metals: Losses electron to become stable
(3) Non-metals: Gains electron to become stable
(4) Noble gases: They have stable configuration
Now let's match each term with its correct response:
A: 1-C, 2-B, 3-D, 4-A - Incorrect
B: 1-B, 2-D, 3-A, 4-C - Incorrect
C: 1-D, 2-A, 3-C, 4-B - Incorrect
D: 1-A, 2-C, 3-B, 4-D - Correct
Therefore, the correct matching is:
(1) Valency - A: Combining capacity of an atom
(2) Metals - C: Losses electron to become stable
(3) Non-metals - B: Gains electron to become stable
(4) Noble gases - D: They have stable configuration
So, the correct answer is D.

Explanation:
The correct answer is B: Cathode rays.
- When a gas at reduced pressure is subjected to an electric discharge, the rays originating from the negative electrode are called cathode rays.
- Cathode rays are streams of electrons that are emitted from the cathode (negative electrode) in a vacuum tube or gas discharge tube.
- These rays are produced when a high voltage is applied across the electrodes, causing electrons to be accelerated towards the anode (positive electrode).
- Cathode rays have several properties:
- They travel in straight lines.
- They can cause fluorescence in certain materials.
- They are deflected by electric and magnetic fields, indicating that they have a negative charge.
- They can produce heat and can cause ionization when they collide with atoms or molecules in the gas.
- The discovery of cathode rays by J.J. Thomson in the late 19th century played a crucial role in the development of modern physics and the understanding of the structure of the atom.
- Cathode rays are not the same as anode rays (A), which are positive ions formed near the anode in a gas discharge tube.
- They are also different from x-rays (C), which are electromagnetic radiation produced by high-energy processes, and radiation rays (D), which is not a specific term in the context of gas discharge.
Therefore, the correct answer is B: Cathode rays.

The maximum number of electrons that can be accommodated in the n^th level is 2n².

Explanation:

1. Introduction:

The maximum number of electrons that can be accommodated in an energy level is determined by the formula 2n², where n represents the principal quantum number or the energy level.

2. Understanding the formula:

• The formula 2n² is derived from the fact that each energy level can hold a maximum of 2 electrons per orbital and there are n² orbitals in a given energy level.


• Since each orbital can hold 2 electrons (one with spin-up and one with spin-down), the maximum number of electrons that can be accommodated in a given energy level is 2n².

3. Example:

Let's take the example of the first three energy levels:

• n = 1: The first energy level can hold a maximum of 2 electrons (2 * 1² = 2).


• n = 2: The second energy level can hold a maximum of 8 electrons (2 * 2² = 8).


• n = 3: The third energy level can hold a maximum of 18 electrons (2 * 3² = 18).

4. Conclusion:

Based on the formula 2n², the maximum number of electrons that can be accommodated in the n^th energy level is 2n². Therefore, the correct answer is B.

Isotopes of an element

Isotopes of an element are atoms that have the same number of protons in their nuclei but differ in the number of neutrons.

Explanation:

• Isotopes: Isotopes are different forms of an element that have the same number of protons in their nuclei but differ in the number of neutrons. This means that isotopes of an element have the same atomic number (number of protons) but different mass numbers.


• Number of protons: Isotopes of an element have the same number of protons in their nuclei. The atomic number defines the identity of an element, so all isotopes of the same element have the same number of protons.


• Number of neutrons: Isotopes of an element have different numbers of neutrons in their nuclei. This means that isotopes of the same element have different mass numbers. The mass number is the sum of the number of protons and neutrons in an atom.

Therefore, the correct answer is option D: Isotopes of an element have the same number of protons in their nuclei.

Number of valence electrons in Ar:
Valence electrons are the electrons found in the outermost shell of an atom. To determine the number of valence electrons in an atom, we need to look at the electron configuration of the element.
1. Electron configuration of Argon (Ar):
- Argon has an atomic number of 18, which means it has 18 electrons.
- The electron configuration of Argon is 1s² 2s² 2p⁶ 3s² 3p⁶, with a total of three electron shells.
2. Determining the number of valence electrons:
- Valence electrons are found in the outermost shell, which in the case of Argon, is the third shell (3s² 3p⁶).
- The third shell can hold a maximum of 8 electrons (2 in the 3s orbital and 6 in the 3p orbitals).
- Therefore, Argon has a total of 8 valence electrons.
3. Answer:
- The number of valence electrons in Argon (Ar) is 8.

Note: The options provided in the question are not correct. The correct answer is 8, not 19 or 20.

Rutherford's α-particle scattering experiment:
The experiment conducted by Ernest Rutherford in 1911 involved firing α-particles at a thin gold foil to study the structure of the atom. The outcome of the experiment revealed several key observations.
Observations:
Rutherford made the following observations during his α-particle scattering experiment:
- Most of the particles pass straight through the gold foil: Rutherford expected the α-particles to pass straight through the gold foil because he believed in the prevailing "plum pudding" model of the atom, where positive charge was thought to be evenly distributed throughout the atom. However, he was surprised to find that the majority of the α-particles passed straight through the gold foil without any significant deflection.
- Some of the particles deflect by their path: Rutherford also observed that some of the α-particles deflected from their original path after passing through the gold foil. This deflection indicated the presence of a concentrated positive charge within the atom, which he later attributed to the atomic nucleus.
- Most of the α-particles rebound after hitting the gold foil: Another unexpected observation was that a small percentage of the α-particles rebounded back towards the source after striking the gold foil. This result led Rutherford to propose that the positive charge of an atom is concentrated in a tiny, dense nucleus at its center.
- Some of the particles do not pass through the gold foil: Rutherford also observed that a small fraction of the α-particles did not pass through the gold foil at all. Instead, they were scattered in various directions or even turned back. This scattering indicated that the majority of an atom's volume is empty space, with the positive charge concentrated in a small region.
Conclusion:
Based on Rutherford's observations, it can be concluded that the only observation not made during the α-particle scattering experiment was that "most of the particles rebound after hitting the gold foil" (option a). The other observations, including some particles deflecting from their path (option b), some particles not passing through the gold foil (option c), and most particles passing straight through the gold foil (option d), were all observed by Rutherford during the experiment. Therefore, the correct answer is option B: (a) and (c) are correct.