Structure Of Atom - Practice Test, Class 9 Science - Class 9 MCQ

The answer is C: nucleus.
Explanation:
The atom is made up of three subatomic particles: protons, neutrons, and electrons. However, the majority of the mass of an atom is concentrated in the nucleus, which is located at the center of the atom. Here is a detailed explanation of why the nucleus contains almost the entire mass of an atom:
1. Protons: Protons are positively charged particles found in the nucleus. They have a mass of approximately 1 atomic mass unit (AMU).
2. Neutrons: Neutrons are neutral particles also located in the nucleus. They have a mass of approximately 1 AMU, similar to protons.
3. Electrons: Electrons are negatively charged particles that orbit the nucleus in specific energy levels. Compared to protons and neutrons, electrons have a much smaller mass, approximately 1/1836 AMU.
4. Relative masses: The mass of an electron is negligible compared to the mass of a proton or neutron. Therefore, when calculating the total mass of an atom, the contribution of electrons is insignificant.
5. Nuclear mass: The combined mass of protons and neutrons in the nucleus determines the majority of the atom's mass. As protons and neutrons are much heavier than electrons, their contribution overwhelms the mass of electrons.
6. Atomic mass: The atomic mass of an element is determined by the sum of the protons and neutrons in the nucleus. Electrons do not significantly contribute to the atomic mass.
7. Isotopes: Isotopes are atoms of the same element with different numbers of neutrons. Since the number of protons determines the element, isotopes have the same number of protons but different numbers of neutrons. This means that the mass of an atom can vary based on the presence of different isotopes.
In conclusion, the nucleus contains almost the entire mass of an atom, while the electrons contribute very little to the overall mass. The protons and neutrons in the nucleus determine the majority of an atom's mass, and the electrons mainly contribute to the atom's charge and chemical behavior.

Electron was discovered by J.J. Thomson

• J.J. Thomson, a British physicist, is credited with the discovery of the electron in 1897.


• Thomson conducted a series of experiments using cathode rays, which are streams of negatively charged particles.


• Through his experiments, Thomson was able to determine that cathode rays were composed of particles that were much smaller and lighter than atoms.


• He proposed that these particles were negatively charged and named them electrons.


• Thomson's discovery of the electron revolutionized the understanding of atomic structure and laid the foundation for the development of modern atomic theory.


• His work earned him the Nobel Prize in Physics in 1906.

To determine the number of protons in an atom, we need to look at its atomic number.
Given:
Mass number = 23
Atomic number = 11
Definition:
- Mass number: The sum of protons and neutrons in an atom.
- Atomic number: The number of protons in an atom.
Steps to find the number of protons:
1. Atomic number = number of protons.
2. Therefore, for the given atom with atomic number 11, the number of protons is 11.
Hence, the correct answer is A: 11.

The atomic number of an atom corresponds to the number of protons in its nucleus. To determine the atomic number of the atom with full K, L, and M shells, we need to analyze the electron configuration.
1. Identify the electron configuration:
- The K shell can hold a maximum of 2 electrons.
- The L shell can hold a maximum of 8 electrons.
- The M shell can hold a maximum of 18 electrons.
2. Calculate the total number of electrons:
- Since the K, L, and M shells are all full, the total number of electrons is 2 + 8 + 18 = 28.
3. Find the element with the atomic number:
- The atomic number corresponds to the number of protons in the nucleus.
- By referring to the periodic table, we can find that an atom with 28 protons corresponds to the element with atomic number 28.
4. Choose the correct option:
- The correct option is A: 18.
Final Answer: The atomic number of the atom with full K, L, and M shells is 28.

The mass of the atom is determined by the neutrons and protons.
Explanation:
The mass of an atom is primarily determined by the number of neutrons and protons it contains. Here is a detailed explanation of why this is the case:
1. Nucleons: Neutrons and protons are collectively known as nucleons. They are located in the nucleus of an atom, which is the dense core at the center of the atom.
2. Neutrons: Neutrons are subatomic particles that have no electric charge. They contribute to the overall mass of the atom but do not participate in chemical reactions. The number of neutrons in an atom can vary, resulting in different isotopes of an element.
3. Protons: Protons are positively charged subatomic particles. They also contribute to the overall mass of the atom. The number of protons in an atom determines its atomic number and, therefore, its identity as a specific element.
4. Electrons: Electrons are negatively charged particles that orbit the nucleus in energy levels called shells. While electrons do have mass, it is extremely small compared to the mass of neutrons and protons. Therefore, electrons do not significantly contribute to the overall mass of the atom.
5. Atomic Mass: The atomic mass of an atom is determined by adding up the masses of its protons and neutrons. It is usually expressed in atomic mass units (amu) or grams per mole (g/mol). The mass of an electron is so small that it is generally not considered when calculating the atomic mass.
In conclusion, the mass of an atom is primarily determined by the number of neutrons and protons it contains. Electrons, although they have mass, do not significantly contribute to the overall mass of the atom.

Appropriate Air Pressure for Production of Cathode Rays in Discharge Tube
The appropriate air pressure for the production of cathode rays in the discharge tube is 0.001 mm Hg. Here's a detailed explanation:
1. Understanding Cathode Rays
- Cathode rays are streams of electrons that are produced when a high voltage is applied across a cathode and an anode in a vacuum tube.
- The vacuum tube, also known as the discharge tube, is filled with a low-pressure gas.
2. Importance of Low Air Pressure
- The presence of air molecules can hinder the movement of electrons and affect the production of cathode rays.
- Therefore, it is essential to maintain a low-pressure environment inside the discharge tube to allow for the unimpeded flow of electrons.
3. Comparing the Given Air Pressures
- A: 1 cm Hg: This air pressure is too high and would not provide a suitable environment for the production of cathode rays.
- B: 1 mm Hg: This air pressure is still relatively high and may hinder the movement of electrons, reducing the efficiency of cathode ray production.
- C: 0.001 cm Hg: This air pressure is too low and would result in an almost complete vacuum, making it challenging to sustain cathode rays.
- D: 0.001 mm Hg: This air pressure provides an appropriate balance, maintaining a low-pressure environment without reaching a complete vacuum.
4. Conclusion
- The most appropriate air pressure for the production of cathode rays in the discharge tube is 0.001 mm Hg.
- This air pressure allows for the efficient movement of electrons while still maintaining a low-pressure environment.

Explanation:
- Cathode rays are a stream of electrons that are emitted from the cathode (negative electrode) in a cathode ray tube.
- These electrons are negatively charged particles.
- When a voltage is applied across the cathode ray tube, the cathode rays are attracted towards the positive electrode (anode) due to the opposite charges.
- This attraction causes the cathode rays to be deflected towards the positive electrode.
- Therefore, the correct answer is option A: positive electrode.
- The positive electrode attracts the negatively charged cathode rays, causing them to move towards it.
Summary:
- Cathode rays are deflected towards the positive electrode (anode) in a cathode ray tube.
- The positive electrode attracts the negatively charged cathode rays, causing them to move towards it.

The proton is heavier than an electron by 1840 times.
Explanation:
- The mass of a proton is approximately 1.6726219 x 10^-27 kilograms.
- The mass of an electron is approximately 9.10938356 x 10^-31 kilograms.
- To find the difference in mass between a proton and an electron, we can calculate the ratio of their masses.
- The mass of a proton divided by the mass of an electron is approximately 1.6726219 x 10^-27 / 9.10938356 x 10^-31 = 1836.15267389.
- Rounding this number to the nearest whole number, we get that the proton is heavier than an electron by approximately 1836 times.
- Therefore, the correct answer is B: 1840 times.
Summary:
The proton is approximately 1840 times heavier than an electron.

It contain 6 protons 6 electrons and 6 neutrons...so its atomic number is 6 and mass number is 12

Chadwick's Nobel Prize for the Discovery of Neutrons
Introduction:
Chadwick, a British physicist, was awarded the Nobel Prize for his significant contribution to the field of nuclear physics. He played a crucial role in the discovery of neutrons, which led to a better understanding of the atomic nucleus and the development of nuclear energy.
Explanation:
Here is a detailed explanation of why Chadwick received the Nobel Prize for the discovery of neutrons:
1. Background:
- In the early 1930s, scientists were aware of the existence of positively charged protons and negatively charged electrons within atoms.
- However, some experimental results indicated the presence of another neutral subatomic particle.
2. Chadwick's Experiment:
- In 1932, James Chadwick conducted a series of experiments to investigate the nature of this neutral particle.
- He bombarded a thin sheet of beryllium with alpha particles (helium nuclei) and observed the resulting radiation.
- Chadwick noticed that this radiation could penetrate materials more effectively than expected, suggesting the presence of a neutral particle with a similar mass to a proton.
3. Discovery of Neutrons:
- Building upon his experiments, Chadwick concluded that the radiation was composed of previously unknown particles, which he named "neutrons."
- Neutrons are electrically neutral particles with approximately the same mass as protons.
- Chadwick's discovery of neutrons revolutionized the understanding of atomic structure and nuclear reactions.
4. Significance:
- Chadwick's discovery of neutrons provided evidence for the existence of the atomic nucleus, a dense central region within atoms.
- It explained why some atomic nuclei were more stable than others, as neutrons help bind protons together through the strong nuclear force.
- The discovery of neutrons also laid the foundation for the development of nuclear energy, as they play a key role in nuclear reactions and power generation.
Conclusion:
James Chadwick was awarded the Nobel Prize in Physics in 1935 for his discovery of neutrons. His groundbreaking research significantly contributed to our understanding of atomic structure and paved the way for advancements in nuclear physics and energy.

Mass number is equal to the number of protons plus the number of neutrons.
Explanation:
The mass number of an atom is the total number of protons and neutrons in its nucleus. Protons and neutrons are the two types of subatomic particles found in the nucleus of an atom. Electrons, on the other hand, are located in the electron cloud surrounding the nucleus.
- Protons: Protons are positively charged particles found in the nucleus of an atom. Each proton has a mass of approximately 1 atomic mass unit (amu).
- Neutrons: Neutrons are neutral particles found in the nucleus of an atom. Like protons, each neutron has a mass of approximately 1 amu.
- Electrons: Electrons are negatively charged particles that orbit the nucleus in specific energy levels. They have a negligible mass compared to protons and neutrons.
- The mass number of an atom is determined by adding together the number of protons and neutrons. It represents the total mass of the atom.
Therefore, the correct answer is B: number of protons number of neutrons.

Fluorescence in discharge tubes
The fluorescence observed on the walls of a discharge tube is a result of the interaction between the particles present in the tube and the gas or vapor within it. Here's a detailed explanation of how each option relates to the fluorescence:
A: Cathode rays
- Cathode rays are streams of electrons emitted from the cathode (negative electrode) in the discharge tube.
- When these high-speed electrons collide with gas atoms or molecules, they transfer energy to them.
- This energy excites the electrons in the gas atoms, causing them to move to higher energy levels.
- As the excited electrons return to their original energy levels, they release energy in the form of light.
- This emitted light is what causes the fluorescence on the walls of the discharge tube.
B: Anode rays
- Anode rays are positively charged ions that are accelerated towards the cathode in the discharge tube.
- These ions do not contribute to the fluorescence observed on the walls of the tube.
- They may interact with the gas atoms, but their effect is not responsible for the fluorescence.
C: Canal rays
- Canal rays are positively charged ions that are accelerated towards the anode in the discharge tube.
- Similar to anode rays, canal rays do not result in the fluorescence observed on the walls of the tube.
- Their interaction with the gas atoms does not produce the emitted light.
D: None of the above
- This option is incorrect because fluorescence in a discharge tube is indeed caused by cathode rays.
In conclusion, the correct answer is A: Cathode rays. When these high-speed electrons collide with gas atoms or molecules, they excite the electrons in the gas, leading to the emission of light and the fluorescence observed on the walls of the discharge tube.

The volume of the nucleus of an atom when compared to the extra nuclear part is smaller.
Explanation:
- The nucleus of an atom is located at its center and contains protons and neutrons.
- The extra nuclear part, also known as the electron cloud, surrounds the nucleus and consists of electrons.
- The nucleus is extremely small compared to the overall size of the atom.
- The size of an atom is primarily determined by the electron cloud, as electrons occupy a large amount of space.
- The nucleus, on the other hand, is much smaller and denser.
- The size of the nucleus is typically on the order of femtometers (10^-15 meters), while the size of the atom can range from picometers (10^-12 meters) to nanometers (10^-9 meters).
- Therefore, the volume of the nucleus is significantly smaller than the volume of the extra nuclear part.
- This size difference is due to the fact that protons and neutrons, which make up the nucleus, have much greater mass than electrons.
- As a result, the majority of an atom's volume is occupied by the electron cloud, while the nucleus occupies a relatively small fraction of the total volume.
- Hence, the correct answer is B: smaller.

Rutherford's Alpha-scattering Experiment
The foil of element that was used in Rutherford's alpha-scattering experiment was gold.
Explanation:
Rutherford's alpha-scattering experiment was conducted by Ernest Rutherford in 1909. This experiment played a significant role in understanding the structure of an atom. Here's a detailed explanation of the experiment:
1. Aim of the Experiment:
The aim of Rutherford's experiment was to investigate the structure of the atom and determine the distribution of positive charge within it.
2. Experimental Setup:
- Rutherford used a thin metal foil as a target for alpha particles.
- The foil was made up of a single element to ensure consistency and accurate observations.
3. Alpha Particles:
- Alpha particles are positively charged particles consisting of two protons and two neutrons, similar to helium nuclei.
- These particles were obtained from a radioactive source, such as radium or polonium.
4. Observations:
- Rutherford expected that the alpha particles would pass through the foil with minimal deflection, as per the prevailing plum pudding model of the atom.
- However, he observed that a small fraction of alpha particles were deflected at large angles and some even bounced back.
5. Conclusion:
- Based on the unexpected observations, Rutherford proposed a new atomic model known as the nuclear model.
- According to this model, atoms have a tiny, dense, positively charged nucleus at the center, surrounded by mostly empty space.
- The deflection and backward scattering of alpha particles indicated that the positive charge and most of the mass of an atom are concentrated in a small region, the nucleus.
6. Choice of Foil:
- Rutherford chose a thin foil made of gold for his experiment.
- Gold was selected because it could be easily hammered into thin sheets without tearing or breaking.
- Additionally, gold has a high atomic number, which means it has a large number of protons in its nucleus, making it suitable for scattering experiments.
Therefore, in Rutherford's alpha-scattering experiment, a foil of gold was used.

Valency of an element with electronic configuration 2, 8, 7
To determine the valency of an element, we need to consider the number of valence electrons it has. Valence electrons are the electrons in the outermost energy level of an atom. In this case, the electronic configuration of the element is 2, 8, 7.
Valence electrons:
The number of valence electrons can be determined by looking at the last digit of the electronic configuration. In this case, the last digit is 7, so the element has 7 valence electrons.
Valency:
Valency is the combining capacity of an atom, which is determined by the number of valence electrons it can gain, lose, or share to achieve a stable electron configuration.
To find the valency of the element, we need to consider the number of valence electrons and their tendency to gain or lose electrons.
In general, atoms tend to gain or lose electrons to achieve a stable electron configuration of 8 electrons in their outermost energy level (except for hydrogen and helium, which achieve stability with 2 electrons).
In this case, the element has 7 valence electrons.
Since it is easier for the element to gain 1 electron to achieve a stable configuration, the valency of the element is 1.
Therefore, the correct answer is A: 1.

During a chemical reaction, atomic number remains the same.
Explanation:
Atomic number is the number of protons in the nucleus of an atom. It is a unique identifier for each element on the periodic table. During a chemical reaction, the arrangement of electrons in an atom may change, but the number of protons in the nucleus remains constant. This is because chemical reactions involve the rearrangement of electrons to form new chemical bonds, but the nucleus and the number of protons within it remain unaffected.
Key points:
- Atomic number is the number of protons in the nucleus of an atom.
- It is a unique identifier for each element on the periodic table.
- During a chemical reaction, the arrangement of electrons may change, but the number of protons remains the same.
- Chemical reactions involve the rearrangement of electrons to form new chemical bonds.
- The nucleus and the number of protons within it remain unaffected during a chemical reaction.
- Therefore, the atomic number remains the same throughout the chemical reaction.

Fixed circular paths around the nucleus:
- The fixed circular paths around the nucleus of an atom are called orbits.
- These orbits were proposed by Niels Bohr in his atomic model.
- Orbits represent the energy levels or shells in which electrons can exist.
- Electrons occupy specific orbits depending on their energy.
- The electrons in the innermost orbit have the lowest energy, while those in the outermost orbit have the highest energy.
Characteristics of orbits:
- Orbits are fixed and well-defined paths.
- Each orbit has a specific energy level associated with it.
- Electrons can jump from one orbit to another by absorbing or emitting energy.
Difference between orbits and orbitals:
- Orbits are found in the Bohr model of the atom, which is a simplified representation.
- Orbitals, on the other hand, are found in the quantum mechanical model of the atom.
- Orbitals are regions in space where electrons are likely to be found.
- Unlike orbits, which are circular, orbitals have different shapes (s, p, d, f).
In conclusion, the fixed circular paths around the nucleus are called orbits. These orbits represent the energy levels or shells in which electrons can exist. It is important to note that the concept of orbits is a simplification and the actual behavior of electrons is described by orbitals in the quantum mechanical model.

Dalton's Atomic Theory:
According to Dalton's atomic theory, which was proposed by John Dalton in the early 19th century, matter is composed of tiny indivisible particles called atoms. This theory laid the foundation for our understanding of the structure and behavior of matter at the atomic level.
The Smallest Particle:
According to Dalton's atomic theory, the smallest particle that is capable of independent existence is an atom. Here's a detailed explanation:
1. Element:
- An element is a pure substance made up of only one type of atom.
- However, atoms are considered as the basic building blocks of elements.
2. Molecule:
- A molecule is a group of two or more atoms chemically bonded together.
- While molecules are composed of atoms, they are not the smallest particle according to Dalton's atomic theory.
3. Ion:
- An ion is an atom or a group of atoms that has gained or lost electrons, resulting in a positive or negative charge.
- Although ions are formed from atoms, they are not considered as the smallest particle based on Dalton's atomic theory.
4. Atom:
- According to Dalton's atomic theory, atoms are the smallest particle that can exist independently.
- Atoms cannot be further divided into smaller particles without losing their chemical properties.
- Each atom consists of a nucleus (containing protons and neutrons) and electrons orbiting around the nucleus.
Conclusion:
In summary, Dalton's atomic theory states that the smallest particle capable of independent existence is the atom. This theory revolutionized our understanding of matter and formed the basis for further advancements in the field of atomic and molecular science.

Dalton’s Atomic Theory:
Explanation:
According to Dalton’s atomic theory, an atom can neither be created nor destroyed. This theory, proposed by John Dalton in the early 19th century, laid the foundation of modern atomic theory and explained the nature of atoms and their behavior.
Key Points:
- Dalton’s atomic theory is based on the following postulates:
- All matter is composed of indivisible and indestructible particles called atoms.
- Atoms of the same element are identical in mass and properties.
- Compounds are formed by the combination of atoms in simple whole number ratios.
- Chemical reactions involve the rearrangement of atoms, but no atoms are created or destroyed during the process.
- Based on these postulates, it is clear that according to Dalton’s atomic theory:
- Atoms cannot be created: This means that atoms cannot be formed out of nothing. They exist in a constant state and cannot be brought into existence.
- Atoms cannot be destroyed: This means that atoms cannot be broken down or eliminated completely. They are indestructible and retain their identity even during chemical reactions.
- The law of conservation of mass, which is a fundamental principle in chemistry, supports the idea that atoms cannot be created or destroyed. This law states that the total mass of substances involved in a chemical reaction remains constant before and after the reaction.
Conclusion:
According to Dalton’s atomic theory, an atom cannot be created or destroyed. This theory provides a fundamental understanding of the behavior and properties of atoms, laying the groundwork for modern atomic theory.

The concept that atoms combine in small whole number ratio was proposed by Dalton.
Explanation:
1. Dalton's atomic theory:
- John Dalton, an English chemist, proposed the atomic theory in the early 19th century.
- According to Dalton's atomic theory, atoms are indivisible and indestructible particles that combine to form compounds.
- Dalton stated that atoms combine in small, whole number ratios to form compounds.
- This means that the ratio of atoms in a compound is always a simple integer ratio.
2. Experimental evidence:
- Dalton's proposal was based on extensive experimental evidence and observations.
- Through experiments, Dalton observed that when elements combine to form compounds, they do so in consistent and predictable ratios.
- For example, in water (H2O), the ratio of hydrogen atoms to oxygen atoms is always 2:1.
3. Importance of Dalton's concept:
- Dalton's concept of atoms combining in small whole number ratios laid the foundation for modern stoichiometry.
- Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions.
- Dalton's ideas helped explain the law of definite proportions, which states that a compound always contains the same elements in the same proportion by mass.
Therefore, it was John Dalton who proposed the concept that atoms combine in small whole number ratios.