Chapter Notes: Structure of the Atom

# Structure of the Atom Class 9 Notes Science Chapter 4

 Table of contents Atomic Structure Thomson's model of an Atom Rutherford's Model of an Atom Bohr's Model of Atom Distribution of Electrons in Different Orbits Valency Atomic Number Mass Number Isotopes Isobars

The structure of an atom comprises protons, neutrons and electrons. These basic components provide the mass and charge of the atoms.

## Atomic Structure

• John Dalton assumed that the atom is indivisible.
• In 1866 E. Goldstein discovered the presence of new radiations in a gas discharge tube and called them canal rays. These rays were positively charged radiations which led to the discovery of a sub-atomic particle called proton.
•  In 1897 J.J. Thomson discovered the sub-atomic particle - the electron with a negative charge.
• The neutron was discovered by Chadwick. Neutrons have no charge.
• Table: Properties of Electrons, Neutrons and Protons

Question for Chapter Notes: Structure of the Atom
Try yourself:Question 1: Which component of atom has a postive charge?

## Thomson's model of an Atom

• In the sphere of protons, electrons are distributed.
• The total positice charge in an atom equal to the total negative charge.
• Atom is electrically neutral.
• Also known as the plum pudding model.

Plum Pudding Model

## Rutherford's Model of an Atom

Rutherford's Experiment

• α-Particles: (+2 charge and 4 mass) when fast-moving α-particles are bombarded on very thin gold foil, the following observations were made:
• Most of the α-particles passed straight through the gold foil.
• Some of the α-particles were deflected by the foil by small angles.
• One out of 12000 particles appeared to rebound.

### Conclusions made by Rutherford

• Most of the space inside the atom is empty because most of the α-particles passed through the gold foil.
• Very few particles were deflected from their path because the +ve charge of the atom occupies very little space.
• A very small fraction of α-particles were rebounded back, showing all + ve charge and mass of the gold atoms were concentrated in a very small volume within the atom.
• The radius of the nucleus calculated was 10⁵ times less than the radius of the atom.
• Nuclear Model of an Atom given by Rutherford
(i) Centre → +ve charge → called nucleus. All mass resides in the nucleus.
(ii) Electrons → revolve around the nucleus in orbits.
(iii) The size of nucleus is very small compared to the size of the atoms.

Rutherford's Nuclear Model of Atom

### Drawbacks of Rutherford's model of the atom

• When an electron undergoes acceleration, it radiates energy. Thus revolving electrons would lose energy and finally fall into the nucleus.
• Due to this atoms should be highly unstable and hence matter would not exist in the form that we know.
• But we know that atoms are quite stable.

Question for Chapter Notes: Structure of the Atom
Try yourself:Question 2: Rutherford’s ‘alpha (α) particles scattering experiment’ resulted in the discovery of

## Bohr's Model of Atom

Postulates of Neil Bohr

• Only special orbits known as discrete orbits of electrons are allowed inside the atom.
• While revolving in discrete orbits the electrons do not radiate energy.
• These orbits are called energy levels.
• Orbits or shells are represented by K, L, M, N or the numbers, n = 1, 2, 3, 4, .......

Bohr's Model

### Drawbacks of Bohr's Model of Atom

• It violates the Heisenberg Uncertainty Principle.
• It could not explain the spectra obtained from larger atoms.

## Distribution of Electrons in Different Orbits

### Rules

• Maximum number of electrons present in a shell is given by 2n2 (n = shell number)

Example: maximum number of electrons present in n = 1 are

n = 1 (K shell)

2(1)2 = 2 electrons

• The maximum number of electrons that can be accommodated in the outermost orbit is 8.
• Electrons are not accommodated in a given shell unless the inner shells are completely filled.

## Valency

• The combining capacity of an atom is called its valency.
• The number of bonds that an atom can form as part of a compound is expressed by the valency of the element.
• Valence electrons are those electrons that are present in the outermost orbit of the atom.

## Atomic Number

• The total number of protons in the nucleus of an atom gives us the atomic number of that atom.
• It is represented with the letter ‘Z.’

## Mass Number

• The number of protons and neutrons combined to give us the mass number of an atom.
• Mass Number is also called Atomic Mass.
• It is represented using the letter ‘A.’

An element is represented by AXZ, where Z is atomic number which is also equal to number of proton, A is mass number and X is symbol of the element. Mass number (A) = Number of protons (Z) + Number of neutrons.

## Isotopes

• Atoms of the same element with the same atomic number but a different mass number, are called isotopes.
• Chemical properties → same
• Physical properties → different
Isotopes of Hydrogen
Applications of Isotopes:
(a) An isotope of Uranium is used as fuel in nuclear reactors.
(b) An isotope of Cobalt is used in the treatment of cancer.
(c) An isotope of Iodine is used in the treatment of goitre.

Question for Chapter Notes: Structure of the Atom
Try yourself:Question 3: The number of electrons in an element X is 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?

## Isobars

• Atoms of different elements with the same mass number but different atomic numbers are called isobars.

Examples of Isobars

### Some Practice Questions:

Ques. How hydrogen atom is different from atoms of all other elements?

Ans. All the atoms are made up of three subatomic particles: electrons, protons and neutrons. Hydrogen atom is made up of only one electron and one proton. It does not contain any neutrons. So, it is different from atoms of all other elements.

Ques. What is mass number?

Ans. The mass number of an element is the sum of the number of protons and neutrons present in the atom of the element.

Mass number = No. of protons + No. of neutrons.

Example: A hydrogen atom has 1 proton but 0 neutron thus the mass number of H is 1.

The mass number of an element is denoted by the letter A. Protons and neutrons present in a nucleus, together known as nucleons.

Hence, Mass number = Number of nucleons.

Ques. What are the general features of isotopes?

Ans. The general features of isotopes are as follows:

1. The isotopes of an element have the same atomic number (i.e., the same number of protons in the   nucleus and the same number of electrons in the extranuclear part)

2. The isotopes of an element have different mass numbers (i.e. different in the number of neutrons present in the nucleus)

3. Isotopes have the same electronic configuration and hence share similar chemical properties.

4. Isotopes of an element have different masses, so they have different physical properties like melting point, boiling point, density etc.

5. Due to differences in the nuclear structure (i.e., number of neutrons), they have different nuclear properties, e.g., the C-14 isotope is radioactive whereas the C-12 isotope is non-radioactive.

Ques. What are isotones?

Ans. Some atoms of different elements have different atomic numbers and different mass numbers but they have a same number of neutrons. These atoms are known as isotones.

Example:12C6 and 16O8.

Both C and O have the same number of neutrons i.e. 8.

The document Structure of the Atom Class 9 Notes Science Chapter 4 is a part of the Class 9 Course Science Class 9.
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## FAQs on Structure of the Atom Class 9 Notes Science Chapter 4

 1. What is the Thomson model of an atom?
Ans. The Thomson model of an atom, proposed by J.J. Thomson in 1904, suggests that an atom is a positively charged sphere with negatively charged electrons embedded in it. According to this model, electrons are distributed uniformly throughout the atom.
 2. What is the Rutherford model of an atom?
Ans. The Rutherford model of an atom, proposed by Ernest Rutherford in 1911, suggests that an atom consists of a small, dense, and positively charged nucleus at the center, surrounded by negatively charged electrons orbiting around it. This model helped to discover the existence of the atomic nucleus.
 3. What is Bohr's model of an atom?
Ans. Bohr's model of an atom, proposed by Niels Bohr in 1913, suggests that electrons move in specific energy levels or orbits around the nucleus. Each energy level has a fixed energy, and electrons can jump from one level to another by absorbing or emitting energy. This model explained the stability of atoms and the spectral lines observed in the emission and absorption of light.
 4. How are electrons distributed in different orbits?
Ans. Electrons are distributed in different orbits or energy levels around the nucleus of an atom. The first energy level closest to the nucleus can hold a maximum of 2 electrons, the second level can hold a maximum of 8 electrons, the third level can hold a maximum of 18 electrons, and so on. Electrons occupy the lowest available energy level before filling the higher energy levels.
 5. What is atomic number and mass number?
Ans. The atomic number of an atom represents the number of protons in its nucleus. It is denoted by the symbol 'Z' and determines the element's identity. For example, hydrogen has an atomic number of 1, indicating it has one proton in its nucleus. The mass number of an atom represents the total number of protons and neutrons in its nucleus. It is denoted by the symbol 'A'. For example, carbon-12 has a mass number of 12, indicating it has 6 protons and 6 neutrons. It is important to note that the number of neutrons can vary in atoms of the same element, leading to the existence of isotopes.

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