Punjab 9th Class Chemistry Chapter 2 – Atomic Structure Online Test, Short Questions Notes

Discovery of Electron: J. J. Thomson discovered electrons through cathode-ray discharge tube experiments. Cathode rays were found to consist of negatively charged particles called electrons.

Discovery of Proton: Goldstein discovered positive rays in a discharge tube. These rays were made up of positively charged particles. The proton was later identified as a fundamental constituent of the nucleus.

Discovery of Neutron: James Chadwick discovered the neutron in 1932. A neutron is a neutral particle present in the nucleus of an atom.

Niels Bohr proposed his atomic model in 1913. According to this model:

Electrons revolve around the nucleus in fixed circular paths called orbits or shells.

Each shell has a fixed energy level.

Electrons do not radiate energy while moving in a permitted fixed orbit.

When an electron absorbs energy, it moves from a lower energy level to a higher energy level.

When an electron falls from a higher energy level to a lower energy level, it emits energy.

The maximum number of electrons in a shell is calculated using the formula:

Maximum number of electrons = 2n²

Here, n represents the shell number.

Atomic Number: The atomic number is the number of protons present in the nucleus of an atom.

Mass Number: The mass number is the total number of protons and neutrons present in the nucleus.

Example 1: Sodium (Na-23)

Atomic number = 11

Mass number = 23

Number of protons = 11

Number of electrons = 11 in a neutral atom

Number of neutrons = 23 − 11 = 12

Example 2: Oxygen (O-16)

Atomic number = 8

Mass number = 16

Number of protons = 8

Number of electrons = 8 in a neutral atom

Number of neutrons = 16 − 8 = 8

Isotopes are atoms of the same element that have the same atomic number but different mass numbers. They differ in the number of neutrons.

Applications of Radioactive Isotopes:

Medical Diagnosis: Technetium-99m is used to obtain images of organs such as the brain and lungs.

Cancer Treatment: Cobalt-60 is used in radiation therapy to destroy cancer cells.

Carbon Dating: Carbon-14 is used to estimate the age of ancient organic materials.

Industrial Testing: Radioactive isotopes are used to inspect materials and detect defects in metals and other structures.

Power Generation: Radioactive materials are used as fuels in nuclear power generation.

Radioactivity: Radioactivity is the spontaneous emission of radiation from an unstable atomic nucleus. This radiation may include alpha particles, beta particles, or gamma rays.

Ionization: Ionization occurs when radiation removes electrons from atoms. The atom becomes a positive ion, while the removed electron becomes a free electron.

Example:

A neutral atom loses an electron:

A → A⁺ + e⁻

Here, A represents a neutral atom, A⁺ represents the positive ion, and e⁻ represents the removed electron.

Relative Atomic Mass is the weighted average mass of the isotopes of an element compared with one-twelfth of the mass of a carbon-12 atom.

Example: Nitrogen

Nitrogen-14 has an abundance of 99.64%.

Nitrogen-15 has an abundance of 0.355%.

Formula:

Relative atomic mass = [(Mass of isotope 1 × Percentage abundance) + (Mass of isotope 2 × Percentage abundance)] ÷ 100

Calculation:

Relative atomic mass = [(14 × 99.64) + (15 × 0.355)] ÷ 100

Relative atomic mass ≈ 14.021

Therefore, the relative atomic mass of nitrogen is approximately 14.021.

Discovery of Electron: J. J. Thomson discovered electrons through cathode-ray discharge tube experiments. Cathode rays were found to consist of negatively charged particles called electrons.

Discovery of Proton: Goldstein discovered positive rays in a discharge tube. These rays consisted of positively charged particles. The proton was later identified as a fundamental constituent of the atomic nucleus.

Discovery of Neutron: James Chadwick discovered the neutron in 1932. A neutron is a neutral particle present in the nucleus of an atom.

Niels Bohr proposed his atomic model in 1913. According to this model:

Electrons revolve around the nucleus in fixed circular paths called orbits or shells.

Each shell has a fixed energy level.

Electrons do not radiate energy while moving in a permitted fixed orbit.

When an electron absorbs energy, it moves from a lower energy level to a higher energy level.

When an electron falls from a higher energy level to a lower energy level, it emits energy.

The maximum number of electrons in a shell is calculated using the formula:

Maximum number of electrons = 2n²

Here, n represents the shell number.

Atomic Number: The atomic number is the number of protons present in the nucleus of an atom.

Mass Number: The mass number is the total number of protons and neutrons present in the nucleus.

Example 1: Sodium-23

Atomic number = 11

Mass number = 23

Protons = 11

Electrons = 11 in a neutral atom

Neutrons = 23 − 11 = 12

Example 2: Oxygen-16

Atomic number = 8

Mass number = 16

Protons = 8

Electrons = 8 in a neutral atom

Neutrons = 16 − 8 = 8

Isotopes are atoms of the same element that have the same atomic number but different mass numbers. They differ in the number of neutrons.

Applications of Radioactive Isotopes:

Medical Diagnosis: Technetium-99m is used to obtain images of organs such as the brain and lungs.

Cancer Treatment: Cobalt-60 is used in radiation therapy to destroy cancer cells.

Carbon Dating: Carbon-14 is used to estimate the age of ancient organic materials.

Industrial Testing: Radioactive isotopes are used to inspect materials and detect defects in metals and concrete.

Power Generation: Radioactive materials are used as fuels in nuclear power generation.

Radioactivity: Radioactivity is the spontaneous emission of radiation from an unstable atomic nucleus. This radiation may include alpha particles, beta particles, or gamma rays.

Ionization: Ionization occurs when radiation removes electrons from atoms. The atom becomes a positive ion, while the removed electron becomes a free electron.

Example:

A neutral atom loses an electron:

A → A⁺ + e⁻

Here, A represents a neutral atom, A⁺ represents the positive ion, and e⁻ represents the removed electron.

Relative Atomic Mass is the weighted average mass of the isotopes of an element compared with one-twelfth of the mass of a carbon-12 atom.

Example: Nitrogen

Nitrogen-14 has an abundance of 99.64%.

Nitrogen-15 has an abundance of 0.355%.

Formula:

Relative atomic mass = [(14 × 99.64) + (15 × 0.355)] ÷ 100

Relative atomic mass ≈ 14.021

Therefore, the relative atomic mass of nitrogen is approximately 14.021.