Class 9 Chemistry Notes – Chapter 2

Subject: Chemistry

Class: 9th

Board: Punjab Board

Chapter 2: Atomic Structure

Exercise: Multiple Choice Questions (MCQs)

(i) Cathode rays can deflect:

Answer: (c) Both in electric and magnetic fields

(ii) Which of the following is correct about the charge on a neutron?

Answer: (a) Zero

(iii) The number of protons in a nucleus is called:

Answer: (b) Atomic number

(iv) The total number of protons and neutrons in a nucleus is called:

Answer: (c) Mass number

(v) Which one of the following is a heavy particle?

Answer: (d) Neutron

(vi) The isotopes of an element have:

Answer: (a) Same atomic number but different mass numbers

(vii) The isotope used in nuclear reactors is:

Answer: (b) Uranium-235

(viii) The mass of an electron is about:

Answer: (b) 1/1836 times the mass of a proton

(ix) Which of the following scientists discovered the proton?

Answer: (a) E. Goldstein

(x) Rutherford’s alpha-particle scattering experiment proved that:

Answer: (c) The nucleus is present at the center of the atom

Exercise: Questions for Short Answers

i. Why is it said that almost all the mass of an atom is concentrated in its nucleus?

Answer:

Almost all the mass of an atom is concentrated in its nucleus because protons and neutrons, which reside inside the nucleus, are each about 1836 times heavier than electrons, which orbit outside the nucleus.

ii. How do cathode rays prove that electrons are negatively charged particles?

Answer:

Cathode rays are deflected toward the positive plate in an electric field. Their deflection in a magnetic field also confirms that they carry a negative electrical charge.

iii. Why have isotopes not been shown separately in the periodic table?

Answer:

Isotopes have not been shown separately in the periodic table because they possess the same chemical properties and electronic configuration, occupying the same position in the table.

iv. What are the main defects of Rutherford’s atomic model?

Answer:

According to classical physics, revolving charged particles should continuously radiate energy and spiral into the nucleus, but atoms are stable. Furthermore, Rutherford’s model should produce a continuous spectrum instead of the observed line spectrum.

v. Why do isotopes of the same element have different physical properties?

Answer:

Isotopes have different physical properties because physical properties depend on atomic mass, and isotopes have different numbers of neutrons, resulting in different masses.

vi. Define atomic number and mass number with examples.

Answer:

Atomic number is the total number of protons in the nucleus of an atom. For example, carbon has an atomic number of 6.

Mass number is the sum of protons and neutrons in the nucleus. For example, carbon-12 has a mass number of 12.

vii. What is the role of radioactive isotopes in medical diagnosis?

Answer:

Radioactive isotopes act as tracers in medical diagnosis to detect tumors, monitor organ function, and study metabolic pathways. For example, Iodine-131 is used for thyroid disorders, while Technetium-99m is used for imaging various organs.

viii. How did Chadwick discover the neutron?

Answer:

Chadwick discovered the neutron by bombarding a beryllium target with alpha particles. This produced highly penetrating, neutral radiation consisting of particles with a mass nearly equal to that of a proton.

Exercise: Constructed Response Questions

i. If an atom has atomic number 11 and mass number 23, calculate the number of electrons, protons, and neutrons.

Answer:

Number of protons = 11

Number of electrons = 11

Number of neutrons = Mass number − Atomic number

Number of neutrons = 23 − 11 = 12

Therefore, the atom has 11 protons, 11 electrons, and 12 neutrons.

ii. Why did Rutherford select gold foil for his scattering experiment instead of other metals?

Answer:

Gold was selected because it can be beaten into an extremely thin foil only a few hundred atoms thick. This minimizes multiple scattering of alpha particles and allows clear observation of their deflection.

iii. How does Bohr’s model overcome the stability defect of Rutherford’s model?

Answer:

Bohr proposed that electrons revolve only in specific stationary orbits or shells corresponding to fixed energy levels. While in these orbits, electrons do not radiate energy, preventing them from spiraling into the nucleus.

iv. Discuss the importance of isotopes using carbon dating and medical imaging as examples.

Answer:

Carbon-14 is used in carbon dating to determine the age of ancient organic artifacts by measuring radioactive decay. Medical imaging utilizes radioactive isotopes such as Technetium-99m and Iodine-131 as tracers to detect diseases and examine internal body structures.

v. Why are cathode rays named so?

Answer:

Cathode rays are named so because they originate from the cathode, which is the negative electrode inside a discharge tube when a high voltage is applied across a low-pressure gas.

vi. What are the characteristics of canal rays?

Answer:

Canal rays consist of positively charged particles whose charge-to-mass ratio depends on the nature of the residual gas in the discharge tube. They are heavier than electrons and travel in a direction opposite to cathode rays.

vii. Explain why isotopes have identical chemical properties.

Answer:

Isotopes have identical chemical properties because chemical behavior is determined by the number and arrangement of electrons, particularly valence electrons. These remain identical for all isotopes of the same element.

viii. How can you calculate the relative atomic mass of an element from its isotopic abundances?

Answer:

The relative atomic mass is calculated by multiplying the isotopic masses by their respective percentage abundances, adding the products, and dividing the total by 100.

Formula:

Relative atomic mass = Σ (Isotopic mass × Percentage abundance) / 100

Exercise: Descriptive Questions

i. Describe the discovery of electrons with the help of a discharge tube experiment.

Answer:

J. J. Thomson used a discharge tube containing gas at low pressure, with a high voltage applied across two electrodes. He observed rays traveling from the cathode to the anode that produced a green glow on the glass wall. These rays were deflected by electric and magnetic fields toward the positive plate, proving the existence of negatively charged subatomic particles called electrons.

ii. Explain Rutherford’s atomic model and discuss its observations and conclusions.

Answer:

Rutherford bombarded a thin gold foil with alpha particles.

Observations:

1. Most alpha particles passed straight through the foil without deflection.

2. A few alpha particles were deflected at various angles.

3. Very few alpha particles bounced straight back.

Conclusions:

1. An atom is mostly empty space.

2. A dense, positively charged center called the nucleus contains almost all the mass of the atom.

3. Electrons revolve around the central nucleus.

iii. Give a detailed account of Bohr’s atomic model. What are its postulates?

Answer:

Bohr’s atomic model is based on quantum theory and includes the following postulates:

1. Electrons revolve around the nucleus in circular orbits called energy levels or shells, designated by the principal quantum number (n = 1, 2, 3, …).

2. Each orbit has a fixed energy. While an electron remains in an orbit, it does not radiate energy.

3. Energy is emitted or absorbed only when an electron jumps from one orbit to another.

The energy change is given by the Planck equation:

ΔE = E₂ − E₁ = hf

Here, ΔE is the change in energy, h is Planck’s constant, and f is the frequency of radiation.

iv. What are isotopes? Explain the isotopes of hydrogen and carbon.

Answer:

Isotopes are atoms of the same element that have the same atomic number but different mass numbers due to different numbers of neutrons.

Isotopes of Hydrogen:

1. Protium (¹₁H): Contains 1 proton and 0 neutrons.

2. Deuterium (²₁H or D): Contains 1 proton and 1 neutron.

3. Tritium (³₁H or T): Contains 1 proton and 2 neutrons. It is radioactive.

Isotopes of Carbon:

1. Carbon-12: Approximately 98.9% abundance.

2. Carbon-13: Approximately 1.1% abundance.

3. Carbon-14: Present in trace amounts and radioactive.

v. Describe the properties of neutrons.

Answer:

Neutrons are neutral subatomic particles with no electrical charge. Their mass is nearly equal to that of a proton, approximately 1.675 × 10⁻²⁷ kg or 1.0087 atomic mass units (amu). They are constituents of all atomic nuclei except ordinary hydrogen (protium). Due to their lack of electrical charge, neutrons can penetrate matter without being deflected by electric fields.

vi. Compare the location, charge, and relative mass of protons, neutrons, and electrons.

Answer:

Electron:

Location: Outside the nucleus in electron shells.

Charge: −1

Relative mass: 1/1836

Proton:

Location: Inside the nucleus.

Charge: +1

Relative mass: 1

Neutron:

Location: Inside the nucleus.

Charge: 0 (neutral)

Relative mass: 1.0087

Exercise: Numerical Problems / Calculation Tasks

Q1. Calculate the number of neutrons, protons, and electrons in the following atoms: Pt-195 and U-238.

Answer:

For Platinum (Pt-195):

Mass number (A) = 195

Atomic number (Z) = 78

Number of protons = Atomic number = 78

Number of electrons = 78 (for a neutral atom)

Number of neutrons = Mass number − Atomic number

Number of neutrons = 195 − 78 = 117

Therefore, Pt-195 has 78 protons, 78 electrons, and 117 neutrons.

For Uranium (U-238):

The calculation uses the following standard formulae:

Number of protons = Atomic number (Z)

Number of electrons = Atomic number (Z), for a neutral atom

Number of neutrons = Mass number (A) − Atomic number (Z)

Q2. Calculate the relative atomic mass of an element given the percentage abundances of its isotopes.

Answer:

The relative atomic mass is calculated by multiplying the mass of each isotope by its percentage abundance, adding all the products, and dividing the sum by 100.

Formula:

Relative atomic mass = Σ (Isotopic mass × Percentage abundance) / 100

Note: The specific isotopic masses and percentage abundances are required to calculate a numerical answer.

Notes by Sir Dilawar

For more notes, visit QuranEducationPoint.com