Punjab textbook board

The detailed, step-by-step solutions to Chapter 2, “Atomic Structure,” from the new Punjab Board 9th Class textbook (2025 edition) by Ahsan Publishers. Aligned with the SLO-based syllabus, this guide provides clear explanations, solved exercises, and conceptual insights tailored to help students excel in their studies. Perfect for students preparing for exams and teachers seeking comprehensive teaching material.

(i) How many electrons can be accommodated at the most in the third shell of the elements?

• Options:
  (a) 8
  (b) 18
  (c) 10
  (d) 32
• Answer: (b) 18
• Explanation:
  The maximum number of electrons in any shell is determined by the formula 2n², where n is the shell number. For the third shell = 2(9) = 18 Thus, the third shell can accommodate a maximum of 18 electrons.

(ii) What information was obtained from discharge tube experiments?

• Options:
  (a) Structure of atom was discovered.
  (b) Neutrons and protons were discovered.
  (c) Electrons and protons were discovered.
  (d) Presence of nucleus in an atom was discovered.
• Answer: (d) Presence of nucleus in an atom was discovered.
• Explanation:
  Rutherford’s experiments using a discharge tube and his gold foil experiment showed that atoms have a small, dense, positively charged nucleus at their center. These discoveries laid the foundation for understanding atomic structure.

(iii) Why have isotopes not been shown in the periodic table?

• Options:
  (a) Periodic table cannot accommodate a large number of isotopes of different elements.
  (b) Some of the isotopes are unstable and they give rise to different elements.
  (c) All the isotopes have the same atomic number; so there is no need to give them separate places.
  (d) Isotopes do not show periodic behavior.
• Answer: (c) All the isotopes have the same atomic number; so there is no need to give them separate places.
• Explanation:
  Isotopes of an element have the same number of protons but differ in the number of neutrons. Since the periodic table is based on atomic number, isotopes are not given separate places—they occupy the same position.

(iv) Which particle is present in a different number in the isotopes?

• Options:
  (a) Electron
  (b) Neutron
  (c) Proton
  (d) Both neutron and electron
• Answer: (b) Neutron
• Explanation:
  Isotopes are variants of the same element that have the same number of protons and electrons but differ in the number of neutrons. This difference in neutron number affects their atomic mass but not their chemical properties.

(v) Predict the boiling point of heavy water (D₂O).

• Options:
  (a) 101.4°C
  (b) 98.2°C
  (c) 100°C
  (d) 105.4°C
• Answer: (a) 101.4°C
• Explanation:
  Heavy water (D₂O) has deuterium atoms instead of regular hydrogen atoms. Because deuterium is heavier, heavy water has slightly stronger intermolecular forces, resulting in a higher boiling point (approximately 101.4°C) compared to normal water (100°C).

(vi) What will be the relative atomic mass of hydrogen given the abundances of its two isotopes, 99.9844% and 0.0156%?

• Options:
  (a) 1.0078
  (b) 1.0784
  (c) 1.0800
  (d) 1.0700
• Answer: (a) 1.0078
• Explanation:
  The relative atomic mass is calculated using the formula: Relative atomic mass=(m1⋅f1)+(m2⋅f2) , m1=1.0078, f1=0.999844 m2=2.0140 f2=0.000156 : (1.0078⋅0.999844)+(2.0140⋅0.000156)≈1.0078

(vii) How is radiocarbon dating useful for archeologists?

• Options:
  (a) It helps determine the age of organic matter.
  (b) It helps determine the composition of matter.
  (c) It helps determine the usefulness of matter.
  (d) It helps determine whether the matter is radioactive or not.
• Answer: (a) It helps determine the age of organic matter.
• Explanation:
  Radiocarbon dating is based on measuring the decay of carbon-14, a radioactive isotope, to determine the age of organic materials such as bones, wood, or fossils. This technique is widely used in archeology to estimate the age of artifacts.

(viii) What does keep the particles present in the nucleus intact?

• Options:
  (a) Particles are held together by strong nuclear force.
  (b) Particles are held together by weak nuclear force.
  (c) Particles are held together by electrostatic force.
  (d) Particles are held together by dipolar force.
• Answer: (a) Particles are held together by strong nuclear force.
• Explanation:
  The strong nuclear force is the fundamental force that binds protons and neutrons in the nucleus, overcoming the electrostatic repulsion between positively charged protons. This force operates only at very short ranges.

(ix) How do electrons keep themselves away from the oppositely charged nucleus?

• Options:
  (a) By keeping themselves stationary
  (b) By revolving around the nucleus
  (c) Due to their wave-like nature
  (d) A magnetic field around the nucleus keeps them away
• Answer: (b) By revolving around the nucleus
• Explanation:
  Electrons revolve around the nucleus due to the balance between the electrostatic attraction to the positively charged nucleus and the centrifugal force from their motion. This concept is based on Bohr’s atomic model.

(x) Rubidium consists of two isotopes 85 and 87 . The percent abundance of the light isotope is 72.2%. What is the percent abundance of the heavier isotope? Its atomic mass is 85.47.

• Options:
  (a) 15%
  (b) 28%
  (c) 37%
  (d) 72%
• Answer: (b) 28%
• Explanation:
  The percent abundances of isotopes must add up to 100%. Given that the light isotope has an abundance of 72.2%, the heavier isotope will have: 100%−72.2%=28%100

2. Questions for Short Answers

(i) Why is it said that almost all the mass of an atom is concentrated in its nucleus?
The nucleus of an atom contains protons and neutrons, which are much heavier than electrons. Since electrons are very light and located outside the nucleus, nearly all the mass of an atom comes from the protons and neutrons in its nucleus.

(ii) Why are elements different from one another?
Elements are different because they have different numbers of protons in their nuclei. The number of protons (also called the atomic number) is unique to each element and determines its properties and behavior.

(iii) How many neutrons are present in ₈₃Bi ²¹⁰
The isotope Bi 210 has a mass number (number of protons and number of neutrons) of 210 and an atomic number (number of protons) of 83. Number of neutrons=Mass number−Atomic number=209−83=126

So, it has 126 neutrons.

(iv) Why is tritium a radioactive element?
Tritium is radioactive because its nucleus, which contains one proton and two neutrons, is unstable. This instability causes it to emit radiation as it breaks down into a more stable form.

(v) How can an atom absorb and evolve energy?
Atoms absorb energy when electrons move to higher energy levels (excited state) after gaining energy. They release energy when electrons return to lower energy levels (ground state). This process is observed as the emission or absorption of light or other forms of energy.

3. Constructed Response Questions

(i) Why does the energy of an electron increase as we move from the first shell to the second shell?
The energy of an electron increases as we move from the first shell (closer to the nucleus) to the second shell (farther from the nucleus) due to the concept of electrostatic attraction. Electrons in the first shell are held more tightly by the positive charge of the nucleus because they are closer to it. To move an electron from the first shell to the second shell, it must overcome the strong attractive force of the nucleus, which requires energy. Additionally, electrons in higher shells have higher potential energy because they are less bound to the nucleus and are more “free” to move.

(ii) Why is it needed to lower the pressure of the gas inside the discharge tube?
In a discharge tube, lowering the pressure reduces the number of gas particles per unit volume. This is important because high-pressure gas would cause frequent collisions between gas particles, preventing the movement of free electrons and ions. At low pressure, the gas becomes less dense, allowing electrons to move freely through the tube and collide with gas atoms. These collisions excite the gas atoms, which then emit light as they return to their ground state, creating a visible glow or discharge. This principle was crucial in experiments like J.J. Thomson’s cathode ray experiments, which led to the discovery of the electron.

(iii) What is the classical concept of an electron? How has this concept changed with time?
The classical concept of an electron, based on the early atomic models such as J.J. Thomson’s and Bohr’s models, described electrons as small particles orbiting the nucleus in fixed circular paths (orbits), similar to planets orbiting the sun. However, with the development of quantum mechanics, this concept changed. The modern quantum mechanical model describes electrons as existing in regions called orbitals, where there is a high probability of finding them. Unlike fixed paths, orbitals represent three-dimensional regions around the nucleus, and electrons exhibit both particle-like and wave-like behavior. This understanding was made possible by advancements in the Schrödinger equation and Heisenberg’s uncertainty principle.

(iv) Why are the nuclei of radioactive elements unstable?
The nuclei of radioactive elements are unstable because of an imbalance in the number of protons and neutrons. A stable nucleus requires an optimal ratio of protons to neutrons, but in radioactive elements, this balance is disrupted. This imbalance results in excessive repulsive forces between protons, or insufficient binding forces among neutrons, causing the nucleus to emit radiation (alpha, beta, or gamma rays) in an attempt to reach a more stable state. The larger the nucleus, the harder it is to maintain stability, which is why heavier elements are often radioactive.

(v) During discharge tube experiments, how did scientists conclude that the same type of electrons and protons are present in all the elements?
Scientists observed that the cathode rays produced in a discharge tube always exhibited the same properties, regardless of the gas used in the tube. These rays were found to consist of negatively charged particles (electrons) with a fixed charge-to-mass ratio, independent of the type of atom or element present. Similarly, the positive rays (protons) generated in these experiments had consistent properties across different gases. This uniformity demonstrated that electrons and protons are fundamental components of all atoms, and their properties do not vary from one element to another.

4. Descriptive Questions

(i) Explain the structure of a hydrogen atom.
A hydrogen atom is the simplest atom, consisting of a single proton in the nucleus and one electron revolving around it. The nucleus, which contains the proton, is positively charged, while the electron is negatively charged. The electron occupies a specific energy level (or shell) around the nucleus. In the modern quantum mechanical model, the electron is described as existing in a spherical orbital around the nucleus, where it is most likely to be found. Since hydrogen has no neutrons, its mass is almost entirely due to the proton.

(ii) How does the theory of atomic structure explain the ionization of atoms by a radioactive isotope?
Ionization occurs when an atom loses or gains an electron, resulting in a charged particle called an ion. Radioactive isotopes emit high-energy particles or radiation (alpha, beta, or gamma rays) that can knock electrons out of atoms. For example, beta particles emitted by radioactive isotopes have enough energy to remove electrons from nearby atoms, creating ions. This process is the basis for many applications of radioactive isotopes, such as radiation therapy and smoke detectors.

(iii) What is radioactivity? Explain any three applications of radioactive isotopes.
Radioactivity is the spontaneous emission of particles or energy from the unstable nucleus of an atom. This occurs as the nucleus seeks to become more stable.
Applications:

1. Medical Applications: Radioactive isotopes like iodine-131 are used in diagnosing and treating thyroid diseases, and cobalt-60 is used in cancer radiation therapy.
2. Archaeology: Carbon-14 dating is used to determine the age of ancient objects like fossils or artifacts by measuring the decay of carbon-14 in organic materials.
3. Energy Production: Uranium-235 and plutonium-239 are used as fuel in nuclear reactors to generate electricity through controlled nuclear fission reactions.

(iv) Find out the relative atomic mass of mercury from the given data.
To calculate the relative atomic mass, use the formula:

A_r=∑ (Isotope mass × Relative abundance fraction)

Substitute the given values:

A_r=(199×0.00146)+(198×0.1002)+(200×0.1322)+(201×0.0685)+(202×0.1634)+(204×0.2313)+(206×0.298)

After performing the calculations, the final relative atomic mass is obtained. This process demonstrates how isotopes contribute to the average atomic mass of an element.

5. Investigative Questions

(i) How can scientists synthesize elements in the laboratory?
Scientists synthesize new elements by bombarding the nuclei of existing elements with high-speed particles such as protons, neutrons, or heavier ions. This process often takes place in particle accelerators, where the nuclei are forced to collide with enough energy to fuse and form a new element. For example, synthetic elements like uranium-235 and transuranium elements like americium and einsteinium were created in this way. These processes require precise conditions and advanced technology.

(ii) A system just like our solar system exists in an atom. Comment on this statement.
The early atomic model, proposed by Niels Bohr, compared the atom to a miniature solar system, with electrons revolving around the nucleus in circular orbits, similar to how planets orbit the sun. While this analogy helped visualize atomic structure, modern quantum mechanics has shown that electrons do not follow fixed orbits. Instead, they exist in probabilistic regions of space called orbitals, which are shaped by their energy levels and interactions. Unlike planets, electrons also exhibit wave-particle duality, a concept that cannot be explained using the solar system analogy.