Chapter 4: Structure of the Atom
This comprehensive study material is fully aligned with the KVS Class IX Science Student Support Material (2025-26). It details all foundational concepts of sub-atomic particles, historical atomic models, Bohr's postulates, electronic configurations, valency, atomic and mass numbers, isotopes, and isobars, with complete solved questions from the official bank.
1. Introduction to Sub-Atomic Particles
All matter is made of very tiny particles called atoms. Initially thought to be indivisible by John Dalton, modern science has established that the atom is divisible and composed of several sub-atomic particles. The three primary sub-atomic particles are electrons, protons, and neutrons.
(a) Electron
- Discovery: Identified by J. J. Thomson in 1897.
- Properties: Represented as $e^-$. It is a negatively charged particle. The mass of an electron is considered to be negligible ($1/2000^{\text{th}}$ the mass of a hydrogen atom), and its relative charge is minus one ($-1$).
(b) Proton
- Discovery: First observed by E. Goldstein in 1886 as positively charged radiations in a gas discharge, which he named canal rays. It was later formally identified as the proton by Ernest Rutherford during 1917–1920.
- Properties: Represented as $p^+$. It is a positively charged particle. Its mass is taken as $1\mathrm{~u}$ (approximately 2000 times that of an electron) and its charge is plus one ($+1$).
Canal rays are positively charged radiations that consist of particles having charges equal in magnitude and opposite in sign to that of an electron, passing from anode to cathode in a gas discharge tube.
(c) Neutron
- Discovery: Discovered by James Chadwick in 1932.
- Properties: Represented as $n$. It is an electrically neutral particle (carrying no charge) with a mass nearly equal to that of a proton ($1\mathrm{~u}$). Neutrons are present in the nucleus of all atoms except Hydrogen-1 (protium).
The mass of an atom is calculated as the sum of the masses of the protons and neutrons present in its nucleus: \[ \text{Atomic Mass } (A) = \text{protons } (p) + \text{neutrons } (n) \] An atom is electrically neutral overall because it contains an equal number of protons and electrons, mutually balancing their positive and negative charges.
2. Historical Models of an Atom
To explain how these charged sub-atomic particles are arranged within an atom, various scientists proposed different atomic models.
A) Thomson's Model of an Atom (Plum-Pudding / Watermelon Model)
J. J. Thomson proposed that the structure of an atom resembles a Christmas pudding or a watermelon. The main postulates are:
- An atom consists of a positively charged sphere, and the electrons are embedded in it like dry fruits (plums) in a pudding or seeds in a watermelon.
- The negative and positive charges are equal in magnitude, making the atom as a whole electrically neutral.
Drawback: Although it explained electrical neutrality, the model was highly speculative. Other scientists' subsequent experiments (especially Rutherford's) yielded results that could not be explained by Thomson's model.
B) Rutherford's Model of an Atom (Alpha Scattering Experiment)
Ernest Rutherford, known as the 'Father of nuclear physics', designed an experiment where fast-moving alpha ($\alpha$)-particles were made to fall on a thin gold foil.
- Why Gold Foil? Gold is the most malleable metal. Rutherford wanted the thinnest layer possible, and the gold foil he selected was only about 1000 atoms thick.
- Why Alpha ($\alpha$) Particles? Alpha particles are double-charged helium ions ($\mathrm{He^{2+}}$) with a mass of $4\mathrm{~u}$ and a $+2$ charge. Because they are fast-moving, they possess a considerable amount of kinetic energy.
Observations of Rutherford's Experiment:
- Most of the fast-moving $\alpha$-particles passed straight through the gold foil without any deflection.
- Some of the $\alpha$-particles were deflected by the foil by small angles.
- Surprisingly, one out of every 12,000 particles appeared to rebound, deflecting backwards by an angle of $180^{\circ}$.
Rutherford's Conclusions:
- Most of the space inside the atom is empty because most $\alpha$-particles passed through without deflection.
- Very few particles were deflected, indicating that the positive charge of the atom occupies very little space.
- A tiny fraction of $\alpha$-particles rebounded, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom, which he called the nucleus.
Rutherford's Nuclear Model Postulates:
- There is a positively charged center in an atom called the nucleus. Nearly all the mass of an atom resides in this nucleus.
- The electrons revolve around the nucleus in circular paths.
- The size of the nucleus is extremely small compared to the size of the atom (the radius of the nucleus is about $10^5$ times smaller than that of the atom).
The revolution of the electron in a circular orbit is not expected to be stable. Any particle moving in a circular orbit undergoes continuous acceleration. During acceleration, charged particles must radiate energy. Thus, the revolving electron would lose energy and eventually spiral into the nucleus. If this were so, atoms would be highly unstable, and matter would collapse. However, we know that atoms are exceptionally stable.
C) Bohr's Model of an Atom
To overcome the objections raised against Rutherford's model, Neils Bohr put forward the following postulates in 1913 (for which he received the Nobel Prize in 1922):
- Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
- While revolving in these discrete orbits, the electrons do not radiate energy. These orbits or shells are called energy levels.
- These shells are represented by the letters $\mathrm{K, L, M, N,...}$ or numbers $n = 1, 2, 3, 4,...$.
3. Bohr-Bury Scheme: Electron Distribution Rules
The distribution of electrons into different orbits of an atom was suggested by Neils Bohr and Charles Bury. The main rules are:
- The maximum number of electrons present in a shell is given by the formula $2n^2$, where $n$ is the shell number.
Shell Number (\(n\)) Shell Name Maximum Electron Capacity (\(2n^2\)) 1 K \(2 \times 1^2 = 2\) 2 L \(2 \times 2^2 = 8\) 3 M \(2 \times 3^2 = 18\) 4 N \(2 \times 4^2 = 32\) 5 O \(2 \times 5^2 = 50\) - The maximum number of electrons that can be accommodated in the outermost orbit is **8**. This is known as the octet rule.
- Electrons are filled in shells in a step-wise manner. Inner shells are completely filled before electrons enter outer shells ($\mathrm{K \rightarrow L \rightarrow M \rightarrow N}$).
4. Key Definitions & Formulas
Valency
The combining capacity of an element is known as its valency. The electrons present in the outermost shell of an atom are called valence electrons. If the outermost shell has close to a full octet, the valency is calculated by subtracting the valence electrons from 8.
Atomic Number (\(Z\))
The total number of protons present in the nucleus of an atom. Since atoms are neutral, this is also equal to the number of electrons. It defines the unique identity of an element.
Mass Number (\(A\))
The sum of the total number of protons and neutrons present in the nucleus of an atom.
Neutrons Calculation:
Notation representing an Element:
5. Isotopes and Isobars
Isotopes
Atoms of the same element that have the same atomic number but different mass numbers. They exhibit identical chemical properties but different physical properties.
- Isotopes of Hydrogen:
- Protium: $\mathrm{^{1}_{1}H}$
- Deuterium: $\mathrm{^{2}_{1}H}$ (represented as \(\mathrm{D}\))
- Tritium: $\mathrm{^{3}_{1}H}$ (represented as \(\mathrm{T}\))
- Isotopes of Carbon: Carbon-12 ($\mathrm{^{12}_{6}C}$) and Carbon-14 ($\mathrm{^{14}_{6}C}$).
- Isotopes of Chlorine: Chlorine occurs in nature in two isotopic forms with mass $35\mathrm{~u}$ and $37\mathrm{~u}$ in a $3:1$ ratio.
Calculation of Average Atomic Mass of Chlorine:
Applications of Isotopes in Real Life:
- An isotope of uranium ($\mathrm{^{235}U}$) is used as a fuel in nuclear reactors.
- An isotope of cobalt ($\mathrm{^{60}Co}$) is used in the treatment of cancer.
- An isotope of iodine ($\mathrm{^{131}I}$) is used in the treatment of goitre disease.
- An isotope of carbon ($\mathrm{^{14}C}$) is used in carbon dating to determine the age of ancient fossils.
Isobars
Atoms of different elements with different atomic numbers but possessing the same mass numbers. They have different chemical properties due to different electron configurations but share identical mass. Examples:
- Argon ($\mathrm{^{40}_{18}Ar}$), Potassium ($\mathrm{^{40}_{19}K}$), and Calcium ($\mathrm{^{40}_{20}Ca}$).
6. Chapterwise Question Bank
Multiple Choice Questions (1 Mark Each)
MCQ 1 The sub-atomic particles like proton is different from electron. How?
(a) An electron is much heavier than a proton
(b) An electron carries more charge than a proton
(c) A proton is easily removed from an atom, but not an electron
(d) A proton is positively charged, whereas an electron is negatively charged.
MCQ 2 An atom has 4 electrons, 4 protons, and 4 neutrons. The atom is electrically neutral. Which difference in properties of protons and electrons affects the electrical neutrality of the atom?
(a) Electrons are 2000 times lighter than protons.
(b) Electrons and protons have the same charge and mass.
(c) Electrons and protons have opposite charges of the same magnitude.
(d) Electrons are present in the outer side of the atom compared to the protons.
MCQ 3 Thomson's Model of an atom failed because?
i. It could not explain the screening of negative charges from that of positive.
ii. It did not tell about the presence of electrons.
iii. It did not give an idea about the discrete energy levels.
iv. It explained the atom as a whole to be electrically neutral.
Choose the correct option: (a) Only (iii) (b) Both (i) & (iii) (c) Only (i) (d) Both (ii) & (iv)
MCQ 4 Which of the following statements is incorrect about the structure of an atom?
i. The whole mass of an atom is concentrated in the nucleus.
ii. The atom is an indivisible particle.
iii. The atom as a whole is neutral.
iv. All the atoms are stable in their basic state.
Choose the correct option: (a) (i) and (iii) (b) Only (ii) (c) (ii) and (iv) (d) None of these
MCQ 5 Which of the following is an incorrect statement in reference with observations in Rutherford's particle scattering experiment?
(a) Some of the \(\alpha\)-particles rebound after hitting the gold foil.
(b) Some of the particles deflected from their path.
(c) Some of the particles do not pass through the gold foil.
(d) Most of the particles pass straight through the gold foil.
MCQ 6 The electronic configuration of Chlorine is:
(a) \(2, 7\) (b) \(2, 8, 8, 7\) (c) \(2, 8, 7\) (d) \(2, 7, 8\)
Assertion-Reason Based Questions (1 Mark Each)
Directions: Choose (a) if both A and R are true and R is correct explanation of A; (b) if both are true but R is not correct explanation; (c) if A is true, R is false; (d) if A is false, R is true.
A/R 7
Assertion (A): For noble gases, valency is zero.
Reason (R): Noble gases have 8 valence electrons.
A/R 8
Assertion (A): The mass of the total number of protons and neutrons is a measure of the approximate mass of an atom.
Reason (R): The mass of an electron is negligible.
A/R 9
Assertion (A): Isotopes are electrically neutral.
Reason (R): Isotopes are species with same mass number but different atomic number.
A/R 10
Assertion (A): The size of the nucleus is very small as compared to the size of the atom.
Reason (R): The electrons revolve around the nucleus of the atom.
Very Short Answer Questions (2 Marks Each)
Q11 An element 'X' has mass number 4 and atomic number 2. Write the valency of this element. Will it react with other atoms of different elements?
Q12 An element 'A' has valency +3, while another element 'B' has valency -2. Give the formula of their compound formed when 'A' reacts with 'B'.
Q13 The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of both these elements is 40. What is the name given to such a pair of elements?
Q14 Find out the valency of the atoms represented by the Figs. (a) and (b).
Q15 Identify the \(\mathrm{Na^+}\) ion from the following figures. What is the valency of sodium atom? Give reason.
Short Answer Questions (3 Marks Each)
Q16 Compare and contrast the atomic models proposed by J.J. Thomson and Ernest Rutherford by identifying at least three key differences in terms of structure, experimental basis, and explanation of atomic behavior.
Q17 Calculate the valency of the following elements:
(i) Element A (Atomic number 5)
(ii) Element B (Atomic number 12)
(iii) Element C (Atomic number 14)
Q18 Complete the table on the basis of information available in the symbols given below:
(a) \(\mathrm{^{35}_{17}Cl}\) (b) \(\mathrm{^{12}_{6}C}\) (c) \(\mathrm{^{81}_{35}Br}\)
Q19 Write the molecular formulae for the following compounds:
(a) Copper (II) bromide
(b) Aluminium (III) nitrate
(c) Calcium (II) phosphate
Long Answer Questions (5 Marks Each)
Q20 (a) Calculate the number of neutrons present in the nucleus of an element X which is represented as \(\mathrm{^{31}_{15}X}\).
(b) An element 'Z' forms the following compounds when it reacts with hydrogen, chlorine, oxygen, and phosphorus: \(\mathrm{ZH_3}\), \(\mathrm{ZCl_3}\), \(\mathrm{Z_2O_3}\), and \(\mathrm{ZP}\).
(i) What is the valency of element 'Z'?
(ii) Is element 'Z' a metal or non-metal? Justify.
Q21 (a) The given figure depicts the atomic structure of an atom of an element 'X'. Write the following information about the element 'X': (i) Atomic number, (ii) Atomic mass, (iii) Valency, (iv) Metal/Non-metal status.
(b) Radhika could not solve the following question in the group; her group mate Aryan explained him and solved his difficulty. Aryan asked: "What is the mass number and valency of element X if it has 5 protons and 6 neutrons? Identify the element."
(i) What is the answer for the above question?
(ii) What value of Aryan's friend is reflected in this behavior?
Case Study Based Questions (4 Marks Each)
Case Study 22 Rutherford (1871–1937) was known as the 'Father' of nuclear physics. Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. Rutherford designed an experiment where fast-moving alpha (\(\alpha\))-particles were made to fall on a thin gold foil. On the basis of his experiment, Rutherford put forward the nuclear model of an atom, which had several notable features including a highly concentrated positive nucleus containing nearly all mass, circular electron orbits, and an extremely small nuclear volume relative to the atom. However, a major drawback of circular orbits was that revolving accelerated electrons should continuously radiate energy and spiral into the nucleus, causing unstable structures.
Questions:
1. Which of the following scientists is known as the 'Father of nuclear physics'?
(a) J.J. Thomson (b) John Dalton (c) E. Rutherford (d) Neils Bohr
2. The positively charged center in an atom is termed as:
(a) Nucleus (b) Molecule (c) Atom (d) Proton
3. Identify the correct statements regarding Rutherford's model:
Statement 1: Positively charged center is called the nucleus.
Statement 2: Electrons revolve around the nucleus in circular paths.
Statement 3: Nearly all the mass of an atom resides in the nucleus.
Statement 4: The size of the nucleus is extremely small compared to the atom.
(a) Only 2 (b) Both 3 & 4 (c) Both 1 & 2 (d) All of the above
4. State two main features of Rutherford's nuclear model.
Case Study 23 Neil's Bohr got the Nobel Prize for his work on the structure of atom in 1922. In order to overcome the objections raised against Rutherford's model of the atom, Neil's Bohr put forward several postulates about the model of an atom, outlining discrete, allowed orbits (energy shells) labeled by letters (K, L, M, N) or numbers (1, 2, 3, 4) in which revolving electrons do not radiate energy.
Questions:
1. The orbits or shells are represented by:
(a) Letters (b) Numbers (c) Both a & b (d) Special symbols
2. These orbits or shells are called:
(a) Energy levels (b) Discrete orbit (c) Atomic levels (d) None of the above
3. Which of the following books/works is written by Professor Bohr?
(a) The Theory of Spectra and Atomic Constitution
(b) The Description of Nature
(c) Both (a) and (b)
4. True or False: "While revolving in discrete orbits, electrons do not radiate energy."
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