Board exam focus — Structure of the Atom (CBSE & HBSE)

CBSE emphasises the historical evolution of atomic models (Thomson to Rutherford to Bohr) with reasoning behind each, plus numerical practice on atomic number, mass number and electronic configuration. HBSE leans more on direct definitions, Bohr-Bury rules and isotope/isobar comparisons asked as short-answer questions.

Atomic Models: Thomson, Rutherford and Bohr

Charged Particles in Matter

Atoms were thought to be indivisible until the discovery of sub-atomic particles. The electron (negative) was discovered by J. J. Thomson, the proton (positive) by E. Goldstein (in canal rays), and the neutron (neutral) by J. Chadwick.

Thomson's Model of the Atom

Thomson proposed that the atom is like a sphere of positive charge with electrons embedded in it (often compared to a Christmas pudding, or watermelon with seeds as electrons).

1. An atom consists of a positively charged sphere and electrons are embedded in it.
2. The negative and positive charges are equal in magnitude, so the atom is electrically neutral.

Trap: Thomson's model could NOT explain how positive charge is distributed and was disproved by Rutherford's experiment.

Rutherford's Alpha-Particle Scattering Experiment

Rutherford bombarded a thin gold foil with fast-moving, positively charged alpha (\u03b1) particles.

Observations:

• Most alpha particles passed straight through the foil undeflected.
• A few were deflected by small angles.
• One in about 12,000 bounced back (deflected by nearly 180\u00b0).

Conclusions:

Rutherford's nuclear model: a tiny, dense, positively charged nucleus at the centre, with electrons revolving around it.

Drawback: According to classical physics, a revolving (accelerating) electron should radiate energy, spiral inward and collapse into the nucleus. Rutherford's model could not explain the stability of the atom.

Bohr's Model of the Atom

Niels Bohr resolved the stability problem with these postulates:

1. Electrons revolve around the nucleus only in certain permitted discrete orbits (shells) called energy levels, designated K, L, M, N (n = 1, 2, 3, 4).
2. While revolving in these special orbits, an electron does not radiate energy. These are called stationary states or energy levels.
3. Energy is emitted or absorbed only when an electron jumps from one orbit to another.

Sub-atomic Particles, Atomic Number, Mass Number, Isotopes and Isobars

Sub-atomic Particles

Protons and neutrons together are called nucleons.

Atomic Number (Z)

The atomic number is the number of protons present in the nucleus of an atom. In a neutral atom, number of protons = number of electrons.

Mass Number (A)

The mass number is the sum of the number of protons and neutrons present in the nucleus.

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

An element X is written as $^{A}_{Z}X$, e.g. carbon as $^{12}_{6}C$.

Isotopes

Isotopes are atoms of the same element having the same atomic number but different mass numbers (same protons, different neutrons).

• Examples: Hydrogen has three isotopes \u2014 protium ($^{1}_{1}H$), deuterium ($^{2}_{1}H$), tritium ($^{3}_{1}H$).
• Chlorine exists as $^{35}_{17}Cl$ and $^{37}_{17}Cl$; its average atomic mass is 35.5 u (in ratio 3:1).

Applications of isotopes:

• An isotope of uranium is used as fuel in nuclear reactors.
• An isotope of cobalt is used to treat cancer.
• An isotope of iodine is used to treat goitre.

Isobars

Isobars are atoms of different elements having different atomic numbers but the same mass number.

• Example: Calcium ($^{40}_{20}Ca$) and Argon ($^{40}_{18}Ar$) \u2014 both have mass number 40 but atomic numbers 20 and 18.

Trap: Why is atomic mass of chlorine 35.5 u even though no atom has mass 35.5? Because it is the average mass of its isotopes (35 and 37) in the ratio 3:1.

Electronic Configuration, Bohr-Bury Rules and Valency

Distribution of Electrons in Shells (Bohr-Bury Rules)

Electrons are arranged in shells K, L, M, N (n = 1, 2, 3, 4) following these rules:

1. The maximum number of electrons in a shell is given by the formula 2n\u00b2, where n is the shell number.

• K shell (n=1): 2 \u00d7 1\u00b2 = 2
• L shell (n=2): 2 \u00d7 2\u00b2 = 8
• M shell (n=3): 2 \u00d7 3\u00b2 = 18
• N shell (n=4): 2 \u00d7 4\u00b2 = 32

1. The maximum number of electrons in the outermost shell cannot exceed 8.
2. Shells are filled in a step-wise manner; a new shell does not start until the inner shells are filled (as far as the above limits allow).

Electronic Configuration Examples

Valence Electrons and Valency

• Valence electrons are the electrons present in the outermost shell of an atom.
• Valency is the combining capacity of an atom \u2014 the number of electrons it gains, loses or shares to achieve a stable (octet/duplet) configuration.

Rules for finding valency:

• If outermost electrons \u2264 4, valency = number of valence electrons (electrons lost).
• If outermost electrons > 4, valency = 8 - number of valence electrons (electrons gained).
• Elements with a complete outermost shell (e.g. He = 2, Ne = 8) have valency 0 (noble/inert gases).

Trap: The valency of magnesium is 2 (it loses 2 electrons), NOT 8 - 2. Use the 8 minus rule only when valence electrons are more than 4.

Frequently asked questions

Are these Structure of the Atom notes free?

Yes — the Structure of the Atom notes for Science (Class 9) on Siksha Sarovar are completely free to read, with no account required.

Do these notes follow CBSE and HBSE?

Yes. The Structure of the Atom notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Structure of the Atom chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Structure of the Atom.