Journey Inside the Atom (Structure of the Atom): NCERT Solutions (2026)

Canal rays are positively charged radiations discovered by E. Goldstein in a gas discharge tube. These rays led to the discovery of the positively charged subatomic particle called the proton.

No, the atom will not carry any charge. It will be electrically neutral.

Explanation: An electron carries a unit negative charge ($-1$) and a proton carries a unit positive charge ($+1$). Therefore, they cancel each other's effect, making the net charge zero.

According to Thomson's model (the "Plum Pudding" model):

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

Because the total positive charge exactly balances the total negative charge, the atom as a whole is electrically neutral.

According to Rutherford's model, the proton (a positively charged subatomic particle) is present in the dense central nucleus of the atom. (Neutrons were discovered later by James Chadwick).

(Note for students: Practice this diagram neatly in your notebook).

• Draw a small solid circle in the center and label it as the Nucleus (contains protons and neutrons).
• Draw three concentric circles around the nucleus.
• Label the first inner shell as K shell ($n=1$).
• Label the second shell as L shell ($n=2$).
• Label the third outermost shell as M shell ($n=3$).

If a metal other than gold is used, the observations would generally remain the same because the basic structure of the atom (a dense positive nucleus and empty space) is identical for all elements.

However, gold was specifically chosen because it is highly malleable and can be beaten into a remarkably thin foil (about 1000 atoms thick), which is necessary for clear scattering results. A thicker foil of another metal would absorb most $\alpha$-particles, making the scattering pattern hard to observe.

Given:
Mass Number ($A$) = 4
Number of Protons ($Z$) = 2

Formula:
$$\text{Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}$$ $$4 = 2 + \text{Number of Neutrons}$$ $$\text{Number of Neutrons} = 4 - 2 = 2$$

Thus, a helium atom has 2 neutrons.

Carbon (Atomic Number = 6):
Total electrons = 6.
Distribution: K shell = 2, L shell = 4.
Electronic configuration = 2, 4.

Sodium (Atomic Number = 11):
Total electrons = 11.
Distribution: K shell = 2, L shell = 8, M shell = 1.
Electronic configuration = 2, 8, 1.

Maximum capacity of K shell = 2 electrons
Maximum capacity of L shell = 8 electrons
If both shells are full, total electrons = $2 + 8 = 10$ electrons. (This corresponds to the Neon atom).

Answer: (a) J.J. Thomson (Difficulty: Easy)

Answer: (b) - Magnesium has an atomic number of 12. (Difficulty: Medium)

Answer: (c) Different number of neutrons (Difficulty: Medium)

Answer: (b) 18 - Using formula $2n^2$ where $n=3$ (Difficulty: Easy)

Answer: (a) Atomic Nucleus (Difficulty: Medium)

Answer: Isotopes of an element have the same atomic number, which means they have the same number of electrons and the same electronic configuration. Since chemical properties depend solely on the valence electrons, isotopes exhibit similar chemical properties.

Answer: Valency is the combining capacity of an atom.

• Silicon (Atomic No. 14): Configuration is 2, 8, 4. It needs 4 electrons to complete its octet, so its valency is 4.
• Oxygen (Atomic No. 8): Configuration is 2, 6. It needs 2 electrons to complete its octet, so its valency is 2. (Difficulty: Medium)

Answer: Isobars are atoms of different elements having different atomic numbers but the same mass number. Example: Calcium (Atomic No. 20) and Argon (Atomic No. 18) both have a mass number of 40.

Answer: Rutherford wanted a foil as thin as possible to ensure that the alpha particles had to pass through only a single layer of atoms. Gold is highly malleable and can be beaten into extremely thin sheets.

Answer:

Experiment: Fast-moving alpha ($\alpha$) particles were made to fall on a thin gold foil.

Observations:

1. Most of the $\alpha$-particles passed straight through the gold foil.
2. Some $\alpha$-particles were deflected by the foil by small angles.
3. Surprisingly, one out of every 12,000 particles appeared to rebound completely.

Conclusions:

1. Most of the space inside the atom is empty.
2. The positive charge of the atom occupies very little space (the central Nucleus).
3. All the positive charge and mass of the gold atom are concentrated in a very small volume within the atom. (Difficulty: Hard)

Answer:

Rutherford's model could not explain the stability of the atom (a moving electron should radiate energy and fall into the nucleus). Neils Bohr proposed:

1. Electrons revolve around the nucleus only in certain special, permitted orbits called discrete orbits.
2. While revolving in these discrete orbits, the electrons do not radiate energy.

These orbits are called energy levels or shells (K, L, M, N...). Because electrons don't lose energy while in these specific orbits, the atom remains stable. (Difficulty: Hard)

a) What are the atomic numbers of X and Y? (Ans: Both have Atomic No. 6)

b) What are the mass numbers of X and Y? (Ans: $X=12$, $Y=14$)

c) What is the relationship between Atom X and Atom Y? (Ans: They are isotopes of Carbon, specifically C-12 and C-14). (Difficulty: Medium)

Answer: (A) Both are true, and R correctly explains A. (Difficulty: Easy)

Answer: (C) A is true, but R is false. Their valency is zero because their outermost shell is completely filled with 8 electrons (octet), not because of 8 protons. (Difficulty: Medium)