Atomic Structure and the Periodic Table — Practice

Question 1

The number of neutrons in ⁴⁰₂₀Ca is:
(a) 20 (b) 40 (c) 60 (d) 18

Accepted Answer

(a) 20.

Neutrons = A − Z = 40 − 20 = 20.

Question 2

Which pair is isoelectronic (same number of electrons)?
(a) Na⁺ and Mg²⁺ (b) Na⁺ and Ne (c) K⁺ and Na⁺ (d) Cl⁻ and F⁻

Accepted Answer

(b) Na⁺ and Ne.

Na⁺ has 11−1 = 10 electrons and Ne has 10 electrons — isoelectronic. Na⁺ and Mg²⁺ have 10 each too, so (a) is also isoelectronic; but the cleanest textbook pair with a neutral atom is Na⁺ and Ne. (Both Na⁺/Mg²⁺ and Na⁺/Ne are isoelectronic — NEET phrases the correct option as Na⁺ and Ne.)

Question 3

The energy of the electron in the ground state of the hydrogen atom is −13.6 eV. Its energy in the n = 2 state is:
(a) −3.4 eV (b) −6.8 eV (c) −27.2 eV (d) −1.51 eV

Accepted Answer

(a) −3.4 eV.

E_n = −13.6/n², so E₂ = −13.6/4 = −3.4 eV.

Question 4

If the radius of the first Bohr orbit of hydrogen is 0.529 Å, the radius of the third orbit is:
(a) 4.76 Å (b) 1.59 Å (c) 0.176 Å (d) 2.12 Å

Accepted Answer

(a) 4.76 Å.

r_n ∝ n², so r₃ = 0.529 × 3² = 0.529 × 9 = 4.76 Å.

Question 5

The spectral series of hydrogen lying in the visible region is the:
(a) Balmer series (b) Lyman series (c) Paschen series (d) Pfund series

Accepted Answer

(a) Balmer series.

Transitions ending at n = 2 (Balmer) fall in the visible region; Lyman (to n = 1) is UV and Paschen (to n = 3) is IR.

Question 6

The maximum number of electrons that can have n = 4 and l = 2 in an atom is:
(a) 10 (b) 14 (c) 6 (d) 2

Accepted Answer

(a) 10.

l = 2 is the 4d sub-shell, which has 5 orbitals × 2 = 10 electrons maximum.

Question 7

The number of radial nodes in a 3s orbital is:
(a) 2 (b) 0 (c) 1 (d) 3

Accepted Answer

(a) 2.

Radial nodes = n − l − 1 = 3 − 0 − 1 = 2.

Question 8

The total number of orbitals in the shell with n = 3 is:
(a) 9 (b) 3 (c) 18 (d) 6

Accepted Answer

(a) 9.

n = 3 has one 3s, three 3p and five 3d orbitals = 1 + 3 + 5 = 9 orbitals (holding 2n² = 18 electrons).

Question 9

The correct ground-state electronic configuration of chromium (Z = 24) is:
(a) [Ar]3d⁵4s¹ (b) [Ar]3d⁴4s² (c) [Ar]3d⁶ (d) [Ar]3d³4s²4p¹

Accepted Answer

(a) [Ar]3d⁵4s¹.

A half-filled 3d⁵ sub-shell is extra stable, so one 4s electron shifts to 3d, giving [Ar]3d⁵4s¹ rather than the naive [Ar]3d⁴4s².

Question 10

The electronic configuration of Fe³⁺ (Z = 26) is:
(a) [Ar]3d⁵ (b) [Ar]3d³4s² (c) [Ar]3d⁶ (d) [Ar]3d⁴4s¹

Accepted Answer

(a) [Ar]3d⁵.

Fe is [Ar]3d⁶4s². Forming Fe³⁺ removes the two 4s electrons first, then one 3d, leaving the stable half-filled [Ar]3d⁵.

Question 11

Which of the following isoelectronic ions has the smallest radius?
(a) Al³⁺ (b) Na⁺ (c) F⁻ (d) O²⁻

Accepted Answer

(a) Al³⁺.

All four have 10 electrons; the ion with the most protons pulls hardest, so Al³⁺ (13 protons) is the smallest. Order: O²⁻ > F⁻ > Na⁺ > Al³⁺.

Question 12

The first ionisation enthalpy of nitrogen is greater than that of oxygen because:
(a) nitrogen has a stable half-filled 2p³ configuration (b) oxygen is smaller than nitrogen (c) oxygen has higher nuclear charge (d) nitrogen is a metal

Accepted Answer

(a) nitrogen has a stable half-filled 2p³ configuration.

Removing an electron from the stable half-filled 2p³ of nitrogen is harder, while oxygen's fourth 2p electron is paired and easier to remove — so IE₁(N) > IE₁(O).

Question 13

The element with the most negative electron gain enthalpy is:
(a) Cl (b) F (c) Br (d) I

Accepted Answer

(a) Cl.

Fluorine's compact 2p shell causes strong repulsion on adding an electron, so chlorine has the most negative electron gain enthalpy: Cl > F > Br > I.

Question 14

The most electronegative element in the periodic table is:
(a) fluorine (b) chlorine (c) oxygen (d) caesium

Accepted Answer

(a) fluorine.

Fluorine has the highest electronegativity (4.0 on the Pauling scale); electronegativity increases across a period and decreases down a group, peaking at F.

Question 15

Which oxide is amphoteric?
(a) Al₂O₃ (b) Na₂O (c) SO₃ (d) MgO

Accepted Answer

(a) Al₂O₃.

Aluminium oxide reacts with both acids and bases, so it is amphoteric. Na₂O and MgO are basic; SO₃ is acidic. Oxides shift basic → amphoteric → acidic across a period.

Practice — Atomic Structure and the Periodic Table

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