(a) The electronic configurations of atoms of elements A, B, C and D are given as follows: A. Is\(^2\)2s\(^2\)2p\(^2\); B. 1s\(^2\)2s\(^2\)2sp\(^1\) ; C. 1s...

Taking the configurations as A: 1s²2s²2p² (carbon, 6), B: 1s²2s²2p⁶3s¹ (sodium, 11), C: 1s²2s²2p¹ (boron, 5), D: 1s²2s² (beryllium, 4).

(a)(i) Increasing atomic size

A < C < D < B (carbon < boron < beryllium < sodium).

Reason: A, C and D are in the same period; across a period the nuclear charge increases and pulls the electrons in the same shell closer to the nucleus, so the atomic size decreases (C, B and Be). B (sodium) is in the next period with an extra electron shell, so it is the largest.

(a)(ii)

• I. Divalent: D (beryllium, 2 outer electrons).
• II. Two unpaired electrons in the ground state: A (carbon, 2p²).
• III. Readily loses one electron: B (sodium, one outer electron).
• IV. Belongs to Group III: C (boron, 3 outer electrons).

(b)(i) An electrovalent bond is formed by the complete transfer of electrons from one atom to another, while a covalent bond is formed by the sharing of electrons between atoms.

(b)(ii) Metallic bond and hydrogen bond (van der Waals forces also acceptable).

(b)(iii) Covalent compounds have low melting and boiling points, and they do not conduct electricity (they are non-electrolytes).

(c)(i) Isotopy is the existence of atoms of the same element having the same atomic number but different mass numbers (different numbers of neutrons).

(c)(ii) Example: chlorine-35 and chlorine-37 (or carbon-12 and carbon-14).

(c)(iii) Relative atomic mass of Z

\[ \text{R.A.M.} = \frac{(1 \times 18) + (2 \times 20)}{1 + 2} = \frac{18 + 40}{3} = \frac{58}{3} = 19.33 \]

(d)(i)

• I. Halogens: fluorine and iodine.
• II. Noble gases: neon and helium.
• III. Alkaline earth metals: calcium and magnesium.

(d)(ii) Nuclear equation

⁷₃Li + ¹₁H → ⁸₄Be + γ

(d)(iii) One use of radioactive isotopes: as tracers in medicine and agriculture (radiotherapy for cancer treatment, or carbon-14 dating, are also acceptable).