(a)(i) Determine the maximum number of electrons that can occupy the principal energy level M of an atom. (ii) Show the changes in the electronic structures...

(a)(i) Maximum electrons in the M shell (principal quantum number n = 3): \( 2n^2 = 2 \times 3^2 = \mathbf{18} \).

(ii) Formation of sodium fluoride

Sodium (2, 8, 1) loses its one outer electron to become Na⁺ (2, 8); fluorine (2, 7) gains that electron to become F^- (2, 8). Both attain the stable octet:

\[ Na \to Na^+ + e^- \qquad F + e^- \to F^- \qquad Na^+ + F^- \to NaF \]

A dot-and-cross diagram showing the single electron transferring from Na to F is expected.

(iii) Properties of NaF (ionic compound): high melting and boiling points; conducts electricity when molten or dissolved in water; a hard, crystalline solid that is soluble in water.

(b)

(i) \[ 4Na + O_2 \to 2Na_2O \]

(ii) \[ Na_2O + H_2O \to 2NaOH \]

(c) Comparing the conductance of 1 mol dm^-3 NaOH and ethanoic acid

Set up a simple conductivity cell: two clean carbon (or platinum) electrodes connected through a battery and a bulb (or ammeter). Dip the electrodes into a fixed volume of 1 mol dm^-3 NaOH and note the brightness of the bulb (or the ammeter reading). Rinse the electrodes and repeat with the same volume of 1 mol dm^-3 ethanoic acid. The bulb glows brightly (large current) with NaOH, which is a strong electrolyte (fully ionised), but only dimly (small current) with ethanoic acid, which is a weak electrolyte (only partially ionised). Hence NaOH conducts better.