Sketch a graph for the variation of first ionization energy against atomic number for the elements across period 3 (Na - Ar) and give the chemical formulae of the stable oxides of these elements.

SECTION B: INORGANIC CHEMISTRY

3. (a) This question concerns the elements across period 3 of the periodic table (Na - Ar).

• (i) Sketch a graph for the variation of first ionization energy against atomic number for the elements. The graph generally shows an increasing trend in first ionization energy across Period 3, with notable dips at Aluminium (Al) and Sulfur (S).

  • X-axis: Atomic Number (11 to 18)
  • Y-axis: First Ionization Energy (kJ/mol)
  • Points to plot (approximate relative values):
    • Na (11): Low
    • Mg (12): Higher than Na
    • Al (13): Lower than Mg (due to 3p orbital shielding)
    • Si (14): Higher than Al
    • P (15): Higher than Si
    • S (16): Lower than P (due to electron-electron repulsion in half-filled 3p orbital)
    • Cl (17): Higher than S
    • Ar (18): Highest

  First Ionization Energy (kJ/mol)
  ^
  |      Ar
  |    Cl
  |  P
  | Si
  |    S
  | Mg
  |  Al
  | Na
  +-------------------------------------> Atomic Number
    11 12 13 14 15 16 17 18
    Na Mg Al Si P  S  Cl Ar
  

  Copy

• (ii) Complete the table below by giving the chemical formulae of the stable oxides of the elements.

  Element	Oxide
  Na	$\boxed{Na_2O}$
  Mg	$\boxed{MgO}$
  P	$\boxed{P_4O_{10}}$
  Cl	$\boxed{Cl_2O_7}$

• (iii) Write a balanced equation for the reaction of silicon tetrachloride with water.

  $$SiCl_4(l) + 2H_2O(l) \to SiO_2(s) + 4HCl(aq)$$

• (iv) Write two equations to show the amphoteric nature of aluminium oxide ($Al_2O_3$).

  1. Reaction with an acid (e.g., hydrochloric acid): $$Al_2O_3(s) + 6HCl(aq) \to 2AlCl_3(aq) + 3H_2O(l)$$
  2. Reaction with a base (e.g., sodium hydroxide): $$Al_2O_3(s) + 2NaOH(aq) + 3H_2O(l) \to 2Na[Al(OH)_4](aq)$$

• (v) What makes sulphur dioxide gas acidic? Sulphur dioxide ($SO_2$) is an acidic gas because it reacts with water to form sulfurous acid ($H_2SO_3$), which can then donate protons.

  $$SO_2(g) + H_2O(l) \rightleftharpoons H_2SO_3(aq)$$

3. (b)

• (i) Give the outermost electronic configuration for the s-block elements (group I and group II).

  • Group I: $\boxed{ns^1}$
  • Group II: $\boxed{ns^2}$

• (ii) For the elements Lithium and Beryllium, write equations representing:

  • A. First ionization energy for Lithium $$Li(g) \to Li^+(g) + e^-$$
  • B. Second ionization energy for Beryllium $$Be^+(g) \to Be^{2+}(g) + e^-$$

• (iii) Which of these two elements in b(ii) will have a higher second ionization energy? Explain.

  • $\boxed{Lithium}$ will have a higher second ionization energy.
  • Explanation: After losing its first electron, Lithium ($Li$) forms $Li^+$, which has a stable electron configuration similar to Helium ($1s^2$). Removing a second electron from $Li^+$ requires breaking into this very stable, full inner electron shell, which demands a significantly large amount of energy. Beryllium ($Be$), after losing its first electron, forms $Be^+$, which still has one electron in its $2s$ orbital ($1s^2 2s^1$). Removing this second electron from $Be^+$ is easier than removing an electron from the stable $1s^2$ core of $Li^+$.