Observation

This question was popular among the candidates, as majority of them responded to it.

In part (a), majority of the candidates deduced the formula of P, but could not draw the structures of two isomers of P.

In part (b), a fair attempt was made at stating the guiding principles which are used to explain the way electrons of the atoms of the elements are arranges in atomic orbitals.

In part (c), majority of the candidates could not describe the nature of the intermolecular forces holding the molecules together. However, majority of the candidates were able to explain what happens when a sample of each of the substances is added to water.

In part (d), majority of the candidates could not relate oxidation and reduction to loss or gain of electrons. Hence, they could not deduce whether J would be a good oxidizing or reducing agent.

The expected answers include:

1. 


2. • electrons enter orbitals of lowest energies first (Aufbau Principle)
   • Pauli Exclusion Principle) – Two electrons in the same orbital will have different spins / no two electrons will have all four quantum numbers the same.
   • (Hund’s Rule of maximum multiplicity) – electrons filling into degenerate orbitals / orbitals of same energy fill in singly before pairing up.
3. 1. NaH: Ionic/electrostatic forces, charges (of opposite sign) holding Na+ and H- ions together.
      Do not accept ionic/electrovalent bond.
      
      HH₂: Van der Waal’s forces weak (instantaneous)dipole – dipole interactions holding molecules together
      
      H₂S: Dipole - dipole interactions / H – bonding ( resulting from charge separation along S-H bond) because of higher electronegativity of sulphur.
      
      NH₄Cl: Ionic/ electrostatic forces between NH₄⁺ and Cl^- holding the NH₄⁺ and Cl^- units
   2. • Redox reaction occurs/NaH dissolves/gas is evolved/alkaline solution formed
      • H₂ to water no reaction occurs / H₂ is slightly soluble
      • H₂S to water, H₃O+ and S^2-is formed/ H₂S dissolves / slightly acidic solution
      • It dissolves/NH₄⁺ hydrolyses/slightly acidic solution
   3. 
4. 1. There are no unpaired electrons/Zero
   2. It will be a good reducing agent
   3. because it loses the two electrons in its outermost shell / It readily gives out electron/electron donor/electropositive