Observation

The candidates showed good understanding of the question and their performance was good.

In part (a) (i), majority of the candidates copied and completed the table correctly.  However, some could not write the electron configuration of X.

In part (a) (ii), majority of the candidates could state the family to which each of the elements belongs.

In part (a) (iii), majority of the candidates exhibited good understanding of chemical bond that can exist between X and Y.

In part (a) (iv), majority of the candidates could state the two properties of compounds which have the type of bond named in 2(a)(iii).

In part (b), majority of the candidates could not explain why the reactivity of Group VII elements decreases down the group.  Some wrote that it was due to atomic number.  They could not arrange the ions 20Ca2+, 17Cl- , 19K+   in order of decreasing size, thereby losing allocated marks.

In part (c), majority of the candidates could not define relative atomic mass correctly. Furthermore, they could not calculate the vapor density of the diatomic gas with RAM of 32.

The expected answers include:

1. (i)             

(ii)        X – alkali metals
Y - halogen
Z – noble/inert/rare gases

      (iii)     ionic or electrovalent bond

(iv)       - they are soluble in water/polar solvent
- they have high melting/boiling point
- they conduct electricity in solution/molten form.
- they produce ions in aqueous solution/are made up of ions
- they are non volatile/usually solids at room temperature
- they are brittle

1. (i)       Group (VII) elements react by gaining electrons

          The smaller the atom the greater the ability to attract electrons/electron affinity
Atomic size increases down the group
Ease of electron gain decreases down the group

                                                                 OR
The halogens are highly electronegative elements.  The smaller the size, the higher the          electronegativity.  Atomic size increase down the group hence reactivity decreases down the group.

 (ii)   Cl- > K+ > Ca2+
AcceptC1-, K+, Ca2+    OR      Ca2+< K+< C1-

(iii) The species are isoelectronic/have same electron configuration.
For isoelectronic species the greater the nuclear charge/proton number
the smaller the size.

1. (i)    Relative atomic mass is defined as mass of an atom compared to 1/12 the mass of

             an atom of 12C/average mass of one atom compared to1/12th mass of one atom of 12C.

(ii)         Diatomic gas Q    = Q2
Rel. atomic mass  = 32
Relative molecular mass = 32 × 2 = 64
Vapour density                = ½ × 64 =32