In(a)(i) and (ii), candidates correctly defined atomic number and mass number as the number of protons/electrons in an atom and the sum of the protons and neutrons in an atom respectively. However, a few of them lost the marks because they confused atom with element.
In (b)(i) I – III, candidates correctly stated that X belongs to group 6, Y belongs to period 3 and that the type of bond formed between X and Y is ionic/electrovalent.
In (b)(ii), most candidates correctly wrote the formula of the compound formed between X and Y as Y2 X. However some lost marks because they wrote the formula as XY2, they did not know that the metallic component of the compound was to be written first
while others wrote the compound as Na2O, identifying the elements.
In (b)(iii), candidates mentioned two characteristic properties of the compound formed between X and Y from among the following properties.
- solid at room temperature
- high boiling/melting point
- dissolves in water readily
- conducts electricity in solution/molten form.
In (c)(i), candidates knew that the phenomenon exhibited by Ne in the table is isotopy.
In (c)(ii), most candidates named two other elements that exhibit such phenomenon from among the following list: carbon, hydrogen, oxygen, nitrogen, chlorine, phosphorus etc. However, some of them lost marks because they wrote the symbols of the elements instead of the names.
In (c)(iii), candidates correctly calculated the relative atomic mass of Ne thus:
20 x 9.48 + 21 x 0.27 + 22 x 9.25
100 100 100
= 18.096 0.0567 + 2.035
= 20.1877 20.2
In (d), majority of the correctly copied and completed the table as follows:
Type of radiation
2+ or + 2
1- or – 1
Some of the candidates however lost marks as they could not correctly give the charge on each type of radiation.
In (e)(i), candidates did not know that in spite of the fact that helium and hydrogen are in the same period, helium has a greater nuclear charge (of x 2) than hydrogen (+1). Hence, the electron to be removed in helium is more tightly held to the nucleus than in hydrogen.
In (e)(ii), most candidates could not draw the electron dot diagram to show the bonding in ammonium ion thus: