(a)(i) The periodic law states that the properties of the elements are a periodic function of their atomic numbers.
(ii) Periodic property of element(s) is any physical or chemical property of an element which changes regularly with increasing atomic number.
(ii) Three properties of an element which show periodicity are: melting/boiling point, ionization energy, electron affinity, electronegativity/electropositivity, atomic/ionic radius, eletrical and thermal - conductivities/metallic and non metallic properties.
(iii) Brief explanation of properties of an element which shows periodicity across the period:
(1) Melting/boiling points of metallic elements increase due to increase in strength of the metallic bond while melting/boiling point for non-metallic elements decreases due to decrease in Van der waals' forces of attraction.
(2) lonizatioin-energy increases due to increase in proton number or increase nuclear charge or decreased atomic radius.
(3) Eleetron affinity increases due to decrease of atomic radius or increase in nuclear charge.
(4) Eleclronegativity of elements increases due to increased nuclear charged or decreased atomic radius.
(5) Electropositivity of elements decreases due to increased nuclear charge
(6) Atomic/ionic radius decreases due to increased nuclear charge or increase in proton number or increase in effective nuclear charge.
(7) Metallic character (electrical/thermal) decreases because of increase in covalent character

(b) Relative atomic mass defined as the number of times the average mass of one atom of an element is heavier than one twelfth the mass of one atom of carbon-l2. OR the ratio of the average mass of one atom of an element to one twelfth the mass of an atom of carbon-12 OR the average mass of one atom of an element on a scale in which one atom of carbon 12 weighs 12 a.m.u.

(c) (i) Phenomenon exhibited by elements Z is isotopy.
(ii) Difference in the number of neutrons accounts for the difference in mass number.
(iii) 75% ( \(^{35}_{17} Z\) )+ \(^{37}_{17} Z\); 100 - 75 = 25%
75% \(^{35}_{17} Z\) + 25% \(^{37}_{17} Z\)
(\(\frac{75}{100} \times 35\)) + (\(\frac{25}{100} \times 37\))
= 26.25 + 9.25
= 35.5

(d)(i) I. CO\(_{2(g)}\) collected by downward delivery or upward displacement of air.
II. HCI\(_{g}\) collected by downward delivery or upward displacement of air.
III. H\(_{2(g)}\) collected by upward delivery or downward displacement of air.
(ii) Reason for answer stated in d(i)I & d(i)II.
I. CO\(_{2(g)}\) is heavier / denser than air.
II. HCl\(_{g}\) is heavier / denser than air.