Question 2
- (i) I. State the periodic law
II. What is meant by the term periodic property of elements?
(ii) List three properties of an element which show periodicity.
(iii) Explain briefly how each of the properties listed in - (ii) varies across the period. [13 marks]
(b) Define relative atomic mass. - (i) What phenomenon is exhibited by an element Z which exist as
(ii) What accounts for the difference in the mass numbers of the element z ?
(iii) Calculate the relative atomic mass of Z if the percentage abundance of
[5 marks]
- (i) State the method used for collecting each of the following gases:
I. CO2;
II. HCl;
III.H2
(ii) Give a reason for your answer in 2 (d)(i) I and 2 (d) (i) II. [5 marks]
Observation
This question was not a compulsory question. A good number of the candidates responded to it.
In part (a), majority of the candidates stated periodic law correctly, but could not define the term periodic property of elements.
In part (b), majority of the candidates defined relative atomic mass.
In part (c), majority of the candidates stated the phenomenon exhibited by the element, and they equally accounted for the difference in the mass numbers of the element.
In part (d), majority of the candidates stated correctly the methods used for collecting the listed gases.
The expected answers include:
- (i) (I) The periodic law states that the properties of the elements are a periodic function of their atomic numbers.
(II) Periodic property is any (physical or chemical) property of an element which changes regularly / shows variation with increasing atomic number.
(ii) - melting / boiling point
- ionization energy
- electron affinity
- electronegativity / electropositivity
- atomic/ ionic radius
- electrical and thermal conductivities / metallic and non- metallic properties
(iii) Melting/ boiling points of metallic elements increase due to increase in strength of the metallic bond melting / boiling point.
OR
melting / boiling point for non metallic elements decreases due to decrease in van der Waals forces of attraction
- Ionization energy increases due to increase in proton number / increase nuclear charge/ decreased atomic radius
- Electron affinity increases due to decrease of atomic radius / increase in nuclear charge
- Electronegativity of elements increases due to increased nuclear charge / decreased atomic radius.
OR
- Electropositivity of elements decreases due to increased nuclear charge
- Atomic/ ionic radius decreases due to increased nuclear charge / increase in proton number / increase in effective nuclear charge
- metallic character (electrical / thermal) decreases because of increase in covalent character - It is 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 – 12.
OR
It is the ratio of the average mass of one atom of an element to one twelfth the mass of an atom of carbon – 12. OR
It is the average mass of one atom of an element on a scale in which one atom of carbon – 12 weighs 12 a.m.u. - (i) Isotopy (1) Do not accept isotope
(ii) (difference in the number of) neutrons
(iii) Percentage abundance of
- (i) (I) - downward delivery / upward displacement of air
(II) - downward delivery / upward displacement of air
(III) - upward delivery / downward displacement of air
(ii) (I) - heavier / denser than air
(II) - heavier / denser than air