(a) The electronic configuration of five elements represented by the letters P, Q, R, S and T are indicated below.
P - \(1s_2 2s_2 2p_2\)
Q \(1s_2 2s_2 2p_4\)
R \(1s_2 2s_2 2p_6\)
S - \(1s_2 2s_2 2p_6 3s_2\)
T - \(1s_2 2s_2 2p_6 3s_2 3p_5\)
without identifying the elements, state which of them
(i) belongs to group VI in the periodic table;
(ii) is strongly metallic in character;
(iii) readily ionizes by gaining one electron;
(iv) contains two unpaired electrons in the ground state atom
(v) readily loses two electrons during chemical bonding
(vi) Does not paricipates in chemical reactions?
(vii) is an s-block element.
(b)(i) Copy and complete the table below as appropriate
| Particle |
Number of Protons |
Number of Electrons |
Number of Neutrons |
| \(^1_1H\) |
1 |
1 |
|
| \(^{27}_{13}\)Al\(^{3+}\) |
|
|
|
| \(^{16}_{8}O^{2+}\) |
|
|
8 |
(ii) Give the reason why atomic radius increases down a group in the periodic table but decreases from left to right in a period.
(c)(i) What is meant by the half-life of a radioactive element?
(ii) The nuclide \(^{210}_{84}PO\) loses an alpha 4° particle to form lead. Write an equation for the reaction.
(d) State the type of chemical bonding which accounts for each of the following observations:
(i) Chlorine exists as discrete molecules
(ii) Sodium chloride dissolves readily in water;
(iii) \(\mathrm{CuSO_{4(aq)}}\) forms a deep blue complex ion with excess \(\mathrm{NH_{3(aq)}}\)
(a) Identifying the elements from configuration (not required, but shown for clarity)
- P: 1s\(^2\)2s\(^2\)2p\(^2\) (Z = 6, group IV)
- Q: 1s\(^2\)2s\(^2\)2p\(^4\) (Z = 8, group VI)
- R: 1s\(^2\)2s\(^2\)2p\(^6\) (Z = 10, group VIII, noble gas)
- S: 1s\(^2\)2s\(^2\)2p\(^6\)3s\(^2\) (Z = 12, group II)
- T: 1s\(^2\)2s\(^2\)2p\(^6\)3s\(^2\)3p\(^5\) (Z = 17, group VII)
(i) Belongs to group VI: Q (six outermost electrons, 2s\(^2\)2p\(^4\)).
(ii) Strongly metallic: S (a group II element that readily loses electrons).
(iii) Readily ionises by gaining one electron: T (needs one electron to complete its octet, forming a 1\(^-\) ion).
(iv) Two unpaired electrons in the ground-state atom: P and Q. In 2p\(^2\) the two electrons occupy separate orbitals (Hund's rule); in 2p\(^4\) one orbital is doubly filled leaving two singly-occupied orbitals - both give two unpaired electrons.
(v) Readily loses two electrons in bonding: S (3s\(^2\) outer electrons lost to form a 2\(^+\) ion).
(vi) Does not take part in chemical reactions: R (a stable, filled-shell noble gas).
(vii) An s-block element: S (its outermost electrons enter the 3s sub-shell).
(b)(i) Completed table
| Particle | Protons | Electrons | Neutrons |
|---|
| \(^{1}_{1}\)H | 1 | 1 | 0 |
| \(^{27}_{13}\)Al\(^{3+}\) | 13 | 10 | 14 |
| \(^{16}_{8}\)O\(^{2-}\) | 8 | 10 | 8 |
Neutrons = mass number - atomic number. For Al\(^{3+}\), three electrons are lost (13 - 3 = 10). For the oxide ion, two electrons are gained (8 + 2 = 10).
(b)(ii) Trend in atomic radius
Down a group the number of occupied electron shells increases and inner-shell shielding increases, so the outermost electrons lie farther from the nucleus and the radius increases. Across a period electrons are added to the same shell while the nuclear charge rises; the greater effective nuclear attraction pulls the shell inward, so the radius decreases.
(c)(i) Half-life
The half-life of a radioactive element is the time taken for half the atoms (nuclei) in a given sample to decay (disintegrate).
(c)(ii) Alpha decay of polonium-210
\[ ^{210}_{84}\text{Po} \rightarrow\ ^{4}_{2}\text{He} +\ ^{206}_{82}\text{Pb} \]
(d) Type of bonding
- (i) Chlorine as discrete molecules: covalent bonding (shared electron pair within each Cl\(_2\) molecule).
- (ii) Sodium chloride dissolving readily in water: ionic (electrovalent) bonding (the polar water molecules hydrate the Na\(^+\) and Cl\(^-\) ions).
- (iii) Deep-blue complex ion with excess ammonia: co-ordinate (dative) covalent bonding (lone pairs on N donate into the Cu\(^{2+}\) ion).
(a) Identifying the elements from configuration (not required, but shown for clarity)
- P: 1s\(^2\)2s\(^2\)2p\(^2\) (Z = 6, group IV)
- Q: 1s\(^2\)2s\(^2\)2p\(^4\) (Z = 8, group VI)
- R: 1s\(^2\)2s\(^2\)2p\(^6\) (Z = 10, group VIII, noble gas)
- S: 1s\(^2\)2s\(^2\)2p\(^6\)3s\(^2\) (Z = 12, group II)
- T: 1s\(^2\)2s\(^2\)2p\(^6\)3s\(^2\)3p\(^5\) (Z = 17, group VII)
(i) Belongs to group VI: Q (six outermost electrons, 2s\(^2\)2p\(^4\)).
(ii) Strongly metallic: S (a group II element that readily loses electrons).
(iii) Readily ionises by gaining one electron: T (needs one electron to complete its octet, forming a 1\(^-\) ion).
(iv) Two unpaired electrons in the ground-state atom: P and Q. In 2p\(^2\) the two electrons occupy separate orbitals (Hund's rule); in 2p\(^4\) one orbital is doubly filled leaving two singly-occupied orbitals - both give two unpaired electrons.
(v) Readily loses two electrons in bonding: S (3s\(^2\) outer electrons lost to form a 2\(^+\) ion).
(vi) Does not take part in chemical reactions: R (a stable, filled-shell noble gas).
(vii) An s-block element: S (its outermost electrons enter the 3s sub-shell).
(b)(i) Completed table
| Particle | Protons | Electrons | Neutrons |
|---|
| \(^{1}_{1}\)H | 1 | 1 | 0 |
| \(^{27}_{13}\)Al\(^{3+}\) | 13 | 10 | 14 |
| \(^{16}_{8}\)O\(^{2-}\) | 8 | 10 | 8 |
Neutrons = mass number - atomic number. For Al\(^{3+}\), three electrons are lost (13 - 3 = 10). For the oxide ion, two electrons are gained (8 + 2 = 10).
(b)(ii) Trend in atomic radius
Down a group the number of occupied electron shells increases and inner-shell shielding increases, so the outermost electrons lie farther from the nucleus and the radius increases. Across a period electrons are added to the same shell while the nuclear charge rises; the greater effective nuclear attraction pulls the shell inward, so the radius decreases.
(c)(i) Half-life
The half-life of a radioactive element is the time taken for half the atoms (nuclei) in a given sample to decay (disintegrate).
(c)(ii) Alpha decay of polonium-210
\[ ^{210}_{84}\text{Po} \rightarrow\ ^{4}_{2}\text{He} +\ ^{206}_{82}\text{Pb} \]
(d) Type of bonding
- (i) Chlorine as discrete molecules: covalent bonding (shared electron pair within each Cl\(_2\) molecule).
- (ii) Sodium chloride dissolving readily in water: ionic (electrovalent) bonding (the polar water molecules hydrate the Na\(^+\) and Cl\(^-\) ions).
- (iii) Deep-blue complex ion with excess ammonia: co-ordinate (dative) covalent bonding (lone pairs on N donate into the Cu\(^{2+}\) ion).
