I Group 15 Elements: The Nitrogen Family

1. In group 15 of the Periodic Table, the elements, nitrogen (₇N), phosphorus (₁₅P), arsenic (₃₃As), antimony (₅₁Sb) and bismuth (₈₃Bi) are present.
2. (a) The elements of this group can exhibit various oxidation states ranging between -3 to+ 5.
(b) Maximum covalency of Nitrogen is four because it does not have d- orbitals to expand its covalence.
3. The atomic (covalent) and ionic radii (in a particular oxidation state) of the elements of nitrogen family (group 15) are smaller than the corresponding elements of the carbon family (group 14).
There is a considerable increase in covalent radius from N to P. However, from As to Bi, only a small increase is observed.
4. Nitrogen displays a great tendency to form pπ-pπ multiple bonds with itself as well as with carbon and oxygen. The tendency to exhibit pπ-pπ multiple bonding decreases as we move down the group.
5. Group 15 elements are more electronegative than group 14 elements. Electronegativity decreases on moving down the group from N to Bi.
6. All elements of group 15 form gaseous hydrides of the type MH₃.
(а) The basic strength of the hydrides decreases as we down the group. Thus, NH₃ is the strongest base.
NH₃ > PH₃ > ASH₃ > SbH₃
(b) The thermal stability of the hydrides decreases as the atomic size increases,.
7. N cannot form NX₅ because of non-availability of d-orbitals. Bi cannot form a BiX₅ because of the reluctance of 6s electrons of Bi to participate in bond formation.
8. Nitrogen forms a number of oxides. The rest of the members (P, As, Sb and Bi) of the group form two types of oxides: E₂0₃ and E₂0₅.

II Group 16 Elements

9. In group 16 of the Periodic Table, elements, oxygen (₈O), sulphur (₁₆S), selenium (₃₄Se), tellurium (₅₂Te) and polonium (₈₄Po) are present.
10. The elements have the electronic configuration ns2np4 for their valence shells. The first element of group 16 differs in its chemical behaviour from that of other members of the group due to its small size and high electronegativity.
11. The metallic character increases with increase in atomic number. The first four elements are non- metallic in character. Non-metallic character is strongest in O and S, weaker in Se and Te while Po is metallic.
12. Atomic and ionic radii increase from top to bottom, due to an increase in the number of shells.
13. Ionisation enthalpy decreases down the group, due to increase in size. Elements of group 16 have lower ionisation enthalpy values as compared to group 15 in the corresponding periods. This is due to the fact that group 15 elements have extra stable half-filled p-orbital electronic configurations.
14. Oxygen has less negative electron gain enthalpy than sulphur due to the compact nature of oxygen atom.
15. Next to F, O has the highest electronegativity value among’the elements. Within the group, electronegativity decreases with increase in atomic number.
16. The tendency for catenation decreases as we go down the group.
17. All the elements of the group form volatile , hydrides.
(a) The volatility increases from water to hydrogen sulphide and then declines.
This is evident in their boiling point. Increasing order-of boiling points of hydrides is H₂S < H₂Se < H₂Te < H₂0. Down 27. the group boiling point increases because of the increase in molecular weight which increases the van der Waal’s forces of 28. interaction. H₂0 has abnormally high b.p. due to hydrogen bonding.
(b) The thermal stability of the hydrides decreases in the order: H₂0>H₂S>H₂Se>H₂Te>H₂Po.
(c) The strength of the hydrides as acids increases in the order: H₂0<H₂S<H₂Se<H₂Te.
18. All the elements of group 16 form binary halides.
19. (a) S, Se and Te form a number of oxo-acids. Among the oxo-acids of S, sulphuric acid is most important.
(b) Sulphurous acid (H₂S0₃) and thiosulfiiric acid (H₂S₂0₃) are unstable and cannot be isolated. They exist only in aqueous solutions or in the form of their salts.

III Group 17 Elements

20. Group 17 of the Periodic Table contain fluorine (₉F), chlorine (₁₇Cl), bromine (₃₅Br), iodine (₅₃I) and astatine (₅₅At).
21. The electronic configuration is ns² np⁵ for valence shells.
22. Halogens have the smallest atomic radii in their respective periods due to maximum effective nuclear change.
23. The first ionisation energies are relatively high but decrease down the group. Iodine can lose an electron and form I⁺ ion.
24. Electron affinity varies as Cl > F > Br > I
25. F is the, most electronegative element known. Electronegativity decreases down the group. Halogens are good oxidising agents. The oxidising power decreases down the group.
27. (a) Reactivity varies as F₂ > Cl₂ > Br₂ > I₂
(b) Order of ionic character in M – X bond is M-F>M-Cl>M-Br>M-I
28. Strength of hydrohalic acids varies as HF<HCl<HBr<HI
The order of B.P is HCl < HBr < HI < HF
29. (a) Hypohalous acids are all weak acids and exist in solution only. Acid strength decreases down the group. HOCl > HOBr > HOI
(b) Acid strength increases as the number of O-atoms increases for a given halogen atom. HOCl < HClO₂ < HClO₃ < HClO₄

IV Group 18 Elements: The Noble Gases

30. In group 18 of the Periodic table, elements helium (₂He), neon (₁₀Ne), argon (₁₈Ar), krypton (₃₆Kr), xenon (₅₄Xe) and radon (₈₆Rn) are present. They are collectively called as noble gases.
31. Noble gases are located at the end of each period. Their valence shell orbitals are fully occupied.
32. They are monoatomic and are sparingly soluble in water.
33. Xe forms fluorides XeF₂, XeF₄ and XeF₆.
34. Xe0₃ is trigonal pyramidal in shape whereas XeOF₄ is square pyramidal.
35. Uses:
(a) He is a non-inflammable gas, lighter than air, therefore, used in filling balloons for meteorological observations.
(b) Ar is used to provide an inert atmosphere.