Question

Explain the following:
(A) Bi(V) is a stronger oxidizing agent than Sb(V).
(B) Unlike phosphorus, nitrogen shows little tendency for catenation?
(C) Halogens have maximum negative electron gain enthalpy in the respective period. Why?

Answer 1

For (A): Lesser is the stability of a cation, more is its ability to gain an electron and so it will be a stronger oxidizing agent. From top to bottom in a periodic table the stability of +5 oxidation state decreases.
For (B): Atoms having small size experience lesser catenation due to more repulsion among the nucleus of the atoms forming a single bond and so their bond weakens.
For (C): With decrease in size of an atom, its effective nuclear charge increases and it gains an electron more easily. So, its electron gain enthalpy becomes more negative.

Complete step by step answer:
(A) Bi(V) is a stronger oxidizing agent than Sb(V).
First of all we will see what oxidising agents are. Any substance which has the ability to oxidise another substance is an oxidising agent. It itself accepts their electrons and gets reduced.
Both Sb and Bi lie in the 15th group. But Sb lies in the 5th period and Bi lies in the 6th period. Since the 6th and 7th period contain f-block and so from top to bottom the inert pair effect increases. Due to the inert pair effect, the stability of the +5 oxidation states of the elements decreases down the group. Hence $S{b^{ + 5}}$ will be more stable as compared to $B{i^{ + 5}}$ and so $B{i^{ + 5}}$ being unstable will have a higher tendency to accept a pair of electron than $S{b^{ + 5}}$.
The element having a higher tendency to accept electron pairs will be a better oxidising agent and hence $B{i^{ + 5}}$ is a stronger oxidizing agent than $S{b^{ + 5}}$.

(B) Unlike phosphorus, nitrogen shows little tendency for catenation?
First of all we will see what catenation is. The tendency of atoms to self link among themselves to form chains, rings, sometimes layers in 2-D and space lattices in 3-D is known as catenation. Some common elements exhibiting catenation are: carbon, silicon, sulphur, boron, etc.
Now coming back to the question, nitrogen atoms are smaller in size and so they experience greater repulsion of electron density between two nitrogen atoms which weakens the single bond between N-N. This is not the case with phosphorus. Hence catenation is much more common in phosphorus compounds than in nitrogen compounds.

(C) Halogens have maximum negative electron gain enthalpy in the respective period. Why?
First of all we will see what electron gain enthalpy is. The energy released when an isolated gaseous atom absorbs an electron to its outermost shell is known as electron gain enthalpy. More negative electron gain enthalpy means that the atom can easily gain an electron.
Getting back to the question, we know that from left to right the size of the atom decreases and hence the effective nuclear charge also increases. With increase in effective nuclear charge, the tendency of the element to gain an electron also increases and so its electron gain enthalpy becomes more negative. Due to these given parameters halogens can easily gain one electron to complete their octet and attain a noble gas configuration.
Hence, halogens have maximum negative electron gain enthalpy in the respective period.

Note: For (A): The inert pair effect is the tendency of the outermost s-electrons to remain unshared or unbounded. This occurs due to the ineffective shielding of the s-electrons by the intervening d and f electrons. From top to bottom the inert pair effect increases.
For (B): Carbon exhibits maximum catenation property because of its smaller size and tendency to form strong covalent bonds. The bond energy of C-C bond is higher than N-N bond. Also we should know that catenation of nitrogen is restricted to 2-3 N atoms.
For (C): Electron gain enthalpy is the property of non-metals. Along a period, from left to right the electron gain enthalpy becomes more negative with increasing non-metallic character. Also from top to bottom, the electron gain enthalpy decreases due to decreasing non-metallic character.