What do you understand by:
A. inert pair effect
B. allotropy
C. catenation

Hint: The ionisation becomes difficult when the outermost electrons (of s-orbital) are tightly bonded and this occurs due to inert pair effect. The presence of two or more elements with the difference between physical composition is called allotropy. Catenation is the self-linking property of the atoms like carbons.

Complete answer:
A. Inert pair effect: Sidgwick gave the inert pair theory and defined it as the effect of the s-orbital electron when they do not react with other species to form the bond.
-The non-participation of the s-orbital electron is due to their high energy need for the separation of them.
-The inert pair effect increases when we down the group due to the poor shielding of f and d orbital electrons, the electrons are more attracted towards the nucleus.
-The effect also occurs because for example, when we move down the group in group 14, +2 oxidation state's stability increases and the stability of +4 oxidation state decreases. After all, the energy required to pair them becomes more than the energy required to separate them.
-For example, Tin and lead from group 4 and 5 forms $\text{S}{{\text{n}}^{2+}}\text{ and P}{{\text{b}}^{2+}}$respectively. And $\text{S}{{\text{b}}^{2+}}\text{ and B}{{\text{i}}^{2+}}$ is also an example of inert pair effect.
-They have low oxidation also.

B. Allotropy: When an element is found to exist in two or more forms with a similar physical state but have a different arrangement of the atoms.
-The term allotropy is used only for elements and not for the compounds.
-For example, there are four allotropes of carbon i.e. diamond, graphite, graphene and fullerenes, they have a different arrangement of atoms.
-In diamond, the arrangement of atoms is done in the tetrahedral lattice and each carbon makes four bonds. In graphite, each carbon makes three bonds with each other and one electron remains free.
-In graphene, it is the part of graphite i.e. the single sheet of the graphite. When graphene aggregates it forms graphite. Whereas in fullerenes, the carbon makes the bond with each other in the shape of spherical or tubular.

C. Catenation: It is a property of the atoms to link with each other and form an open or closed chain compound.
-In carbon, catenation has taken place readily and forms a covalent bond with each other to form a long chain.
-Catenation can also be seen in the silicon, boron and sulphur.

Note: The inert pair effect can be seen in group 13, 14, 15 and 16. The stability of different allotropes is different under certain conditions. The trend of the catenation has been observed in the periodic table as to when we move down the group then the tendency of catenation decreases.