Question

Order the following elements according to increasing size $C{s^ + },B{a^{2 + }},T{e^{2 - }},X{e^ - }$ .

Answer 1

Here, the size is the atomic size. Atomic size is defined as the distance of the center of the nucleus from the outermost electrons of the orbital. Atomic size changes when an electron is added or removed from the outermost orbital.

Complete step by step answer:
Now we will understand the basic trends of atomic size. In the question, we have given cations as well as anions. So, in general, the cations are formed when an electron is removed from its outermost shell or orbital. This removal of an electron leads to a decrease in the size of an atom. Let us consider an example of the formation of cations. The reaction is as follows, $Cs \to C{s^ + } + {e^ - }$ . Here, one electron is removed which leads to the formation of the cation. Now we will discuss the formation of anions. The anion is formed when the extra electron is added or received from another atom in the outermost shell or orbital of the atom. We can understand it with the help of an example of the formation $Te + 2{e^ - } \to T{e^{2 - }}$ . We can observe that the addition of two electrons leads to the formation of an anion.
According to Fajan’s rule, anions are greater in size as compared to cations or neutral molecules. For cations, the greater the positive charge, the smaller the size of an atom. So, according to the rule, the correct order of increasing size will be $B{a^{2 + }} < C{s^ + } < X{e^ - } < T{e^{2 - }}$ . Anion with a greater negative charge will be the largest.

Note:
For the atomic size we follow a general rule that is $anion > neutral > cation$ . If any of the two species have an identical number of electrons then they are called isoelectronic species. For the size of isoelectronic species, we use the concept of effective nuclear charge.