Question

The decreasing order of the size of ${H^ - },{O^ - },{F^ - },C{l^ - }$
A. ${H^ - } < {O^ - } < {F^ - } < C{l^ - }$
B. ${H^ - } > {F^ - } > {O^ - } > C{l^ - }$
C. ${H^ - } > {O^ - } > C{l^ - } > {F^ - }$
D. $C{l^ - } > {F^ - } > {O^ - } > {H^ - }$

Answer 1

Hint: To answer this question, you must recall the trend of change in the atomic radius of an atom in a periodic table. As we move from left to right in a period, the atomic size decreases and when we move down the group, an increase in the size is observed.

Complete step by step answer:
- In order to explain the behaviour of an atom or ion or molecule, we must have an idea about its size. Atomic radius is the most common way of expressing the size of an atom and similarly the size of an ion is given by the ionic radius.
- Anions are obtained when a neutral atom gains an additional electron and carries a negative charge. Since the number of electrons increases, the interelectronic repulsion increases and the effective pull on each electron decreases. As a result, the size of anions is greater than parent atoms. The size of an ion is determined by the effective nuclear pull on the valence electrons.
- Hydrogen contains only one proton and thus has a very low nuclear charge. When it gains an electron, its outer electrons are relatively free and maximize the ionic radii. Thus, the correct order is given by
${H^ - } > {O^ - } > C{l^ - } > {F^ - }$

Thus, the correct option is C.

Note:
Atomic radius can be termed as half of the distance between the nuclei of two identical atoms bonded together. When we move from left to right in a period, protons and electrons are added to the same energy level. As the nuclear charge increases, the pull on the electrons increases and thus the size decreases. But ultimately in the last elements of the period, the size increases due to inter electronic repulsions. As we move down the group, the number of shells keeps on increasing and thus the size of the atom increases.