Question

Which is the correct order of size of ${O^ - }$,${O^{2 - }}$, ${F^ - }$ and $F$?
(A) ${O^{2 - }} > {O^ - } > {F^ - } > F$
(B) ${O^ - } > {O^{2 - }} > {F^ - } > F$
(C) ${O^{2 - }} > {F^ - } > F > {O^ - }$
(D) ${O^{2 + }} > {F^ - } > {O^ - } > F$

Answer 1

Hint: The atomic radius of an atom exhibits a periodic trend as well as a group trend. It goes on decreasing as we move along the row, and keeps on increasing as we move along the column.

Complete step-by-step solution: When we are trying to explain the behavior of an atom or molecule or compound, we need to have an idea about their size. Atomic radius is one of the ways of expressing the size of an atom. For measuring the atomic radius, we consider the distance between the nuclei of two identical atoms which are bonded together. The atomic radius can thus be defined as one-half of the distance between the nuclei of identical atoms that are bonded together. Let us try to understand the trend of atomic radius across a period and along a group.
When we move across a period, protons are being added to the nucleus and electrons are being added to the same principal energy level. The increase in the number of protons in the nucleus increases the nuclear charge and thus it pulls the electrons with more force than before. These results in a decrease in the size of the atom. This effect will further go on decreasing, if we move to the right along the period. This is because of electron-electron repulsion, which will ultimately cause atomic size to increase.
When we move down in a group, atomic number increases. There is an increase in the positive nuclear charge due to addition of protons and also there is an increase in the number of occupied principal energy levels, with addition of electrons. As the number of principal energy levels increases, the size of orbitals also increases. So, for lower principle energy levels, the orbitals are also lower in size. This effect of the greater number of principal energy levels becomes dominant over the increase in nuclear charge and as a result of which, atomic radius increases down a group.
So, the correct order will be ${O^ - } > {F^ - }$

The correct option is B $({O^{2 - }} > {O^ - } > {F^ - } > F)$.

Note: Anions are formed by adding an additional electron to the neutral atom and cations are formed by removing an electron from the parent neutral atom. Anions carry negative charge and cations carry positive charge. When size is compared, anions are having a greater size than the parent neutral atom and cations will have smaller size compared to their parent atom.