Question

Which of the following has the highest first ionization energy?
A.Lithium
B.Beryllium
C.Boron
D.Carbon

Answer 1

Hint: Ionization energy is the minimum amount of energy required to remove the most loosely bound electron of an isolated neutral gaseous atom or molecule. The ionization energy decreases from top to bottom in groups and increases from left to right across a period.

Complete step by step answer:
The first ionization energy is the energy required to remove one mole of the most loosely held electron from one mole of gaseous atom to produce one mole gaseous ion each with a charge of +1.
As we move across the period, the first ionization energy increases. This is due to the fact that the atomic radius decreases i.e. the atom is small in size. Further, the outer electrons are closer to the nucleus and are more strongly attracted to the center.
Therefore, the order of ionization energies is given below:
 $$Li<Be<B<C$$
Therefore, on moving from left to right, the ionization energy increases as effective nuclear charge increases and the size decreases and hence, the removal of electrons becomes more and more difficult.
So, carbon has the highest first ionization energy.

Hence, option D is correct.

Note:
Since the nuclear charge is necessarily diminished with respect to the valence shell, the alkali metals display the lowest ionization energies. Further, the ionization energy of carbon is 1086.45 Kj $mo{l}^{-1}$ . It is not as high as its neighbor nitrogen (with its half-filled 2p subshell), but it is higher than that of its other neighbor boron (which has only one electron in its 2p subshell).