Question

Which of the following represents the correct order of increasing first ionization enthalpy for Ca, Ba, S, Se, and Ar?
a- S < Se < Ca < Ba < Ar
b- Ba < Ca < Se < S < Ar
c- Ca < Ba < S < Se < Ar
d- Ca < S < Ba < Se < Ar

Answer 1

Hint: The first ionization enthalpy is the energy required to remove 1 electron from the neutral gaseous atom. It depends upon the effective nuclear charge of the atom.

Complete answer:
Ionization enthalpy is the minimum amount of energy required to remove the most loosely bound electron or the most outer shell electron of an isolated neutral gaseous atom or molecule. As one moves across a period i.e. left to right in the periodic table, the ionization enthalpy of an element increases. This is due to the fact that the electrons are more tightly bound by the higher effective nuclear charge. As we move down into the group the ionization energy of the elements decreases, because the electrons are held in an outer orbital which is away from the nucleus and therefore, are less tightly bound. As a result the ionization enthalpy decreases.
- Ar has higher ionization enthalpy because it is a noble gas of period 3.
- In case of Ba it is in 6 period and more metallic character that makes its ionization energy lowest.
- Among Ca, S and Se, Ca has lower ionization energy as it is a metal.
- From S and Se, S has higher Ionization energy as it is of period 3 and Se belongs to period 4.

Hence , by the above properties we can conclude that the correct order of increasing first ionization enthalpy for Ca, Ba, S, Se, and Ar is option (B) Ba < Ca < Se < S < Ar .

Note:
In periodic table noble gases have highest ionization energy value and s block elements have lowest ionization energy value. Find out them first then examine the remaining elements by their period and group number.