Answer:
The first  ionization energy will increase from sodium to aluminium to chlorine (on moving from left to right in the period).
Explanation:
The ionization energy is the energy required to remove an electron from the valence shell of an isolated gaseous atom of an element.
Now the ionization energy depends upon
a) size of an element ,larger the size more lesser the effective nuclear charge on electron thus lower the ionization energy.
b) the electronic configuration of an element. If on losing an electron an element attains noble gas configuration or half filled stability then it will easily lose an electron.
In case of Sodium, if it loses an electron it will attain the noble gas configuration of Neon.
also the size of sodium is more than that of aluminium and the size of aluminium is more than that of chlorine.
So it is most easy to remove electron from sodium than from aluminium and than from chlorine.
Relation will be:
First ionization energy will increase as:
Na < Al < Cl
