The alkali metals have low ionization enthalpies that decrease down the group from lithium to cesium. This is because the increasing atomic size outweighs the effect of the increasing nuclear charge, so the outermost electron is well shielded from the positive charge of the nucleus.

2. The
s-block
elements

6. IONIZATION ENTHALPY
• The ionization enthalpies of
the alkali metals are
considerably low and
decrease down the group
from Li to Cs.
• This is because the effect of
increasing size outweighs
the increasing nuclear
charge, and the outermost
electron is very well
screened from the nuclear
charge.