The electronic structure

the increase of the first ionization enthalpy cross a period is due to the increase in the effective nuclear charge, which cause a decrease in atomic radius and hence a decrease in the the size of the electron cloud. As the electron cloud draws closer to the nucleus, if is more difficult to remove an electron. Noble gases have maximum first ionizatoin enthalpies because of their satble octet electronic configurations. A large amount ot energy is requied to remove an electron from a fully-filled electron shell.

Completely filled electrons subshell has extra stability upon themselves. It is very difficult to remove them when compared to other electrons in the same electron shell. Hence there is a large increase in I.E. in a same period when electrons are removed from a completely filled electron subshell.