Assertion: Generally, ionization enthalpy increases from left to right in a period. Reason: When successive electrons are added to the orbitals in the same principal quantum level, the shielding effect of the inner core of electrons does not increase very much to compensate for the increased attraction of the electron to the nucleus.

Ionization enthalpy of the elements increases from left to right in a period as when we move from left to right in a period, the electrons are added in the same principal quantum number. So, the nuclear force on the electrons by the nucleus increases causing the decrease in the size, and also the shielding effect provided by the inner core electrons is not enough to compensate for the increase in attractive force on the electrons to the nucleus. Therefore, both assertion and reason are correct with reason being the correct explanation of the assertion.