Why is the first ionization enthalpy of chromium lower than that of zinc?

The first ionization enthalpy is the energy required to remove the outermost (valence) electron from an atom. In the case of chromium (Cr) and zinc (Zn), there is a difference in their electron configurations that explains why the first ionization enthalpy of chromium is lower than that of zinc.

Chromium has an electron configuration of [Ar] 3d⁵ 4s¹, while zinc has an electron configuration of [Ar] 3d¹⁰ 4s².

Now, let's consider the removal of the outermost electron in both cases:

Chromium (Cr):

Chromium has one valence electron in the 4s orbital.
When you remove this electron to form a Cr⁺ ion, the electron is being removed from a higher energy level (4s) compared to the 3d orbital.
Removing an electron from a higher energy level requires less energy because electrons in higher energy levels are farther from the nucleus and are shielded by inner electron shells. Thus, the first ionization enthalpy of chromium is lower.
Zinc (Zn):

Zinc has two valence electrons in the 4s orbital.
When you remove one of these electrons to form a Zn⁺ ion, the electron is being removed from the same 4s orbital.
Since the electron is being removed from the same energy level (4s), it requires more energy compared to removing the single valence electron from chromium's 4s orbital.
In summary, the lower first ionization enthalpy of chromium compared to zinc is due to the fact that the electron being removed from chromium is in a higher energy level (4s) and is shielded to some extent by the 3d electrons, making it easier to remove. In contrast, removing an electron from zinc requires breaking the stability of a filled 4s orbital, which requires more energy.