why first ionisation energy of magnesium are smaller than beryllium?

The atomic number of magnesium is 12 and its ground state of electronic configuration is
Mg : 1s² 2s² 2p⁶ 3s²
Each magnesium atom has 12 nuclear charges and 10 electrons in inner shells.
Hence, effective nuclear charge of magnesium = 12 - 10 = 2

The atomic number of beryllium is 4 and its ground state of electronic configuration is
Be : 1s² 2s²
Each beryllium atom has 4 nuclear charges and 2 electrons in inner shells.
Hence, effective nuclear charge of beryllium = 4 - 2 = 2

Magnesium and beryllium have equal effective nuclear charges. Magnesium has a greater atomic size than beryllium. This is because magnesium atom has one more electronic shell than beryllium atom. Therefore, the attractive force between the nucleus and the electrons in the outermost shell of magnesium atom is smaller than that of beryllium atom, because of a longer distance between the nucleus and the electrons in the outermost shell of magnesium. This leads to the fact that less energy is needed to remove an electron in the outermost shell from a magnesium atom than from a beryllium atom. In other words, the first ionization energy of magnesium is smaller than that of beryllium.