Question

Why is there a drop in value from Beryllium to Boron for period 2 and why is there a drop in value from Magnesium to Aluminium in period 3?

Answer 1

While moving along the periods of periodic table, the properties of elements are changing.
In the 2nd period Boron has one more electron on the other shell comparatively to Beryllium (the same as Alluminium has one more electron comparatively to Magnesium in the 3rd period). This influences the charge, atom’s diameter and mass, and, consequently, properties.