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shielding increases as u go across a period from left to right. this is because electrons are added but the core shell stays the same. The Zeff increases and the radii of the atom decreases because the valence electrons are more tightly bound.
going down a group the shells increase in size but the Zeff stays the same and this is related to the increase in atomic radii. going up the shells decrease and the radii becomes smaller.
Zeff- for Na has 11 electrons. in the row before it the core electrons are 10. 11-10= + 1, so Na valence electrons feel a +1 shielding effect.
Zeff- for Mg has 12 electrons. in the row before it the core electrons are 10. 12-10= +2, so Mg valence electrons feel a +2 shielding effect.