The valence electrons of an atom of which element would feel a smaller effective nuclear charge than the valence electrons of a calcium (ca) atom?

Any element with to the right on the periodic table, or down below Ca on the periodic table, will feel a smaller effective nuclear charge. Elements to the right of Ca have more valence electrons to 'share' the pull of the protons in the nucleus. Elements below Ca have additional, filled shells of electrons, shielding the valence electrons from the nucleus.

Ido not understand. 

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