The nuclear charge an electron actually experiences is called the ___ nuclear charge (Zeff). This charge is always ___ than the actual nuclear charge since in many-electron systems the electrons ___ each other from the nucleus.

1. Effective nuclear charge (Zeff)

2. Less

3. Shield

Explanation:

The effective nuclear charge (Zeff) is defined as the nuclear charge that is experienced by an electron of an atom having atomic number > 1.

The effective nuclear charge experienced by a given electron is always less than the actual nuclear charge of the nucleus of the atom. This is because of the shielding effect of the inner electrons of the atom.

The shielding effect is defined as the shielding of an electron from the positive nuclear charge of the nucleus by the inner electrons of an atom.  

Therefore, the outer electrons of an polyelectronic atom are shielded by the inner electrons from the nuclear charge of the nucleus.