The effective nuclear charge is the amount of charge experienced by an electron taking into account any shielding effects from other electrons. Electrons between the nucleus and the electron of interest cause shielding and reduce the actual charge felt by the electron of interest. The outermost electron of boron experiences a lower effective nuclear charge than carbon. Why?

The nuclear charge increases from boron to carbon, but there is no additional shielding (that is no additional shells).

Explanation:

First of all, we must know the electron configuration of carbon and boron.

Boron - 1s2 2s2 2p1

Carbon - 1s2 2s2 2p2

Moving from boron to carbon, the effective nuclear charge increases without a corresponding increase in the number of shells. Remember that shielding increases with increase in the number of intervening shells between the outermost electron and the nucleus. Since there isn't an increase in shells, boron experience a lower screening effect.

From

Zeff = Z - S

The Z for carbon is 6 while for boron is 5 even though both have the same number of screening electron S (4 screening electrons). Hence it is expected the Zeff (effective nuclear charge) for boron will be less than that of carbon.