Effective Nuclear Charge
Effective nuclear charge, the charge an electron
experiences after accounting for the shielding due to other electrons, increases
from left to right across a given period, thus an electron in a 2p orbital of a
nitrogen atom experiences a greater Zeff (3.83) than an electron in a 2p
orbital of a carbon atom (3.14). This trend makes sense if we consider what happens to
atomic structure as we move from left to right across a period. Every time we move one
group farther to the right, we add one more proton to the nucleus (i.e., the
actual nuclear charge, Z, increases by one) and we add one more electron to the valence
shell. This additional electron will not be perfectly shielded from the nucleus by the
other electrons, nor will it do a perfect job of shielding the other electrons from the
nucleus. This means that S, the shielding constant, will increase by less than one and, as
a result, Zeff increases.
Effective nuclear charge is most useful when we are comparing electrons in the same shell and subshell for two different atoms (for example we might compare Zeff for a 2p electron in nitrogen with Zeff for a 2p electron in carbon). It is less useful to compare say, Zeff for a 2s electron of lithium with Zeff for a 3s electron of sodium, and since we are rarely concerned with the core electrons of an atom, we will not generally compare Zeff for atoms in different periods. We will use the concept of Zeff to rationalize the trends for some other physical properties, so make sure you are comfortable with this explanation for the periodic trend for Zeff before proceeding.