Question

Which of the following statements about the effective nuclear charge (Z_eff) is correct?

	a.	Zeff increases across a period and is relatively constant down a group.
	b.	Zeff decreases across a period and increases down a group.
	c.	Zeff increases with the size of the atom.
	d.	Zeff increases as the value of the principal quantum number increases.
	e.	Zeff is greater for hydrogen than for any other element.

Answer 1

Z_eff increases across a period and is relatively constant down a group.

Z is the number of protons in your atom. Every atom has a different Z, Hydrogen has 1, Helium has 2, Lithium has 3, etc.

Zeff is the charge felt by the valence electron. If the valence electron always lies outside all of the electrons below it, the Zeff would always be 1. For example, Lithium has 3 electrons and a 3+ nucleus. If the valence electron always lies outside the 2 other electrons, its Zeff would always be 1 (3 -2).

However, this is not always the case. In a row, the valence electron can often fall below the other electrons. This happens in a row, because as electrons are filled in a row, they tend to have similar energies. So, let's say for Beryllium, the valence electron lies outside 2 other electrons, but is 50% outside the third electron. Therefore, the Zeff would be higher and be 1.5 (3 -2 - 0.5).

As you move to the right on the periodic table, the Zeff will increase because the valence electron will have about the same energy as many more electrons. (just use the analogy I showed you). The Zeff values may not be completely right, but you get the picture.

Boron Zeff = 2.0
Carbon Zeff = 2.5
Nitrogen Zeff = 3.0
Oxygen Zeff = 3.5
Fluorine Zeff = 4.0