Which of the following statements about the effective nuclear charge (Zeff) 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.

A

Explanation:

The effective nuclear charge refers to the attraction that a particular electron in its orbit feels for the nuclear charge. Across the period, there is an increase in nuclear charge without any corresponding increase in the number of shells. Hence the effective nuclear charge increases as outermost electrons feel the attractive force of the nucleus all the more. However, down the group, the increase in nuclear charge is counteracted by increase in the number of shells. The effective nuclear charge then remains fairly constant down the group.