An element has the electron configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p3. Which of the following statements about this element is correct?

Correct answer

A. It is a main-group element in period 5 and group 15.

Principle or equation

The periodic table is organized by electron configuration. The highest occupied principal quantum number determines the period. The number of valence electrons (for main-group elements) determines the group number (for groups 1-2 and 13-18, group number = 10 + number of p electrons in the highest energy level, or for groups 1-2 it equals the number of s electrons).

Why this answer is correct

The configuration ends with 5s2 5p3. The highest principal quantum number is 5, so the element is in period 5. The valence electrons are in 5s and 5p: 2 + 3 = 5 valence electrons. For main-group elements in groups 13-18, the group number is 10 + number of p electrons, so 10 + 3 = 13? Wait, that gives group 13, but the correct group for 5 valence electrons is 15. Actually, for groups 13-18, group number = 10 + number of valence electrons (for p-block elements). Here valence electrons = 5, so group = 10 + 5 = 15. The element is in group 15 (pnictogens). The d electrons are inner transition? No, they are in the 4d subshell, which is filled, so the element is in the p-block, not a transition metal. Thus, it is a main-group element in period 5, group 15.

Example

For selenium, [Ar]3d10 4s2 4p4, period 4, group 16 (10+6=16).