Which of the following is the trend down a period?
a. the number of complete electron shells increases by one.
b. the number of complete electron shells stays the same.
c. the number of outer shell electrons decreases by one.
d. none of these.

B. The number of complete electron shells stays the same.

Explanation:

The periods on a periodic table consists of those elements that are arranged in the horizontal row in the periodic table.

Elements in the same period have the same number of electron shells and this corresponds to the period. This implies that elements of period 2 have two electron shells, those of period 3 have three shells and so on.

The number of Valence electrons of the elements on the same period increases progressively by one across the period from left to right.

A group is the vertical column to which elements are arranged on the periodic table. Elements in the same group have the same number of Valence electrons and the number of shell increases down a group.

When we go down a period, the number of shell stays the same.