Compared to the average diameter of a hydrogen atom, the average diameter of a helium atom is:

The average diameter of a helium atom is smaller.

Explanation:

Step 1: Data given

An atom gets larger as the number of electronic shells increase; therefore the radius of atoms increases as you go down a certain group in the periodic table of elements.

In general, the size of an atom will decrease as you move from left to the right of a certain period.

Step 2:

Since hydrogen is more left on the periodic table than helium, the diameter of the atom will decrease moving from hydrogen to helium.

Hydrogen has a bigger diameter than helium.

The average diameter of a helium atom is smaller.