The table shows the atomic number of four atoms.
Which one is NOT a metal?

Based on the periodic table of chemical elements, the atom which is not a metal is the atom with an atomic number (number of protons) of 18.

Given the following data:

An chemical element can be defined as a pure substance which is made up of atoms that are having the same atomic number (number of protons) in its atomic nucleus.

This ultimately implies that, an atomic number is equal to the number of protons in the nucleus of an atom of a chemical element.

In Chemistry, the following chemical elements have this atomic number:

Note: Both group 1 and 2 chemical elements are metals.

However, an atom with an atomic number of 18 is in group 8 of the periodic table and as such isn't a metal but a noble gas.

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A. Argon

Explanation:

Argon is a nonmetal. It is a noble gas with an atomic number of 18.