The question “which of the following is not an element?” cannot be answered completely without seeing the actual options.

What an element is

In chemistry, an element is a pure substance made of only one type of atom, all with the same number of protons in the nucleus.

Examples of elements include hydrogen, oxygen, carbon, nitrogen, and calcium.

Common types of answers

In typical exam or forum questions with this wording, the options often mix:

  • Names of actual elements (for example: oxygen, iron, sodium)
  • Names of compounds (for example: water, salt, sugar)
  • Names of mixtures (for example: air, brass, steel)

Anything that is a compound (made of more than one type of atom chemically combined) or a mixture is not an element.

How to decide in your question

To determine which option is not an element in your specific problem:

  1. Identify which options are listed on the periodic table (those are elements).
  2. Check if any option is:
    • A compound name (like water, carbon dioxide, sodium chloride).
    • A mixture name (like air, bronze, steel, milk).
  3. The compound or mixture among the options is the one that is not an element.

If you share the exact multiple‑choice options, a precise answer can be given for your particular “which of the following is not an element?” question.