Which is not a compound?

sugar
water
silicon dioxide
gold

gold

Explanation:

Element is a pure substance which is composed of atoms of similar elements. Example: Argon with atomic symbol

Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass.

Example: Water with formula and contains hydrogen and oxygen in a fixed ratio by mass

1. Sugar has chemical formula of [texC_{12}H_{22}O_{11}[/tex] and thus is a compound.

2. Water has chemical formula of and thus is a compound.

3. Silicon dioxide has chemical formula of and thus is a compound.

4. Gold has chemical formula of and thus is an element.

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it means there are 3 atoms of oxygen