An element is a substance made of atoms that all contain the same number of protons.

An element is a substance made of atoms that all contain the same number of protons.

• A. A substance made of atoms with the same number of protons
• B. A substance formed by chemical bonds between molecules
• C. A mixture of substances
• D. A single element

Answer: (A)

Elements are substances made of atoms that all have the same number of protons—the atomic number defines which element it is. Because protons determine an element’s identity, any substance that contains only atoms with that same proton count qualifies as that element, even if those atoms are bound together in molecules. For example, oxygen gas (O2) is still the element oxygen, since both atoms have eight protons. So the statement exactly describes the defining feature: a substance made of atoms with the same number of protons. The other ideas describe different things: bonding between molecules would form compounds or polymers, not a pure element; a mixture contains more than one substance; and calling something a “single element” ignores the fact that elements can exist as molecules rather than as just individual atoms, even though they remain the same element.

Elements are substances made of atoms that all have the same number of protons—the atomic number defines which element it is. Because protons determine an element’s identity, any substance that contains only atoms with that same proton count qualifies as that element, even if those atoms are bound together in molecules. For example, oxygen gas (O2) is still the element oxygen, since both atoms have eight protons.

So the statement exactly describes the defining feature: a substance made of atoms with the same number of protons.

The other ideas describe different things: bonding between molecules would form compounds or polymers, not a pure element; a mixture contains more than one substance; and calling something a “single element” ignores the fact that elements can exist as molecules rather than as just individual atoms, even though they remain the same element.