Asubstance that is composed entirely of atoms which all have the same number of protons in their nuclei is known as

a. an element
b. a molecule
c. a neutron
d. a compound

An element consist on atoms which may have different number of electrons or neutrons, but the same number of protons. Atoms with the same number of protons have the same name. For example, all carbons have 6 protons, even though some of them have 6 neutrons and others have 7 or 8 neutrons. Atoms belonging to the same element but with a different number of neutrons are called isotopes. Besides, the number of electrons may vary during chemical reactions – that’s how ions are formed.