Which of the following is the best definition of an element?
a. chemical substance whose atoms all have the same number of protons
b. chemical substance that has lost or gained electrons
c. chemical substance made up of different types of atoms
which of the following is the best definition of an element?
a. chemical substance whose atoms all have the same number of protons
b. chemical substance that has lost or gained electrons
c. chemical substance made up of different types of atoms

A. chemical substance whose atoms all have the same number of protons

Explanation:

An element is a substance which contains identical atoms that have the same number of protons in the nucleus.

Elements are arranged in the periodic table according to their atomic number (= number of protons): so atoms of different elements have a different number of protons in their nuclei.

For a neutral atom, the number of electrons around the nucleus is also equal to the number of protons.

Moreover, atoms of the same element can have a different number of neutrons, despite having the same number of protons - these atoms are called isotopes.