# How are two atoms of the same element alike?

They contain the same number of $\text{protons}$..............
They contain the same number of $\text{protons}$, positively charged, massive, nuclear particles. This definition allows for the existence of isotopes, whose atoms contain the same number of $\text{protons}$ (and must do because they are isotopes of the same element), but DIFFERENT numbers of neutrons, neutrally charged, massive $\text{nucular particles}$.