If there are more electrons than protons the charge of the atom is negative. Likewise, if there are less electrons than protons the charge of the atom is positive. In both cases, the exact charge is determined by subtracting protons from electrons. All atoms of a specific element have the same number of protons in their nuclei. Atomic mass number is an atom's total number of neutrons and protons.
Many elements have unequal numbers of neutrons and protons in their nucleus.
An element's atomic weight refers to the total weight of neutrons, protons, and electrons. Atomic number describes the number of protons found in an atom. Some elements can have variants containing different numbers of neutrons but similar numbers of protons. We call these variants isotopes . Carbon has two isotopes.