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The atomic mass of an element is based on the mass of the protons, neutrons, and electrons of the atoms of that element. The mass of the proton and neutron are about the same, but the mass of the electron is much smaller \(about 1/2000 the mass of the proton or neutron\). The majority of the atomic mass is contributed by the protons and neutrons. For any element in the periodic table, the number of electrons in an atom of that element always equals the number of protons in the nucleus.
But this is not true for neutrons. Atoms of the same element can have different numbers of neutrons than protons. Atoms of the same element with different numbers of neutrons are called isotopes of that element. The atomic mass in the periodic table is an average of the atomic mass of the isotopes of an element. For the atoms of the first 20 elements, the number of neutrons is either equal to or slightly greater than the number of protons.

• Electrons do not add the mass to the atom.
• Isotopes are atoms of the same element which contain another number of neutrons.
• The mass of an atom is based on the mass of the protons.
• For the first 20 elements the number of protons are absolutely different than the number of neutrons.