Based on atomic properties and structure:
Isotopes are atoms of an element having the same atomic number but different mass numbers.
The difference in mass number is dues to different neutron numbers.
However, because they have the same proton number, they are atoms of the same element.
Neutral atoms contains equal number of protons and electrons.
An atom of barium has 56 protons and 56 neutrons in order to maintain electrical neutrality.
The mass number of an atom is determined by the sum of protons and neutrons.
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