what are isotopes and why do they have the same properties?

Isotopes are atoms of the same element that have the same proton number but different nucleon number. TRAIN OF THOUGHT --- Isotopes have the same proton number --- uncharged (neutral/unionised) molecules have the same number of protons and electrons --- Therefore isotopes have the same number of electrons, causing them to have the same properties. As seen in the periodic table, elements with the same number of electrons in their orbit (same group), display the same characteristics because they have the same capacity to lose or gain electrons