1.2.3—Isotopes of the same element have the same

Isotopes of one element have the same number of electrons and the same electron arrangement. Because chemical behaviour depends mainly on electron configuration, especially the outer-shell electrons, the isotopes have similar chemical properties.

The shared electron arrangement means isotopes form the same types of bonds, react with the same elements and show essentially the same chemical reactivity. Their different neutron numbers do not change the bonding electron arrangement.

Isotopes can differ in physical properties because different neutron numbers change the mass of the nucleus without adding charge. This supports differences in relative atomic mass and slight differences in density.

Keep the two explanations separate: electron arrangement explains similar chemical behaviour, whereas neutron number explains mass-related physical differences. Do not infer identical physical properties or different chemical reactivity from isotope mass alone.