1.2.4—Isotopes of the same element have different

Isotopes have different numbers of neutrons. Neutrons add mass to the nucleus but carry no charge, so isotope mass can change physical properties without changing the element or its bonding electron arrangement.

The different isotope masses give different relative atomic masses and can cause slight differences in density. These are physical differences linked to nuclear mass, not to a change in chemical identity.

All isotopes of one element have the same number and arrangement of electrons, so they show similar chemical properties even though their physical properties are not identical.

Do not treat a different mass number as a different element or as evidence of different bonding. Keep the isotope-mass explanation for physical-property variation separate from the electron-arrangement explanation for chemical reactivity.