1.2 Isotopes

Isotopes are atoms of the same element. They have the same number of protons, so they have the same atomic number, but they contain different numbers of neutrons.

Because isotope proton numbers are the same but neutron numbers differ, their mass (nucleon) numbers are different. The nuclear identity can therefore be compared using proton number and neutron number separately.

In isotope notation, the lower-left atomic number stays constant for isotopes of one element, while the upper-left mass number changes. This lets the isotope be distinguished without changing the element symbol.

Changing neutrons makes an isotope; changing electrons makes an ion. Do not use a different mass number as evidence that the proton number, and therefore the element, has changed.

Read isotope notation in the form ^A_ZX by locating the mass (nucleon) number A at the upper left and the atomic (proton) number Z at the lower left. X is the chemical symbol, so Z identifies the element.

Use a fixed sequence: (1) copy A and Z, (2) calculate neutron number as A − Z, and (3) check the symbol against Z. The mass number counts protons plus neutrons; it is not the number of electrons.

For isotopes of one element, Z and X stay the same while A changes because the neutron number changes. Thus a different upper-left number does not by itself mean a different element.

Only change the electron count when a charge is shown: a positive ion has lost electrons and a negative ion has gained electrons. Keep this charge adjustment separate from the nuclear counts A, Z and A − Z.

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.

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.