Isotopes

All atoms of the same element have the same number of protons (same atomic number). The element cannot change as long as the proton count stays fixed.

When atoms of the same element have different numbers of neutrons, they have different mass numbers but the same atomic number. These variants are called isotopes.

The term that describes atoms of the same element with different neutron counts is isotopes. The other terms refer to different concepts entirely.

The question asks for the name given to atoms of the same element that differ only in their number of neutrons.

Ions are atoms that have gained or lost electrons, giving them a charge. Changing electron count does not create a different variant of the same element in the same way — this is not the right term.

Allotropes are different structural forms of the same element (e.g., diamond and graphite for carbon). Isomers are molecules with the same molecular formula but different structures. Neither describes atoms differing in neutron count.

Isotopes is the correct term — atoms of the same element (same proton number) with different numbers of neutrons and therefore different mass numbers.

From ${}_{16}^{34}\text{S}$: the mass number $A=34$ (top) and the atomic number $Z=16$ (bottom).

Using the formula: $\text{Neutrons}=A-Z=34-16=18$

Sulfur-34 contains 18 neutrons. Note that 16 is the number of protons, not neutrons — a common mistake to avoid.

We need the number of neutrons in ${}_{16}^{34}\text{S}$. The formula is: $\text{Neutrons}=A-Z$.

16 is the atomic number — the number of protons. Confusing protons with neutrons is the most common mistake here.

34 is the mass number (protons + neutrons combined), not just neutrons. 50 would result from adding $A+Z$, which has no physical meaning for neutron count.

$34-16=18$ neutrons. This is the only value obtained by correctly applying $A-Z$.