Atoms of the same element can differ in neutron count, which explains non-whole-number atomic masses.

• Isotopes have the same Z but different A (¹²C and ¹⁴C).
• Isotopes behave identically chemically because the electron arrangement is the same.
• Relative atomic mass is the weighted average of the isotopes by abundance.
• Weighted average = Σ(mass × percentage abundance) ÷ 100.

• Isotope — same element, different number of neutrons.
• Relative atomic mass — the abundance-weighted average mass.