What defines an isotope?

• A. Atoms of the same element with varying numbers of neutrons
• B. Atoms of different elements with varying numbers of neutrons
• C. Ions with a charge
• D. Molecules with different bonding

Answer: (A)

Isotopes are versions of the same element that differ in the number of neutrons. The identity of an element is determined by how many protons it has, and isotopes share that same proton count, so they are the same element. Changing the number of neutrons changes the mass but not the charge, so the nucleus becomes a bit heavier or lighter without altering the element’s basic identity. For example, carbon-12 and carbon-14 each have six protons, but carbon-14 has eight neutrons. Because chemical behavior is governed mainly by electron structure (which depends on the number of protons/electrons), isotopes behave very similarly chemically. In XRF analysis, the characteristic X-ray energies come from electron transitions in the atom and are the same for all isotopes of an element, so isotopic variation doesn’t change the spectral lines for that element. The other statements mix up different concepts: atoms of different elements with varying neutrons would be different elements; ions are charged forms of atoms; and molecules with different bonding describe isomers or different compounds, not isotopes.