Isotopes refer to atoms of the same element that differ in which property?

• A. Number of neutrons
• B. Number of protons
• C. Number of electrons
• D. Number of energy levels

Answer: (A)

Isotopes are atoms of the same element that differ in the number of neutrons. The number of protons in an atom determines what element it is, so isotopes share the same protons and the same chemical identity. What changes is the neutrons in the nucleus, which changes the atom’s mass but not its charge, since neutrons don’t carry charge. Because the electron count in a neutral atom matches the proton count, isotopes also have the same number of electrons, so their chemical behavior is basically the same. The difference in neutrons can affect nuclear properties, including stability and radioactivity.

For example, carbon-12 and carbon-14 both have 6 protons and, when neutral, 6 electrons, but carbon-14 has 8 neutrons while carbon-12 has 6. That extra mass comes from the additional neutrons, influencing physical properties and stability without changing the element’s identity.