Two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons are called what?

• A. Isotope
• B. An Ion
• C. An Atom
• D. Electron

Answer: (A)

Isotopes are different forms of the same element that have the same number of protons but different numbers of neutrons. Protons determine the element’s identity, so these variants are the same element, but adding or removing neutrons changes the mass without changing the basic character of the element. For example, carbon-12 and carbon-14 both have six protons, but differ in neutrons. An ion differs by charge due to a different number of electrons, not by neutron count. An atom is the basic unit that can exist in various isotopic forms, and an electron is a subatomic particle, not a form of the element with a different neutron number. So the described forms are isotopes.