Ontario Grade 9 Science Class Practice Exam

An isotope of an atom differs from the atom itself because it has a different number of neutrons. Electrons and protons cannot vary in an atom because they determine the element's chemical properties and identity. Isotopes, on the other hand, have the same number of protons and electrons as the atom, but their varying number of neutrons can affect their stability and radioactivity.

Option A is incorrect because electrons are negatively charged particles that orbit around the nucleus of an atom. Though they play a role in an atom's chemical properties, they do not affect its identity or differentiation from an isotope.

Option B is incorrect because the number of protons determines the atomic number of an element, which remains constant for all isotopes of that element.

Option D is incorrect because the atomic mass is the sum of the number of protons and neutrons. While different isotopes of an element