A. Define “isotope” using mass number, atomic number, number of protons, neutrons and electrons.

An isotope is a variant of a chemical element which has the same number of protons (atomic number) but a different number of neutrons in its nucleus. Isotopes are identified by their mass number, which is the sum of the number of protons and neutrons in the nucleus. For an isotope with atomic number Z , mass number A , number of protons ( p ), number of neutrons ( n ), and number of electrons ( e ):

1. Atomic number ( Z ) = number of protons ( p )
2. Mass number ( A ) = number of protons ( p ) + number of neutrons ( n )
3. Number of neutrons ( n ) = mass number ( A ) - atomic number ( Z )
4. Number of electrons ( e ) = number of protons ( p ) = atomic number ( Z )

\textbf{Answer:} An isotope is a variant of a chemical element characterized by the same number of protons (atomic number), but a different number of neutrons in its nucleus, identified by its mass number.