Isotope atoms
Isotopes are atoms of the same element that have the same number of protons (atomic number) but different numbers of neutrons. This means that isotopes have the same number of electrons and the same chemical properties, but they differ in their atomic weight.

Atomic number
The atomic number of an element is the number of protons in the nucleus of an atom. It determines the identity of the element and is unique for each element. All atoms of the same element have the same atomic number.

Atomic weight
The atomic weight of an atom is the average mass of all the isotopes of that element, taking into account their abundance in nature. It is calculated by multiplying the mass of each isotope by its abundance and then summing these values. Atomic weight is expressed in atomic mass units (amu) and is a weighted average.

Number of neutrons
Neutrons are subatomic particles found in the nucleus of an atom, along with protons. Isotopes of an element have different numbers of neutrons, which leads to variations in their atomic weights. The number of neutrons in an atom can be determined by subtracting the atomic number (number of protons) from the atomic weight (mass number).

Explanation
Isotopes of an element have the same number of protons (atomic number) because they belong to the same element. However, they have different numbers of neutrons, which results in different atomic weights. This is because isotopes are defined by their mass number, which is the sum of protons and neutrons in the nucleus. Therefore, the correct answer is option 'A' - isotopes differ in the number of neutrons.

Summary
Isotopes of an element have the same atomic number (number of protons) and number of electrons but differ in the number of neutrons. This difference in neutrons leads to variations in the atomic weight of isotopes.