Neutrons are considered when determining the mass number of atoms because they contribute to the overall mass of the atom. The mass number of an atom is the sum of its protons and neutrons.
Protons and neutrons both have a mass of approximately 1 atomic mass unit (amu). In contrast, electrons have a much smaller mass and are often neglected when calculating the mass number.
Since the atomic number of an element represents the number of protons in its nucleus, adding the number of neutrons to the atomic number gives the mass number. The neutrons provide additional mass without significantly affecting the overall charge or chemical properties of the atom.
Different isotopes of an element have the same number of protons but differ in the number of neutrons. Therefore, isotopes of an element have different mass numbers. By considering the neutrons along with the protons, the mass number provides a more accurate representation of an atom's mass.