How does the nuclear model explain atomic mass?

The nuclear model explains atomic mass as the combined total of protons and neutrons in an atom's nucleus.

The nuclear model of an atom was proposed by Ernest Rutherford in 1911. This model suggests that an atom is mostly empty space, with a tiny, dense nucleus at its centre. The nucleus contains protons and neutrons, while electrons orbit around the nucleus in shells. The atomic mass, also known as the atomic weight, is determined by the total number of protons and neutrons in the nucleus.

Protons and neutrons are much heavier than electrons. In fact, the mass of an electron is so small that it is almost negligible when calculating the atomic mass. Therefore, the atomic mass is essentially the sum of the number of protons and neutrons. This is why the atomic mass is always a whole number (or very close to a whole number when averaged across isotopes).

For example, consider a carbon atom. A carbon atom has 6 protons and 6 neutrons in its nucleus. Therefore, the atomic mass of carbon is 12 atomic mass units (amu).

It's important to note that the number of protons in an atom (the atomic number) identifies the element. For instance, any atom with 6 protons is a carbon atom, regardless of the number of neutrons. However, atoms of the same element can have different numbers of neutrons. These are called isotopes. The atomic mass of an element listed on the periodic table is the weighted average of the atomic masses of its naturally occurring isotopes.

In summary, the nuclear model explains atomic mass as the sum of the protons and neutrons in an atom's nucleus. The mass of the electrons is so small that it is generally ignored in this calculation.