How is the atomic number different from the mass number?

The atomic number represents the number of protons in an atom's nucleus, while the mass number is the total number of protons and neutrons.

The atomic number, also known as the proton number, is a fundamental property of an atom that defines its chemical identity. It is denoted by the symbol 'Z' and represents the number of protons in the nucleus of an atom. This number is unique for each element and is used to arrange elements in the periodic table. For example, hydrogen has an atomic number of 1, helium has an atomic number of 2, and so on. The atomic number also determines the number of electrons in a neutral atom, which in turn influences the atom's chemical behaviour.

On the other hand, the mass number, denoted by 'A', is the total number of protons and neutrons in an atom's nucleus. Neutrons are particles in the nucleus that have roughly the same mass as protons but carry no electric charge. The mass number gives an estimate of the atomic mass, which is measured in atomic mass units (amu). For instance, carbon has six protons and six neutrons, so its mass number is 12.

It's important to note that while the atomic number is always the same for a particular element, the mass number can vary. This is because atoms of the same element can have different numbers of neutrons. These variations are known as isotopes. For example, carbon-12 and carbon-14 are isotopes of carbon. They both have an atomic number of 6 (meaning they both have 6 protons), but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons. Therefore, their mass numbers are 12 and 14, respectively.

In summary, the atomic number and mass number are fundamental properties of atoms that describe their structure and identity. The atomic number indicates the number of protons and defines the element, while the mass number represents the total number of protons and neutrons, providing an estimate of the atom's mass.