What is an Atomic Number

Also known as the proton number, chemists and physicists describe it as the number of protons in the nucleus of any atom. This gives the nucleus its charge since protons are positively charged.

The atomic number gives each atom its uniqueness since the number acts as an identifier. For neutrally charged elements, the atomic number will be equal to the number of electrons in the atom. The protons in the nucleus are the force that keeps the electrons held close in the orbiting mode around the nucleus.

Atomic mass number

  • This should not be confused with atomic number; this defines the total number of neutrons and protons in the nucleus of an atom. The neutron number plus the atomic number makes the atomic mass.
  • Given the fact that protons and neutrons have approximately equal mass and the electron mass being regarded as negligible, atomic mass will be roughly equal to the mass of neutrons and protons.
  • There are instances where elements have the same atomic number but carries different neutron number. They are usually referred to as isotopes of the element. Almost all naturally occurring elements exists in isotopes for example carbon 13 carbon 14 among other isotopes.
  • For isotopes, the average atomic mass will be used as the element’s atomic weight.

Standard atomic weight

  • It’s basically the mean relative atomic mass of a given element in its naturally occurring environment. This environment covers the earth’s crust and atmosphere.
  • These are the standards used in periodic tables and everyday calculations.
  • For synthetic elements, the atomic weight of the mot stable isotope is placed in brackets

Relative atomic mass

  • Can refer to atomic weight or average atomic mass but not atomic mass.
  • It means the weighted mean of atomic masses of all the atoms constituting a chemical element in any given sample measured by the abundance of the isotopes.
  • It can also refer to standard atomic weights even though specificity will be lost.

Relative isotopic mass

  • Simply means the relative mass of a specific isotope. It involves rounding off the atomic mass to the nearest whole number; this gives a simple number of nucleons. From here, the neutron number is then arrived at by subtracting the atomic number.
  • The total number of nucleons in the nucleus gives the nuclide mass number. The numbers are usually in whole numbers due to the rounding off. A good example is the isotope Oxygen 16 which contains 16 nucleons meaning 8 neutrons and 8 protons.


In most cases, an atomic number of an element will equal the electron number hence the neutral charge. A charged atom indicates an imbalance in the number of protons and electrons like in radical elements. One element can exist in nature in different atomic mass hence isotopes.