How can the atomic mass of an individual element be calculated given only the atomic number?
The atomic mass number `m_a `, atomic number `Z ` (number of protons in the atom, by which the periodic table is ordered - also equal to the number of electrons in the atom) and neutron number `N ` (number of neutrons in the atom) are related by the equation
`m_a = Z + N `
so that the atomic mass number `m_a ` is equal to the number of nucleons in the atom, that is the number of protons plus neutrons in the nucleus.
If we are only given/only know the atomic number `Z ` and not the neutron number `N ` we can approximately calculate the atomic mass number `m_a ` from `Z ` . This can be modelled as a piecewise linear relationship,
`m_a = 2Z ` for smaller `Z` (Helium, proton number = 2. NB Hydrogen has no neutrons, so has atomic mass = 1)
` = ((2 +2.5)/2) Z ` for medium `Z ` (Lithium (3) to Tin (50))
` = 2.5Z ` for larger `Z ` (Tin (50) to Ununoctium (118))
but a better fit (see link attached below) may be a power-law function given by
`m_a = 1.61Z^1.1 `
See periodic table for atomic number and atomic mass number