The relative atomic mass of an element is the average atomic mass of atoms of the element. The units of atomic mass are 1/12th of the mass of the Carbon – 14 atom. A single unit of atomic mass is given the label
and has the value
A comprehensive periodic table will contain all the elements, their atomic numbers and relative atomic masses. The atomic number is the number of protons in an atom – the number of protons definesthe element, with different elements having different numbers of protons and different chemical properties. The relative atomic mass is the average mass of an atom of that element. It is often quite close to a whole number, and often taken to be equal to a whole number. For accurate work, the most accurate value of the relative atomic mass must be used.
There are three man reasons the relative atomic mass is not a whole number.
Often the mass of a proton and neutron are both taken to be equal to
but in fact they are both very slightly different from
A proton has a mass of
equivalent to
A neutron has a mass of
equivalent to
Every atom of an element is not identical. Atoms of a single atom are divided into isotopes, with different isotopes of an element having the same number of protons but a diffent number of neutrons. The relative atomic mass is a average of all the atoms of an element.
When protons and neutrons join to make an atomic nucleus, the mass of the atom is less than the mass of the sum of the protons and neutrons. The difference is called the 'mass defect' and is converted into energy via the equation
to hold the nucleus together.