A complete description of an atom can be simplified by means of using the following notation:
where E is the atom symbol shown in the periodic table, Z is the number of protons and A is the atomic mass number.
The mass number is the sum of protons and neutrons. The number of neutron, N, can be worked out as:
Number of neutron = Atomic mass number - number of protons
N= A - Z
The example given above is the atomic symbol for iron. The iron atom has 26 protons and with a mass number of 56. This means there are 56 - 26 = 30 neutrons in the atomic nucleus.
Since Z is unique for each atom, it is common to write as 56Fe or 'iron-56' in the text. In other words, the atomic number is left out since the symbol Fe is sufficient to identify the element as iron.
Usually, the atom symbols are closely related to the elements in English. For example, the symbol Li refers to lithium; the symbol Ti refers to titanium.
However, this is not always the case.
Certain elements, especially those which had discovered since ancients, bear the atom symbols that refer to the elements in Latin. One good example is iron, which has the symbol of Fe (as shown above),
which refers to ferrum in Latin. Other examples are potassium, symbol K (kalium in Latin); gold with the symbol Au (aurum in Latin).