Each element has its own atomic number.
Elements are arranged in the periodic table in order of increasing atomic number. For example hydrogen has the atomic number of one, helium two, lithium three etc.
The atomic number of an element tells you how many protons that the element has. This is written at the bottom left hand side of the symbol. Since atoms are neutral, we know then that sodium atoms must also have 11 electrons to cancel the charge from 11 protons.
The electron arrangement of all atoms can be found in the data booklet. All the electrons are arranged into energy levels. These energy levels can only hold a certain number of electrons.
The first energy level (the one nearest the nucleus) can hold a maximum of 2 electrons with the others being able to hold up to a maximum of 8 electrons (only true for the first 20 elements).
Example: sodium has the electron arrangement 2,8,1.
An atom of sodium has 11 electrons. The first two fill the innermost energy level. The second energy level is also full, holding eight electrons and one electron remains in the outer energy level.
Elements in the same group of the periodic table have the same number of outer electrons.
It is the number of outer electrons that give an element its chemical properties. This is why elements in the same group of the periodic table have similar properties.