The periodic table 2

​The modern periodic table provides a list of all the known elements categorized in increasing order of increasing atomic number. 

Each row across the periodic table is known as a period.

Each column is known as a group.

The period number tells us the number of occupied electron shells ion a given atom. The group number shows the number of electrons in the outermost shell of the atom.

The electrons in an atom arrange themselves in shells ( or levels) around the nucleus. The electrons fill the innermost shells first . Each shell can contain a maximum number of electrons. For GCSE and IGCSE we use a simplified model of electron shells. 

This model works well for explaining the properties of the first 20 elements in the periodic table  - a more complex model is needed to explain the structure of atoms above atomic number 20.