An element's electronic configuration tells you where it sits in the periodic table. The number of shells that contain electrons is the period number. Sodium, 2.8.1, has three shells, so it is in Period 3. The number of electrons in the outer shell is the group number for Groups 1 to 7. Sodium has one outer electron, so it is in Group 1. Chlorine, 2.8.7, has seven outer electrons, so it is in Group 7 and Period 3. Oxygen, 2.6, has six outer electrons, so it is in Group 6 and Period 2.
Elements in the same group have the same number of outer electrons, which is why they have similar chemical properties. Lithium (2.1), sodium (2.8.1) and potassium (2.8.8.1) each have one outer electron and are all Group 1 metals.
The noble gases are in Group 0. Their atoms have a full outer shell: 2 electrons for helium and 8 for the others. A full outer shell is a very stable arrangement, so these elements are unreactive.