Electron Configuration of First 20 Elements

This lesson looks at the electronic configurations of elements in the periodic tables. Electrons are arranged in energy levels or shells around the nucleus of an atom. As we increase the atomic number and progress through the first twenty elements, the electronic structure builds up, one electron at a time. In the Periodic Table, when we get to Helium the outer shell of electrons is full. On the Periodic Table, we have reached the end of the first row or period. Potassium has an outer shell with only one electron. On the Periodic Table, Potassium is on the far left, at the start of a new row. Calcium is next on this row, followed by Scandium, Titanium and so forth. Calcium has 2 electrons in its outer shell. Scandium, it is in Group 3 on the Periodic Table and has 3 electrons in its outer shell. Iron in Group 8 and Krypton in Group 18 both have a full outer shell with 8 Electrons. For the groups’ whose number is composed by two digits, for instance group 18, it is the second number - in this case 8 - which indicates the number of electrons in the outer shells of elements located in this group. Each additional shell corresponds to a new row or period on the Periodic Table. For example, the most reactive metals have just one outer electron. They are the Group 1 Alkali Metals: Lithium, sodium, potassium, rubidium, caesium and francium. The most reactive non-metals have seven electrons in their outer shell. These are the halogens in Group 18: Fluorine, chlorine, bromine, iodine and astatine, fluorine and chlorine being in the first twenty elements. Full outer electron shells are particularly stable, which helps explain why Group eighteen, the noble gases, are unreactive. The electronic configuration helps explain trends amongst elements, such as their reactivity and also whether they are likely to form covalent or ionic bonds.