The periodic table is the arrangement of elements in a pattern that shows that elements with similar chemical properties appear at regular intervals or periods. As a result, elements in each vertical column have similar chemical properties and similar electronic configurations. Their atoms have the same number of electrons in their outermost shells.
Elements arranged horizontally have chemical properties which change from metallic on the left to non-metallic on the right. Moreover, the number of electrons in the outermost shells of their atoms increase progressively from left to right. The horizontal rows are known as periods.
Elements in the same vertical column, called groups, have the same number of valence electrons in the outermost shells of their atoms. Elements in group I have one valence electron, those in group Il have two valence electrons and those in group III have three electrons in their outermost shells, etc. An example is the case of lithium, sodium and potassium that are similar and highly reactive metals; each has one electron in its outermost shell. They belong to group I. Fluorine and chlorine are similar and reactive non-metals with each of their atoms having seven electrons in their outermost shells. They belong to group VII. Helium, neon and argon are rare (or inert) gases. They have the same number of electrons in their outermost shells. They belong to group VIII.
These trends indicate that electrons in the outermost shells of atoms are probably the ones responsible for the chemical properties of elements.