are discussed elsewhere in Group Number of valence electron alence , , , , this book. There are many elements which exhibit variable valence. This is particularly characteristic of transition elements and actinoids, which we shall study later. (b) Anomalous Properties of Second Period Elements The first element of each of the groups (lithium) and (beryllium) and groups - (boron to fluorine) differs in many respects from the other members of their respective group.
For example, lithium unlike other alkali metals, and beryllium unlike other alkaline earth metals, form compounds with pronounced covalent character; the other members of these groups predominantly form ionic compounds. In fact the behaviour of lithium and beryllium is more similar with Table . Periodic Trends in Valence of Elements as shown by the Formulas of Their Compounds Group Formula of hydride LiH NaH KH CaH B H AlH CH SiH GeH SnH NH PH AsH H O H S H Se H Te HF HCl HBr HI Formula of oxide Li O Na O K O MgO CaO SrO BaO B O Al O Ga O In O CO SiO GeO SnO PbO N O , N O P O , P O As O , As O Sb O , Sb O Bi O – SO SeO TeO – – Cl O – – Property Element Metallic radius M/pm Be Mg B Al Ionic radius M + /pm Be Mg the second element of the following group i.e., magnesium and aluminium, respectively. This sort of similarity is commonly referred to