Attraction between atoms or ions leads to a chemical bond. According to the types of bonds contained in a molecule, the physical properties including melting point, hardness, electrical and thermal conductivity and solubility are determined. Chemical bonds involve only the outermost or valence electrons of atoms. Using the example of the simplest element, hydrogen, its two atoms on approaching each other, cause electon-electron and proton-proton repulsions to attempt separation of the atoms. But counterbalance by proton-electron attraction fuses the two hydrogen atoms forming a bond.

This example demonstrates the gain, loss and sharing of electrons by atoms for possession of the same number of electroons as the noble gas in closest proximity on the periodic table. With eight valence electrons (s2p6), all noble gases are chemically stable in a phenomenon called the octet rule. But certain exceptions are possible. Among them, one group of atoms has less than eight electrons like hydrogen with only one. BeH2 has just four valence electrons around Be, with Beryllium contributing two electrons and each hydrogen with one. The second exception applies in elements in periods 4 and above. Their atoms can have over four surrounding valence pairs in certain compounds.