Flexi Says: The number and arrangement of electrons in an atom determine the chemical and physical properties of an element. For example, metals are generally shiny in appearance because they consist of loosely bound electrons that can reflect light. When sodium metal piece is put into the water, there will be an explosion. Sodium has one valence electron in its outmost shell and thus to complete its octet, sodium can easily give up its valence electron. Now, water consists of hydroxide ions and hydronium ions. When sodium comes in contact with water, it can immediately give its valence electron to the hydronium ion due to which high energy is released that we see in the form of explosion. Also, everything you see around is generally not pure elements. The electron configuration of the combination of atoms is very much different from the single element. For example, when sodium chemically combines with chlorine, it forms a sodium chloride compound that has a different structure than sodium and chlorine elements and it does not explode in water. On the contrary, sodium chloride is soluble in water.