Sodium (Na) is a group 1 element. As such it is a metal but unlike other metals like iron or copper, elemental sodium is incredibly reactive, especially with water. The mixture of sodium and water produces sodium hydroxide, hydrogen gas, and an incredible amount of heat. The real danger here is that the heat will cause the hydrogen gas to explode. The reason this reaction occurs is because as a group 1 metal, sodium readily loses its outermost valence electron to produce the sodium cation (Na+). The loss of this single electron gives the sodium ion a complete octet of electrons in its valence shell, thus giving it the highly stable electronic configuration of the nearby noble gas neon. Unlike elemental sodium, the sodium ion is very stable and readily forms numerous ionic salts with numerous anions. In terms of mass, the sodium ion has almost the same mass as elemental sodium since they only differ by one electron.