Sodium is a chemical element with atomic number 11 and the element symbol Na, which belongs to the group of alkali metals. Sodium is a monovalent cation , which is important in water treatment , especially in water softening and water de-iron removal through ion exchange . Due to its monovalent nature, the exchange resin in a softening system strives to release the sodium into the water and exchange it for divalent calcium and magnesium . This is how water softening works because the two substances calcium and magnesium are removed from the water. This removes the limescale from the water, so to speak.