Hydrogen is the first element with one electron. It resembles alkali metals and halogens due to its ability to gain or lose electrons. Hydrogen forms covalent, ionic, and interstitial compounds called hydrides with other elements. Water has many unique properties due to hydrogen bonding between molecules. Hard water contains calcium and magnesium ions that can be removed by boiling, using limewater, or sodium carbonate to produce soft water suitable for household use.