The s-block of the periodic table of elements consists of the first two groups: the alkali metals and alkaline earth metals, plus hydrogen and helium. These elements are distinguished by the property that in the atomic ground state, the highest-energy electron is in an s-orbital. Except in hydrogen and helium, these electrons are very easily lost to form positive ions. The helium configuration is chemically exceedingly stable and thus helium has no known stable compounds; thus it is generally grouped with the noble gases.