A group of elements in the periodic table: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). They are known as the alkaline-earth metals. Formerly, they were classified in group II, which consisted of two subgroups: group IIA (the main group, see alkaline-earth metals) and group IIB. Group IIB consisted of the three metals zinc (Zn), cadmium (Cd), and mercury (Hg), which have two s-electrons outside filled d-subshells. Moreover, none of their compounds have unfilled d-levels, and the metals are regarded as nontransition elements. They now form group 12 and are sometimes called the zinc group. Zinc and cadmium are relatively electropositive metals, forming compounds containing divalent ions Zn²⁺ or Cd²⁺. Mercury is more unreactive and also unusual in forming mercury(I) compounds, which contain the ion Hg₂²⁺.