Default: Be, Mg, Ca, Sr, Ba, Ra respectively.
Neutral atoms: 2 bonds are accepted.
Charged atoms: Each added charge decreases by one the number of possible bonds.
Radical: Each added radical decreases by one the number of possible bonds
Be2+, Mg2+, Ca2+, Sr2+, Ba2+ Ra2+ are accepted
Radicals: ·Me+ is accepted