Although the value of second ionisation potential of alkaline earth metals is higher than that of 1st ionisation potential yet these metals form M^"+2" ions but no M^+ ion?

1 Answer
Sep 30, 2017

All alkaline earth metals can form +1 cations. They don't have to, they usually don't, and they might not, but they can...

"M"(g) + DeltaH_(IE1) -> "M"^(+)(g) + e^(-)

"M"^(+)(g) + DeltaH_(IE2) -> "M"^(2+)(g) + e^(-)


Here's a list of first and second ionization energies from NIST (simply search the chemical symbol of the atom):

"Mg": likes to form "Mg"^(2+)

DeltaH_(IE1) = "7.646 eV"
DeltaH_(IE2) = "15.035 eV"

"Ca": likes to form "Ca"^(2+)

DeltaH_(IE1) = "6.113 eV"
DeltaH_(IE2) = "11.872 eV"

"Sr": likes to form "Sr"^(2+)

DeltaH_(IE1) = "5.695 eV"
DeltaH_(IE2) = "11.030 eV"

"Ba": likes to form "Ba"^(2+)

DeltaH_(IE1) = "5.212 eV"
DeltaH_(IE2) = "10.004 eV"

If you supply the exact energy, I see no reason why one cannot form the +1 cation, as long the positive charge is sufficiently stabilized by negative charge. A lot of odd molecules and compounds can be formed in the gas phase.

For instance, there is such a thing as lead monofluoride, "PbF" (probably in the gas phase), with "Pb"^(+) cation, although lead is typically known to form a +2 or +4 cation.