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
Truong-Son N.
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.

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