# Ca energy release when gaining an electron

Why does $\ce{Ca}$ have a negative electron affinity, i.e. energy is released when it gets an electron (ca. $\pu{-2 kJ/mol}$), when $\ce{Be}$ and $\ce{Mg}$ have positive values?

I know that the electron will be settled in the $\ce{3d}$ orbital which has a lower principal quantum number than $\ce{4s}$, but still, why would energy be released?

Two sign conventions: (1) the more common one states that a positive electron affinity value represents energy release when an electron is added to an atom; (2) the other states that a negative electron affinity represents a release of energy.

http://www.rose-hulman.edu/~brandt/Chem251/GenChem_Review_v3.pdf

Recommended values for these electron affinities, in the units commonly employed in introductory texts and with the sign convention used here, are 2.37, 5.03, and 13.95 kJ/mol for Ca, Sr, and Ba, respectively. The endothermic electron affinities often quoted for Be and Mg are also too large and should be reported simply as ">0". An argument for a return to the original sign convention for the electron affinity is presented in this paper.

http://pubs.acs.org/doi/abs/10.1021/ed074p123

OP: I know that the electron will be settled in the 3d orbital

Actually, as explained in the above article, the electron occupies 4p.

OP: but still, why would energy be released?

Very little energy is released and the lifetime of Ca- is on the microsecond scale in practical experiments. The reference Contributions to the electron affinity of calcium and scandium may be the best source for theoretical models that do and don't predict a slight energy release. According to Atomic negative ions: structure, dynamics and collisions, core electrons need to be considered in addition to the valence electrons in calculating the stability of Ca-. Early calculations that considered only valence electrons predicted that Ca- was unstable. Configuration-interaction study of differential correlation energies in Ca-, Ca, and Ca+ presents detailed calculations of energy levels.

• Can't access it. – RBW Nov 10 '14 at 18:57
• can you access sciencedirect.com/science/article/pii/S0370157304000316 ? – DavePhD Nov 10 '14 at 19:07
• I can, but I couldn't find a satisfying explanation (probably there isn't one). – RBW Nov 10 '14 at 19:27
• – DavePhD Nov 10 '14 at 19:32
• I read that they relativistic effects i.e. electrons have bigger speed, lower mass, the orbital contracts, the electrons spend more time near the nucleus and have lower energies. Did I figure it out correctly, or is the explanation different? – RBW Nov 10 '14 at 19:44

$\ce{Be}$ has negative electron affinity (precisely -2.4 eV).