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Why will $\ce{K}$ have a greater electron affinity than $\ce{Ca}$? I know that $\ce{Ca}$ has a greater first ionization energy, and I also know that $\ce{K}$ has a greater second ionization energy, but I am confused on how to explain this.

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Neither K nor Ca "wants" to ionize. Both ionization energies are endothermic. This question refers to eelctron affinity, which is gaining an electron.

K has a more exothermic electron affinity because the electron gained fills the 4s orbital. In Ca the electron goes to 3d. The energy gap between 4s and 3d is enough to make the process barely exothermic.

Note that the numbers given in tables are positive but that the process is exothermic.

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  • $\begingroup$ But don't K and Ca want to ionize (aka lose electrons, since it's in the context of ionization energy)? K loses 1 electron to form K+, and Ca loses 2 to form Ca2+. $\endgroup$ – user3932000 Oct 29 '16 at 17:23
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the octet rule. All elements want 8 electrons in their outer shell. K normally has only one electron in its outer shell and removing it exposes the full octet shell underneath which is favorable and spontaneously occurs, which is why K exists as K+ in solution. So K actually wants to ionize to +1 but no further. However Ca has two electrons in its outer electron shell. Therefore it wants to loose two and expose the next full shell of eight electrons, which is comparable to the electron configuration of Argon (Ar).

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