Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: helpplsbro on October 16, 2019, 08:27:22 PM
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Hello. My question is essentially the title.
My reasoning as to why electron gain enthalpy should be positive is that upon losing an electron, a group 1 atom will reach a noble gas configuration, as opposed to having 2 electrons in its outer shell if it gains an electron. Of course, gaining an electron will fill the corresponding s orbital fully, but is that the only reason?
Help will be appreciated, thanks.
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No atom loses an electron spontaneously. It costs energy, always.
An alkaline atom may lose an electron (where "lose" is already a simplification) if some other atom(s) gains this electron, and the net change is favourable.
The energy for a neutral atom to lose an electron and to gain one aren't related simply neither.
A table of the electron affinity is there:
https://en.wikipedia.org/wiki/Electron_affinity_(data_page)
and of ionization energy:
https://en.wikipedia.org/wiki/Ionization_energies_of_the_elements_(data_page)
after a brief look, alkaline elements can all attract one more electron. It's more the noble gases and the group II elements that don't.
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The energy for a neutral atom to lose an electron and to gain one aren't related simply neither.
Thanks for your reply. From your reply, what I gather is that the alkali metals would be able to "give" an electron to another element if and only if it was energetically favorable (overall process was exothermic, causing an increase in stability of the system).
However, experimentally (?) it was found that it isn't the case, and it is more energetically favorable (higher stability in the system) for an alkali metal to gain an electron (from another atom), hence it's electron affinity is positive.
Is this correct?
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The ease to give an electron and to gain one should not be compared.
In a chemical reaction, a "given" electron doesn't go far, as opposed to a physical ionization. It happens in solids and solvents, where negative charges stay in the vicinity. And this happens only because an other atom grasped this electron.
Most atoms can bind one more electron. Being initially neutral, they don't repel it, and all electrons can rearrange a bit to minimize the energy. I've seen a dozen exception in the table and no alkaline metals among them.
As an attempted rule, I see that atoms with a closed subshell like p6 (noble gases) or s2 (He, Be, Mg, Zn, Cd, Hg), plus some transition elements but not all, don't bind one more electron. Ca, Sr, Ba, Ra fail narrowly: the ones that precede transition elements, because the energy of the next subshell that can accommodate one more electron is close to s.
Corrected address with parenthesis:
https://en.wikipedia.org/wiki/Electron_affinity_(data_page) (https://en.wikipedia.org/wiki/Electron_affinity_(data_page))
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Most atoms can bind one more electron. Being initially neutral, they don't repel it, and all electrons can rearrange a bit to minimize the energy.
This line answered my question fully. Thanks a lot for your help :)