What is the origin of the energy measured by electron affinity? I\'m studying in

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What is the origin of the energy measured by electron affinity?

I'm studying introductory quantum and atom physics, and I'm having trouble picturing the origin of energy in electron affinity. I understand the energy involved in ionization: we have an electron in state (n,l, ml, s) in an atom. If we take 0 potential energy at infinity, then the work needed to take that electron out to infinity is the ionization energy. In a Bohr model of an atom, that'd be the energy to transition from state n=x to n=infinity (for example, if n=1, then the ionization energy for hydrogen is 13.6 eV).

So far so good (I hope). For electron affinity I picture the inverse process. We have an electron at infinity, with U=0, and we approach it to the atom. For example, in the case of H, I'd expect the new electron to be in 1s state (with different spin to the atom's first electron), so the work involved to bring the electron to the atom is -13.6 eV (probably you'll have to take the spin-orbit effect into account). But the electron affinity of hydrogen os not in the vicinity of -13.6 eV. It's, according to Wikipedia, 0.754195 eV. So my question is, what is that energy? What's the process involved?

Explanation / Answer

Electron affinity of an atom measures the tendency to take extra electron and becoming negative charge ion. When an electron is added to an atom, then amount of energy released is the electron affinity of an atom.

It is different from ionisation energy of an atom. Ionisation energy of an atom is the amount of energy required by an atom to remove an electron from the atom and making it positive charge ion. This can be deduced from Bohr Model of Hydrogen atom, because an electron is bounded in atom due to Nuclear force of attraction and it has negative potential energy. To make electron free from an atom we need to provide energy from outside. But when electron is added to the atom then it will come under nuclear force of attraction so energy will be released. Energy released is the difference in potential energy of ion and it's atom. Like for Hydrogen

Electron affinity = E(neutral) - E(anion)

Electron affinity decrease as the atomic size increases because added electron in outer most shell will be far away from nucleus. Also electron electron repulsion increases with increase in no of electrons.

Also when atomic size is very small, then electron electron repulsion will be more and electron affinity decreases

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