Electron Affinity: Adding Electrons to Valance Shells

[SSG-E]

Homework Statement

How does an electron add up(enters) in the valance shell of an atom? Why is energy released when an electron adds up in the valance shell of an isolated atom.

Relevant Equations

X(g) + e = X(g) = electron affinity

I think that an electron adds up in the valance shell of an atom because an atom tends to achieve the nearest noble gas configuration. But I don't understand how and why electron enters the valance shell. Energy is released when an electron adds up in the valance shell. But why is energy released. I think because photon are emitted. Is it correct?

 

[trurle]

Homework Statement:: How does an electron add up(enters) in the valance shell of an atom? Why is energy released when an electron adds up in the valance shell of an isolated atom.
Relevant Equations:: X(g) + e = X(g) = electron affinity

I think that an electron adds up in the valance shell of an atom because an atom tends to achieve the nearest noble gas configuration. But I don't understand how and why electron enters the valance shell. Energy is released when an electron adds up in the valance shell. But why is energy released. I think because photon are emitted. Is it correct?

During electron capture in gas, photon emission (or to be exact, a series of photons) is typical. Initially electron enters highly excited orbit, which decay nearly immediately. For liquids and solids, phonons (lattice vibrations, or simply heat) are likely to be produced instead of photons.

The energy during formation of negative ion is released (in case of chlorine and lithium, but not for berillium or nitrogen, for example) because the atom electric field is not a point-source. In some electron shell configurations, repulsion from already orbiting electrons may be weaker than attraction from the positively charged nucleus. Because electrons in average (again, not for all atoms) will be farther away from added electron than nucleus. Another factor is polarizability of electron shell. Easily polarizable atoms tends to have higher electron affinity (release more energy during formation of negative ion) - because added electron "push away" existing electrons, reducing the mutual repulsion. This trend is complicated by the effect of atomic radius though.

 

[SSG-E]

During electron capture in gas, photon emission (or to be exact, a series of photons) is typical. Initially electron enters highly excited orbit, which decay nearly immediately. For liquids and solids, phonons (lattice vibrations, or simply heat) are likely to be produced instead of photons.

The energy during formation of negative ion is released (in case of chlorine and lithium, but not for berillium or nitrogen, for example) because the atom electric field is not a point-source. In some electron shell configurations, repulsion from already orbiting electrons may be weaker than attraction from the positively charged nucleus. Because electrons in average (again, not for all atoms) will be farther away from added electron than nucleus. Another factor is polarizability of electron shell. Easily polarizable atoms tends to have higher electron affinity (release more energy during formation of negative ion) - because added electron "push away" existing electrons, reducing the mutual repulsion. This trend is complicated by the effect of atomic radius though.

If added electron "push away" existing electrons then doesn't it mean that mutual repulsion is increased?

 

[trurle]

If added electron "push away" existing electrons then doesn't it mean that mutual repulsion is increased?

If you mean repulsion between existing electrons, then yes. Just this increase of repulsion must be smaller than attraction to the additional electron in negative ion, for negative ion to form.

 

[SSG-E]

If you mean repulsion between existing electrons, then yes. Just this increase of repulsion must be smaller than attraction to the additional electron in negative ion, for negative ion to form.

you mean attraction of nucleus to additional electron?

 

[trurle]

you mean attraction of nucleus to additional electron?

Yes.

 

[DrDu]

There are only few atoms which will bind an additional electron in the gas phase, e.g. O + e -> O-. In these cases, the extra electron is bound because the other electrons in the valence (not valance) shell are not efficiently shielding the positive nuclear charge. But most ions relevant to chemistry are stabilized by either the crystal lattice or by hydration (in solution).