Electron Affinity: Adding Electrons to Valance Shells

In summary: The attractive force of the nucleus towards the added electron is what leads to the formation of negative ion.
  • #1
SSG-E
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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?
 
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  • #2
SSG-E said:
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.
 
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  • #3
trurle said:
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?
 
  • #4
SSG-E said:
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.
 
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  • #5
trurle said:
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?
 
  • #6
SSG-E said:
you mean attraction of nucleus to additional electron?
Yes.
 
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  • #7
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).
 

FAQ: Electron Affinity: Adding Electrons to Valance Shells

What is electron affinity?

Electron affinity is the amount of energy released or absorbed when an electron is added to an atom's valence shell. It is a measure of an atom's tendency to gain electrons and form negative ions.

How is electron affinity measured?

Electron affinity is measured in units of energy, typically kilojoules per mole (kJ/mol). It is measured experimentally by adding an electron to a gaseous atom and measuring the energy change.

What factors affect electron affinity?

The main factors that affect electron affinity are the atomic radius and the nuclear charge of an atom. Smaller atoms with a larger nuclear charge tend to have a higher electron affinity because the added electron experiences a stronger attraction to the nucleus.

What is the trend for electron affinity on the periodic table?

Generally, electron affinity increases from left to right across a period and decreases from top to bottom down a group on the periodic table. This is due to the changing atomic radius and nuclear charge of the elements.

Why do noble gases have a low electron affinity?

Noble gases have a full valence shell and are therefore stable and unreactive. Adding an electron to their already full shell would require a significant amount of energy, making their electron affinity low.

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