7.5 Electron 7.5 Electron AffinitiesAffinities

By: Ginny Edwards, Rikin By: Ginny Edwards, Rikin Marfatia, Dave Rodak, & Marfatia, Dave Rodak, &

Dave NguyenDave Nguyen

What Are Electron What Are Electron Affinities?Affinities?

Opposite of ionization energyOpposite of ionization energy

Electron Affinity – The energy change resulting from Electron Affinity – The energy change resulting from an atom at neutral state gaining an electron and an atom at neutral state gaining an electron and becoming an anionbecoming an anion Ex: ClEx: Cl(g)(g) + 1 e + 1 e- - Cl Cl--

(g)(g) ΔE = -349 kJ/mol ΔE = -349 kJ/mol

Atom must be in a gaseous state Atom must be in a gaseous state

The greater the attraction between the atom and the The greater the attraction between the atom and the added electron, the more negative the electron added electron, the more negative the electron affinity will beaffinity will be A more negative electron affinity also shows a stable A more negative electron affinity also shows a stable

ion will be formedion will be formed

Periodic Trends Periodic Trends (Groups)(Groups)

There aren’t very There aren’t very clear patterns but clear patterns but nevertheless there nevertheless there are someare some

As you go down the As you go down the group (column) the group (column) the electron affinity electron affinity decreases decreases

Placed in larger Placed in larger orbitals further away orbitals further away from the nucleus from the nucleus (less nuclear charge)(less nuclear charge)

Decrease minimal Decrease minimal due to a due to a counterbalance counterbalance These values are all negative

ExceptionsExceptions

VIA and VIIAVIA and VIIA

Oxygen and Fluorine Oxygen and Fluorine have relatively lower have relatively lower electron affinitieselectron affinities These two atoms have a These two atoms have a

high force of repulsion, high force of repulsion, making an added making an added electron more electron more unfavorable than with unfavorable than with Sulfur and ChlorineSulfur and Chlorine Repulsion due to small Repulsion due to small

volumevolume

Nitrogen, Noble Gases, Nitrogen, Noble Gases, and all IIA elements and all IIA elements have a positive Electron have a positive Electron AffinityAffinity

Periodic Trends Periodic Trends (Period)(Period)

From IIIA through VIIA, as you go across a period, an From IIIA through VIIA, as you go across a period, an atom will release more energy to gain an electronatom will release more energy to gain an electron The closer you get to the noble gases, the more filled The closer you get to the noble gases, the more filled

your valence orbitals will becomeyour valence orbitals will become

IA elements release energy to gain an electron, but IA elements release energy to gain an electron, but not easilynot easily

Noble Gases are very stable atoms, therefore their Noble Gases are very stable atoms, therefore their electron affinity is close to zero; they do not accept electron affinity is close to zero; they do not accept electrons easilyelectrons easily

IIA atoms are similar to the Noble Gases in terms of IIA atoms are similar to the Noble Gases in terms of electron affinityelectron affinity Have a filled “s” subshell, so adding an electron would Have a filled “s” subshell, so adding an electron would

place it alone in the “p” subshell (highly unfavorable)place it alone in the “p” subshell (highly unfavorable)

Periodic Trends Periodic Trends (Period) Cont.(Period) Cont.

VA group is similar to IIA VA group is similar to IIA ““p” subshell has all p” subshell has all

occupied orbitals (1 occupied orbitals (1 electron each)electron each)

Being half-filled, these Being half-filled, these elements are more stable elements are more stable than IVA and VIA elementsthan IVA and VIA elements

Representative ElementsRepresentative Elements Generally, the electron Generally, the electron

affinity gets more affinity gets more negative as you go across negative as you go across the representative the representative elementselements