2/21/2013
Electron Affinities and
Electronegativity
by Sam Harrison, Maddie Verniero,
Abby Grunden, Hannah Kose and
Jimmy Macedo
Electron Affinity
The energy change that occurs when an electron is
added to a gaseous atom or ion
• example:
• Cl(g) + e- ------> Cl-(g) ∆E= -349 kJ/mol
Ionization Energy
The energy required to remove an electron from a
gaseous atom when the atom is in its ground state
• example:
• Cl(g)------> Cl+(g) + e- ∆E= 1251 kJ/mol
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Differences
★Electron affinity measures the ease
an atom GAINS an electron
★Ionization energy measures the ease
an atom LOSES an electron
Trends for Electron Affinity
★The greater the attraction between a given
atom and an added electron, the more
negative the atom’s electron affinity
★An electron affinity > 0 indicates the
negative ion is higher in energy than the
separated atom and electron
• example:
• Ar(g) + e- ------> Ar-(g)
∆E > 0
• For some elements, such as the noble
gases, the electron affinity has a positive
value, meaning the anion is higher in energy
than are separated atom and electron
• Because the electron affinity is a positive
number an electron will not attach itself to
an Ar atom; the Ar- ion is unstable and does
not form
Trends for Electron Affinity
★The halogens (one electron shy of a filled p-shell) have the most negative
electron affinities
• By gaining one electron a halogen forms a stable negative ion that has a
noble-gas configuration
• Because occupying a higher-energy subshell is energetically very unfavorable,
the electron affinity is highly positive
★The electron affinities of the group 5A (N, P, As, Sb) are interesting
• They have half filled p subshells and the added electron must be put in an
already occupied orbital resulting in larger electron-electron repulsions
• Their affinities are either positive (N) or less negative to their neighbors to the
left (P, As, Sb)
★Electron affinities do not change greatly as we move down a group
• A lower electron nucleus attraction is counterbalanced by lower electronelectron repulsions
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Electronegativity
Electronegativity- A measure of the ability of an atom that is bonded to
another atom to attract electrons to itself
•
Greater Electronegativity, greater ability to attract electrons to itself
•
Ionization energy measures how strongly an atom holds its electrons
★ Electron affinity is similar to electronegativity because it also measures how
strongly an atom attracts additional electrons
•
An atom with a very negative electron affinity and high ionization energy will
both attract electrons from other atoms and resist having its electrons taken
away; it will be highly electronegative
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