Electron Affinity (IB Chem HL 1 Unit 6)

First electron Affinity (symbol ΔH°ea)

The energy change occurs when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous ions.

X (g) + e^- → X⁻ (g)

Since the added electron is attracted to the positively charged nucleus, the process is generally exothermic.

Trends across a period (Group 2)

• e^- configuration ends in ns² so next being np¹ is further

• reduced attractive force due to a larger radius

Trends across a period (Group 5)

Going from np³ to np⁴ will be the first to experience repulsion from the added electron trying to occupy and already occupied orbital

Trends across a period (Group 17)

• high effective charge (+7) with a barely incomplete outer energy level

• most likely to attract electrons to themselves

Second Electron Affinity

The energy change that occurs when one mole of electrons is added to one mole of gaseous ions to form one mole of gaseous doubly charged ions

X^- (g) + e^- → X^2- (g)

Since the added electron is repelled by the negative ion, the process is generally endothermic

Subsequent electron affinities

As the charge of the ion increases, so does the repulsion-so the reaction becomes more and more endothermic