Ionization Energy and Electron Affinity

Section 7.4-7.5

Objectives

• Examine periodic trends in ionization energy

• Examine periodic trends in electron affinity

Key Terms

• Ionization energy

• Electron Affinity

Ionization Energy

• Ease at which electrons can be removed from an atom or ion• First ionization energy, I1, is the energy

required to remove the first electron from neutral atom

• Second ionization energy, I2, is the energy required to remove the second electron from a +1 ion

• Greater the I, the more difficult to remove the electron

Trends of Ionization Energy

• I1< I2 < I3 < I4

• With each removal, the ion becomes more positive • Strengthens nucleus’s hold on its electrons

• Sharp increase in I when inner-shell electrons are removed• Table 7.2 on page 271

Periodic Trends in I1

• Within rows, I1 increases with atomic number• Slight irregularities

• Within columns, I1 decreases with increasing atomic number

• Representative elements show larger range of I1 values than transition metals• Transitions slowly increase across rows

Factors Affecting Trends

• More energy is needed to remove an electron as:• Effective Nuclear Charge increases• Atomic Radius decreases

Irregularities

• I does not always increases across rows• Decrease from Be to B

Be: [He] 2s2

B: [He]2s2 2p1

• The most easily removed electron in B is that in the 2p orbital, higher in energy than the 2s electrons in Be

• 2p electron of B needs less energy for total removal than does the 2s electron of Be

Irregularities

• O is lower than N• N: [He]2s2 2p3• O: [He]2s2 2p4

• Important difference in the way electrons are distributed

• Hund’s rule: 3 e- in the 2p orbitals of a nitrogen atom all have the same spin, but e- are paired in one of the 2p orbitals of oxygen• Electrons try to stay as far apart as possible to minimize

repulsion

• Force of repulsion between these electrons is minimized to some extent by pairing electrons• Slightly easier to remove an electron

Electron Affinities

• Energy change that occurs when electron are added to a gaseous atom

• Measure of the attraction of an atom to an electron

• Negative values (energy is released during process)

• kJ/mol

• More negative affinity = greater attraction

Trends in Affinities

• Not as evident as that of ionization energy

• Halogens have the greatest affinities (1 e- shy of a filled p subshell)

• Noble gases have affinities of 0

• Group 2A and 5A have very little affinity

Electron Affinities (kJ/mol)

Putting it all together

Homework

• 7.35, 7.42, 7.44, and 7.46 on page 294