Periodic Trends: Ionization Energy, Electron Affinity, and Radii, Study notes of Chemistry

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Lecture 23: Periodic Trends• Reading: Zumdahl 12.15-12.16• Outline– Periodic Trends• Ionization Energy, Electron Affinity, and Radii– Information in a periodic table

Periodic Trends

-^ The valence electron structure of atoms can beused to explain various properties of atoms.•^ In general, properties correlate down a group ofelements.•^ A warning: such discussions are by nature verygeneralized…exceptions do occur.

Periodic Trends: Ionization• Generally done using photons, with energymeasured in eV (1 eV = 1.6 x 10

-19^ J).

-^ The greater the propensity for an atom to “holdon” to its electrons, the higher the ionizationpotential will be.•^ Koopmans’ Theorem: The ionization energy of anelectron is equal to the energy of the orbital fromwhere the electron came.

Periodic Trends: Ionization• One can perform multiple ionizations:+^ Al(g)^ Al(g) + e

-^ I= 580 kJ/mol^1

first

+^ Al(g)^ Al 2+^ -^ (g) + e^ I= 1815 kJ/mol^2

second

2+^ Al(g)^ Al 3+^ -^ (g) + e^ I= 2740 kJ/mol^3

third

3+^ Al(g)^ Al 4+^ -^ (g) + e^ I= 11,600 kJ/mol^4

fourth

• Periodic Trends: Ionization• First Ionization Potentials:Column 8 Column

Periodic Trends: Ionization• First Ionization Potentials:

• Increases as one goes fromleft to right.• Reason: increased Z• Decrease as one goes downa group.

• Reason: increased distancefrom nucleus

226 D. 1s 2s^ 2p3s

26210 3p^ 4s3d^ 4p

6 1 5s

Which atom would you expect to have the lowestionization energy?

22 3 A. 1s2s2p^226 B. 1s^ 2s^ 2p3s

25 3p^

226 C. 1s 2s^ 2p3s

262 3p^ 4s

Periodic Trends: Electron Affinity•^ Electron Affinity: the energy change associatedwith the addition of an electron to a gaseous atom.Z- +Z

(Z+1)-+Z e-

Energy

Periodic Trends: Electron Affinity•^ Elements that have high electron affinity:^ • Group 7 (the halogens) and Group 6 (O and S specifically).

Periodic Trends: Electron Affinity•^ Some elements will not form ions:^ • Orbital configurations can explain both observations.

N?

Periodic Trends: Electron Affinity•^ How do these arguments do for O?O(g) + e

-^ -^ O^ (g)^

EA = -140 kJ/mol (^2 24) 1s 2s^ 2p

(^2 25) 1s 2s^ 2p

-^ What about the second EA for O?-^ O^ (g) + e -^ 2-^ O^ (g)^

EA > 0 (unstable) (^2 25) 1s 2s^ 2p

Bigger Z (^2 26) 1s 2s 2p [Ne] configuration, but electronrepulsion is just too great. +^ overcomes- (^) e repulsion.

Which diagram indicates the evolution in electronaffinity from high affinity to low affinity?

Atomic Radii

• Decrease to right duedue increase in Z

• Increase down columndue to population oforbitals of greater n.

Looking Ahead

• We can partition the periodic table into generaltypes of elements.

Metals: tend to give up e

• non-Metals: tend to gain e

Metalloids: can do either