CHEM 201 Lecture Notes - Lecture 4: Electron Affinity, Electron Configuration

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Published on 2 Nov 2019
Electron Configuration Summary of Activity 3 PART II
CHEM 211 - F2019
Learning Objectives:
Determine the electron configurations for stable
Explain differences in size for different atoms and
Order a given series of elements or ions by size and
justify their ranking.
Use the distance the valence electrons are from the
nucleus (n) and the pull of the nucleus on these
electrons (Zeff) to rationalize the chemical properties
of atoms.
Explain differences in ionization energy, and electron
affinity for different atoms and ions.
Explain differences in ionization energy for an atom
compared to its ions.
Order a given series of elements or ions by ionization
energy and/or electron affinity and justify their
Stable ions Shorthand, ground state
spdf e- configuration Relative size
Mg+2 [Ne] Mg+2 < Mg
Al+[Ne] 3s2Al+3 < Al+< Al
Al+3 [Ne]
O+1 (½ filled subshell)
O+4 (filled subshell)
O+6 (filled shell)
O-2 (filled shell, most stable ion)
[He] 2s2 2p3
[He] 2s2
[He] 2s2 2p6= [Ne]
O < O-2
Part II Key points:
Stability look for: 1) full shell; 2) full subshell; 3) half-filled subshell.
Successive ionization energies increase as you remove more and more electrons; there is a large
increase in ionization energy when you start removing core electrons.
Mg has a low electron affinity (doesn’t want to attract extra electrons, small pull from the nucleus
Z* = 2); O has a high electron affinity (is desperate to attract extra electrons, big pull from the
nucleus Z* = 6).
C has a lower electron affinity than O (since smaller Z*eff; so given a choice, O accepts e- rather
than C); C has a larger ionization energy than Mg (doesn’t want to lose its e-). Hence, C will not be
involved in electron transfer in the “Mg+CO2reaction.
Challenge question: Rank these ions in order of increasing size: Mg+2,Al+3, O-2.
Key points:
The three species have 10 e-, same electron configuration = [Ne]… so we cannot justify
size based on n values and orbitals.
The nuclear pull is not the same!
O has 8 protons; Mg has 12 protons and Al has 13 protons. (we can’t really use Z* as this
applies to valence electrons and Mg+2, Al+3 have none left)
Correct order: Al+3 (smallest, nucleus has 13 protons to attract 10 e-, huge nuclear pull); Mg+2; O-2
(biggest, nucleus has 8 protons to attract 10e-, outnumbered, weak nuclear pull)
Building Blocks - Part 3
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