Electron Counting and the Eighteen-Electron Rule · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Counting to eighteen at a metal centre

Science · Chemistry · ages 20-21
Name ______________________   Date ____________

  1. Chromium is in group 6 and each carbonyl donates 2 electrons. How many electrons do six carbonyls donate in total?

    Answer: ______________

  2. Why do chemists expect eighteen electrons around a stable metal complex?

    • Nine outer orbitals fit eighteen electrons
    • Every metal atom always carries eighteen protons
    • Eighteen is the largest number any molecule can hold

  3. Counting a complex by the neutral method and by the ionic method must give the same total.

    Circle one:   True   False

  4. Iron sits in group 8. How many carbonyl ligands bring it to eighteen?

    • Four carbonyls, giving 8 donated electrons
    • Five carbonyls, giving 10 electrons
    • Six carbonyls, giving 12 donated electrons

  5. Chromium (group 6) binds six carbonyl ligands. What is the total electron count at the metal?

    Answer: ______________

  6. A student counts a complex as 16 by the neutral route and 18 by the ionic route. What should they do?

    • Accept 17 as a compromise between the two totals
    • Keep the ionic total, since that route is always right
    • Recheck each pile, since both routes must agree

  7. A new complex counts to seventeen. What is the sound proposal?

    • Declare seventeen a new stable target for all complexes
    • Ignore the count, since counting never predicts reactions
    • Gain one electron

  8. A square planar complex counts to sixteen and refuses an extra ligand. How do you judge its stability?

    • Stable; square planar stops at sixteen
    • Unstable, since every complex must reach eighteen
    • Unstable, since sixteen is never seen in any complex
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Counting to eighteen at a metal centre W1-mt_uab5c5pODY-s1

  1. 12 · Six ligands times 2 electrons each gives 12 donated electrons.
  2. Nine outer orbitals fit eighteen electrons · One s plus five d plus three p orbitals each hold a pair, giving eighteen.
  3. True · Both routes describe the same electrons, so agreement is the check that the count is right.
  4. Five carbonyls, giving 10 electrons · Eight from the metal plus ten from five carbonyls totals eighteen.
  5. 18 · The metal gives 6 and the ligands give 12, which sums to the stable eighteen.
  6. Recheck each pile, since both routes must agree · Disagreement always flags a miscount in one of the piles.
  7. Gain one electron · Seventeen is one short of eighteen, so gaining one electron reaches the full shell.
  8. Stable; square planar stops at sixteen · The rule has recognised exceptions, and square planar sixteen electron complexes are one of them.
Worksheet · LightMySky