Configurations, oxidation states and who counts as a transition element
Week 2 Ā· Chapter 1 (B): electron configurations of the first series with the chromium and copper exceptions, oxidation states (why iron(II) oxidises easily and manganese(II) does not), the definition of a transition element, coinage metals in and zinc, cadmium and mercury out Ā· 3 lessons
1
Configurations of the first series and the exceptions
Chemistry Sec 3 Term 1, Chapter 1: pp. 8ā11
GoalWrite the electron configuration of each first-series atom, including the chromium and copper exceptions, draw its 3d boxes and count the unpaired d electrons.
New words
Hundās rule Ā· electrons spread over the empty orbitals of a sublevel, one in each, before any orbital gets a second electronunpaired electron Ā· an electron alone in its orbital (box)half-filled sublevel Ā· a sublevel with one electron in each orbital, such as 3dāµ; completely filled is 3dĀ¹ā°
Explanation
After argon, [Ar], the next electrons go into 4s, so calcium is [Ar] 4sĀ². From scandium onward the five 3d boxes take electrons: one in each box up to manganese (3dāµ, Hundās rule), then pairs form from iron on, Fe 3dā¶, Co 3dā·, Ni 3dāø, and zinc 3dĀ¹ā°. The regular pattern is therefore [Ar] 3dāæ 4sĀ², with n rising from 1 at Sc to 10 at Zn. Two atoms break it. Chromium is [Ar] 3dāµ 4sĀ¹ and copper is [Ar] 3dĀ¹ā° 4sĀ¹: moving one electron from 4s into 3d gives a half-filled or a completely filled 3d sublevel, which lowers the energy a little. This is a useful rule but not the whole story, because 4s and 3d are close in energy, so treat the two exceptions as facts to remember. To count unpaired d electrons, draw five boxes and fill them by Hundās rule: with n electrons, the unpaired number is n up to 5 and 10 ā n after that.
Worked examples
Cobalt, Z = 27. Argon accounts for 18 electrons, so 27 ā 18 = 9 are left. Two go to 4s and seven to 3d: [Ar] 3dā· 4sĀ². In the five 3d boxes, the first five electrons go one to a box and the last two pair up with two of them, leaving 3 unpaired d electrons.
Chromium, Z = 24. The pattern would give [Ar] 3dā“ 4sĀ², but the real atom is [Ar] 3dāµ 4sĀ¹, with a half-filled 3d sublevel. Its unpaired electrons: five in 3d and one in 4s, six in all.
Atom
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
d electrons
1
2
3
5
5
6
7
8
10
10
unpaired d electrons
1
2
3
5
5
4
3
2
0
0
Unpaired d electrons along the series. The d electron count follows the pattern, and the unpaired count rises to 5 at Cr and Mn and then falls back to 0 at Cu and Zn.
Class activity
Boxes and coins, on paper. Draw five boxes in a row for the 3d sublevel and one box for 4s. Use coins as electrons: put 6 coins in the 3d boxes for iron, one in each box first and then the sixth on top of the first, and 2 coins in the 4s box. Count the boxes that hold a single coin. Now rebuild chromium: 5 coins in 3d and 1 in 4s. Which arrangement has more single coins, iron or chromium? Write what you notice about chromiumās 3d boxes.
Practice
1
Titanium has Z = 22. Write its configuration and count its unpaired d electrons.
22 ā 18 = 4 electrons beyond argon: [Ar] 3dĀ² 4sĀ². The two 3d electrons sit in different boxes, so 2 unpaired d electrons.
2
Write the configuration of nickel (Z = 28) and count its unpaired d electrons.
[Ar] 3dāø 4sĀ². Eight d electrons: five boxes get one each and three pair up, leaving 5 ā 3 = 2 unpaired.
3
Manganese is [Ar] 3dāµ 4sĀ². How many d electrons are unpaired, and what is special about that number?
All 5 are unpaired, one in each 3d box: the largest possible number for a d sublevel, a half-filled sublevel.
4
Why is copper [Ar] 3dĀ¹ā° 4sĀ¹ and not [Ar] 3dā¹ 4sĀ²?
Moving one electron from 4s to 3d completes the 3d sublevel (3dĀ¹ā°), which is a little more stable. 4s and 3d are close in energy, so this small gain decides.
