Chapter 6.2

Electron configurations

Key Vocabulary

Sub-level: energy subdivision of an energy level.

Valence electron: electrons in the outermost shell of the atom determining the reactivity of the element and group it is in.

Group: columns on the periodic table; same group means same valence electron with the exception of helium.

Period: the horizontal row of the periodic table.

Actinoid series/ lanthanoid series: the strand of elements at the bottom of the periodic table which has their highest energy in the f sub level.

Knowing Electron Configurations by Reading the Periodic Table

Different properties of elements have different electron configurations. You can determine an element's electron configuration just by looking at the periodic table. The Periodic table has been disposed in a way that matches elements together with similar electron configurations. For example, in Groups 1- 2 and 13-.18, all the atoms have their highest energy level in the s or p sub levels. (1-2 has highest energy in s, 13- 18 in p) The group these elements are in determines how the electron configuration ends. For example, groups 1 and 2 end in s1 and s2, and groups 13-18 end in p1-p6. S stands for the s sub-levels, and the number after that represents the group number. Clearly, s sub-level only holds 2 electrons, which is why when we reach 13 we move to the second sub-level which holds 6 electrons- p. Additionally, the coefficient (number before the group number) is simply the period the element is in. For example, Group 1's element's coefficients will range from 1-7. (1s, 2s, 3s,...7s.) This is why, let's say Lithium's electron configuration is 2s1- because it's in the 2nd period in the very first group. The same rule applies in groups 13-18, where group 13 is p1 and 18 is p6.

Special Cases

There are few Groups that have tricky electron configurations. In Groups 3-12, (the d sub-level) the coefficient is one quantum number lower than the actual period. For these transition elements, the lower sub-level precedes the actual sub-level. So in this case the complete electron configuration (for iron) is 4s2 3d6. The 4s2 part exemplifies the sub-level that comes before the group placement of iron. To check, iron is in period 4, (plus the preceding sub-level s) group d6.

There are also the lanthanoids and actinoids who are in groups f1-f14. Similarly to the d sub-level, the f sub-level overlaps by 2 in this case, which means the coefficient is now 2 less than the actual period number.

Chemistry in society

In Chernobyl, Ukraine, a nuclear disaster occurred resulting in a massive release of nuclear material, including the isotope Strontium-90. Grass soil frequently absorbs calcium, which has two valence electrons. Strontium-90 contains the same amount of valence electrons as calcium (2) thus being in the same group and allowing an easy reaction between the radioactive Strontium 90 and the grass. When the cows of Chernobyl eat this grass, they take in the Strontium-90 rather than calcium. Later on, humans milk and slaughter these cows and consume their goods. When the humans consume the cow's products, rather than having the calcium from the soil go into their bones, they have the radioactive Strontium-90 in their bones, thus weakening and damaging the bones.

Minute Lab


  1. Place 1g on iron filings into a test
  2. Add 4 cubic centimeters of 6M hydrochloric acid to the test tube
  3. Place 1g of iron (III) chloride into a separate test tube
  4. Add 4 cubic centimeters of Water to the test tube
  5. Put stoppers in both of the test tubes and shake until some of the solid has dissolved
6. wait until mixture settles and record observations

What happens?

Iron loses two electrons to form the pale green substance in the first test tube. Iron loses three electrons to form the yellow iron(III). The electrons lost are the outermost electrons on the valence electron shell. It would take electrons from the D sublevel before it would take from the S sublevel