The term electronic configuration refers to the arrangement of the electrons around the central nucleus from the inner energy shell to the outer energy shell.
Syllabus reference S3.1.2Structure 3.1.2 - The period number shows the outer energy level that is occupied by electrons.
- Elements in a group have a common number of valence electrons.
- Deduce the electron configuration of an atom up to Z = 36 from the element’s position in the periodic table and vice versa.
Guidance
- Groups are numbered from 1 to 18.
- The classifications “alkali metals”, “halogens”, “transition elements” and “noble gases” should be known.
Tools and links
- Nature of science, Structure 1.2 - How has the organization of elements in the periodic table facilitated the discovery of new elements?
Electronic configuration
The number of outer electrons can be obtained from the periodic table using the group number, it is always the same. This does not apply to the transition metals.
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The valency of an element from the main groups can be obtained from the group number. For elements in groups 1 & 2 the valency is the same as the group number. For elements in groups 15-17 the valency is equal to the 18 - group number
Example: What is the valency of selenium (group 16)
Groups 15 to 17 have a valency = 18 - group number
Valency of selenium = 18 - 16 = 2
Formula
Formulae of binary (two element) compounds can be found using the valencies of the elements. If the group numbers of the elements combining are known, then the formula is easy to determine.
Hydrogen (valency 1) combines with all of the elements from Na to Cl in period 3. The formula of the hydrides formed follows a pattern.
| Group |
1
|
2
|
13
|
14
|
15
|
16
|
17
|
| Hydride |
NaH
|
MgH2
|
AlH3
|
SiH4
|
PH3
|
H2S
|
HCl
|
Example: What is the formula of the compound formed between sodium (group 1) and selenium (group 16)
sodium valency = group number = 1
selenium valency = 18 - group number = 2
Formula of sodium selenide = Na2Se
Electronic configuration
The full electronic configuration is expected to include the sub-shells s, p, d, up to element 54. The configuration is written from the inner (lower energy) shells outwards.
Show all the configurations from 1-36
| Hydrogen | 1s1 | Potassium | 1s2 2s2 2p6 3s2 3p6 4s2 |
| Helium | 1s2 | Calcium | 1s2 2s2 2p6 3s2 3p6 4s2 |
| Lithium | 1s2 2s1 | Scandium | 1s2 2s2 2p6 3s2 3p6 4s2 3d1 |
| Beryllium | 1s2 2s2 | Titanium | 1s2 2s2 2p6 3s2 3p6 4s2 3d2 |
| Boron | 1s2 2s2 2p1 | Vanadium | 1s2 2s2 2p6 3s2 3p6 4s2 3d3 |
| Carbon | 1s2 2s2 2p2 | Chromium | 1s2 2s2 2p6 3s2 3p6 4s1 3d5 |
| Nitrogen | 1s2 2s2 2p3 | Manganese | 1s2 2s2 2p6 3s2 3p6 4s2 3d5 |
| Oxygen | 1s2 2s2 2p4 | Iron | 1s2 2s2 2p6 3s2 3p6 4s2 3d6 |
| Fluorine | 1s2 2s2 2p5 | Cobalt | 1s2 2s2 2p6 3s2 3p6 4s2 3d7 |
| Neon | 1s2 2s2 2p6 | Nickel | 1s2 2s2 2p6 3s2 3p6 4s2 3d8 |
| Sodium | 1s2 2s2 2p6 3s1 | Copper | 1s2 2s2 2p6 3s2 3p6 4s1 3d10 |
| Magnesium | 1s2 2s2 2p6 3s2 | Zinc | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 |
| Aluminium | 1s2 2s2 2p6 3s2 3p1 | Gallium | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p1 |
| Silicon | 1s2 2s2 2p6 3s2 3p2 | Germanium | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p2 |
| Phosphorus | 1s2 2s2 2p6 3s2 3p3 | Arsenic | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p3 |
| sulfur | 1s2 2s2 2p6 3s2 3p4 | Antimony | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4 |
| Chlorine | 1s2 2s2 2p6 3s2 3p5 | Bromine | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5 |
| Argon | 1s2 2s2 2p6 3s2 3p6 | Krypton | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 |
Each member of the same group has an identical outer electronic arrangement in terms of subshells, that can be represented by nsx npy.
Example: The electronic configuration of the elements in group 14 are as follows:
- Carbon = [He] 2s2 2p2
- Silicon = [Ne] 3s2 3p2
- Germanium = [Ar] 3d10 4s2 4p2
In all cases the outer energy shell can be expressed as ns2 np2