Periodicity: 5.12 - Electronic configuration, valency and formula

Syllabus ref: 3.1

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.

Electronic configuration - shells
Formula
Electronic configuration - orbitals

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.

Group 1 - has 1 electron in the outer shell
Group 2 - has 2 electrons in the outer shell
Group 13 - has 3 electrons in the outer shell
etc

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

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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

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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 ns^x np^y.

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

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