1.6. Electron Configurations

Last Updated: 18 August 2019 | Print

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The periodic table

Middle of the 19^th century: Some 60 elements discovered.

Mendeleev organizes them by atomic mass.

Dmitri Mendeleev.

Mendeleev's periodic table.

A modern periodic table.

The order in the periodic table

Atoms are ordered by atomic number.

The electrons in the atoms are ordered in electron shells, so they have the lowest possible energy.

The closer to the nucleus, the lower the energy.

Models of some atoms

Model of a hydrogen atom.The electron configuration of hydrogen

Atomic number: 1
Number of p⁺ in the nucleus: 1
Number of n in the nucleus: 0
Mass number: 1
Number of e^– around the nucleus: 1

Model of a helium atom.The electron configuration of helium

Atomic number: 2
Number of p⁺ in the nucleus: 2
Number of n in the nucleus: 2
Mass number: 4
Number of e^– around the nucleus: 2

Model of a lithium atom.The electron configuration of lithium

Atomic number: 3
Number of p⁺ in the nucleus: 3
Number of n in the nucleus: 4
Mass number: 7
Number of e^– around the nucleus: 3

How many e^– are there room for in the different shells?

Shell	1 (K)	2 (L)	3 (M)	4 (N)	\(n\)
Max. number of e^–	2	8	18	32	\(2n^2\)

Electron configurations for some atoms

Element	Atomic #	K	L	M	N
Hydrogen, H	1	1
Helium, He	2	2
Litium, Li	3	2	1
Beryllium, Be	4	2	2
Fluorine, F	9	2	7
Neon, Ne	10	2	8
Sodium, Na	11	2	8	1
Sulfur, S	16	2	8	6
Argon, Ar	18	2	8	8
Potassium, K	19	2	8	8	1
Zinc, Zn	30	2	8	18	2
Krypton, Kr	36	2	8	18	8

All the electrons marked in red are valence electrons.

Example

Give the electron configuration for oxygen, ₈O.

Solution

Oxygen has 8p⁺ and 8e^–

	K	L	M
8p⁺	2e^–	6e^–	0

Valence electrons

Valence electrons decide much of an element's properties.

These properties were what Mendeleev noticed!
The farther away from the nucleus, the easier they may be released.
The fewer they are, the easier they may be released.
Eight valence electrons are particularly stable!

Rule of octets – noble gas configuration

All the noble gases except helium have eight valence electrons.
Eight valence electrons (or two, for helium) is called noble gas configuration.
In chemical reactions: The atoms strive to attain noble gas configuration.

Mr. Ehinger's Chemistry

1. States of Matter. The Atom and the Periodic Table