Lewis Structures (electron dot diagrams)

Lewis Structures of Atoms

The chemical symbol for the atom is surrounded by a number of dots corresponding to the number of valence electrons.

Number of Valence Electrons	1		2		3	4	5	6	7	8
Example	Hydrogen	Group 1 (Alkali metals)	Helium	Group 2 (alkali earth metals)	Group 13	Group 14	Group 15	Group 16	Group 17 (Halogens)	Group 18 except Helium (Noble Gases)
Lewis Structure (electron dot diagram)

The chemical symbol for the element is surrounded by the number of valence electrons present in the ion.
The whole structure is then placed within square brackets, with a superscript to indicate the charge on the ion.
Atoms will gain or lose electrons in order to achieve a stable, Noble Gas (Group 18), electronic configuration.
Negative ions (anions) are formed when an atom gains electrons.
Positive ions (cations) are formed when an atom loses electrons.

Charge on Ion	1+		2+	3+	4+	4-	3-	2-	1-
No. electrons gained or lost	1e lost		2e lost	3e lost	4e lost	4e gained	3e gained	2e gained	1e gained
Example	H⁺	Group 1 ⁺ (Alkali metals)	Group 2 ²⁺ (alkali earth metals)	Group 13 ³⁺	Group 14 ⁴⁺	Group 14 ^4-	Group 15^3-	Group 16 ^2-	Group 17 ^- (Halogens)	H^- (hydride)
Lewis Structure (electron dot diagram)	OR H⁺	OR Li⁺	OR Be²⁺	OR B³⁺	OR C⁴⁺

The overall charge on the compound must equal zero, that is, the number of electrons lost by one atom must equal the number of electrons gained by the other atom.
The Lewis Structure (electron dot diagram) of each ion is used to construct the Lewis Structure (electron dot diagram) for the ionic compound.

Lithium fluoride, LiF

Lithium oxide, Li₂O

Each lithium atom loses one electron to form 2 cations Li⁺ (2 electrons in total are lost)
Oxygen atom gains two electrons to form the anion O^2-
Lithium oxide compound can be represented as2Li⁺ OR Li⁺Li⁺ OR

In a covalent compound, electrons are shared between atoms to form a covalent bond in order that each atom in the compound has a share in the number of electrons required to provide a stable, Noble Gas, electronic configuration.
Electrons in the Lewis Structure (electron dot diagram) are paired to show the bonding pair of electrons.
Often the shared pair of electrons forming the covalent bond is circled
Sometimes the bond itself is shown (-), these structures can be referred to as valence structures.

hydrogen fluoride, HF

Hydrogen atom has 1 valence electron
Fluorine atom has 7 valence electrons
Hydrogen will share its electron with fluorine to form a bonding pair of electrons (covalent bond) so that the hydrogen atom has a share in 2 valence electrons (electronic configuration of helium) and fluorine has a share in 8 valence electrons (electronic configuration of neon)
Lewis Structure (electron dot diagram) for hydrogen fluoride OR
Valence Structure for hydrogen fluoride

ammonia, NH₃

Nitrogen atom has 5 valence electrons
Hydrogen atom has 1 valence electron
Each of the 3 hydrogen atoms will share its electron with nitrogen to form a bonding pair of electrons (covalent bond) so that each hydrogen atom has a share in 2 valence electrons (electronic configuration of helium) and the nitrogen has a share in 8 valence electrons (electron configuration of neon)
Lewis Structure (electron dot diagram) for ammonia OR
Valence Structure for ammonia

oxygen molecule, O₂

Each oxygen atom has 6 valence electrons
Each oxygen will share 2 of its valence electrons in order to form 2 bonding pairs of electrons (a double covalent bond) so that each oxygen will have a share in 8 valence electrons (electronic configuration of neon).
Lewis Structure (electron dot diagram) for the oxygen molecule OR
Valence structure for the oxygen molecule