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 I (Alkali metals)	Helium	Group II (alkali earth metals)	Group III	Group IV	Group V	Group VI	Group VII (Halogens)	Group VIII except Helium (Noble Gases)
Lewis Structure (electron dot diagram)

Lewis Structures for Ions of Elements

• 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 VIII), 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 I + (Alkali metals)	Group II 2+ (alkali earth metals)	Group III 3+	Group IV 4+	Group IV 4-	Group V 3-	Group VI 2-	Group VII - (Halogens)	H- (hydride)
Lewis Structure (electron dot diagram)	OR H+	OR Li+	OR Be2+	OR B3+	OR C4+

Lewis Structures for Ionic Compounds

• 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.
• Examples
  • Lithium fluoride, LiF
    Lithium atom loses one electron to form the cation Li⁺
    Fluorine atom gains one electron to form the anion F^-
    Lithium fluoride compound can be represented asLi⁺ OR
  • 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

Lewis Structures for Covalent Compounds

• 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.
• Examples
  • 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

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