Chemical Bonding

Valence electrons & Lewis dot structures

How do valence electrons play a role in bonding and making compounds? NaCl MgO MgBr2 AlP BF3 Al2O3 CCl4

Three types of Bonds Ionic Covalent Metallic Metal + Non-metal Two non-metals Metallic Metallic atoms in an element

Metallic Bonds Any metal in its’ elemental form Nuclei held together by “sea” of mobile electrons shared by all atoms Explains shiny, malleable and ductile properties of metals

Ionic Bonding Metal + Non-metal Sometimes polyatomic ions replace either Electrons transferred from cation to anion Electrostatic attraction holds ions together or (+) charges and (-) charges attract

Drawing Lewis structures of Ionic Compounds NaCl MgO MgBr2 AlP Al2O3

Lewis Structures of Molecular Compounds – see packet Count all of the valence electrons in the compound. Draw a “shell” of the atoms. Connect all atoms with single bonds. Give every atom a full octet with lone pairs. Check your structure shows the correct number of electrons counted in step one.

Double and Triple Bonds - packet If the number of electrons shown on Lewis structure is too high: Erase a lone pair from each atom Add another bond between atoms This decreases the electrons shown by 2e- If this STILL is too many electrons, do it again. Maximum of three bonds between any two atoms.

Incomplete Octets - packet Beryllium Has a full shell with 4e- (two bonds) Ex: BeCl2 Boron, Aluminum Has a full shell with 6e- (three bonds) Ex: BF3 or AlCl3

Expanded Octets - packet Any atom in the 3rd period or lower, when the CENTRAL atom in a Lewis structure can have 10 or 12 e- completing its octet. If Lewis structure has too few electrons shown, add lone pairs to the central atom and expand its octet. Examples: PF5, SF6, ClF3, XeF2

Lewis Structures of Polyatomic Ions Count up the valence electrons from each atom and adjust it based on charge of ion. Draw Lewis structure as normal. Ex: NH4+, H3O+, ClF4+, NO3-, CO3-2 OR

Two Types of Covalent Bonds Covalent Bond - each atom contributes 1e- Coordinate Covalent Bond - one atom contributes both e- in bond - most often in polyatomic ions

Bond Polarity Describes how electrons are shared. The sharing of e- can be equal or unequal. Consider electronegativity differences.

Ionic Bonds - Review Electronegativity Difference is > 1.7 SHOULD NOT BE USED TO IDENTIFY IONIC MATERIAL - used to determine strength of bond & determine the degree of ionic character

Non-polar Covalent Bonds Diatomic Molecules Made from two identical nonmetals – equal attraction for e- Therefore, equal sharing between atoms 0 difference in electronegativity Ex: H2, N2, O2, F2, Cl2, Br2, I2 Can be single, double or triple bonds (varying number of electrons shared)

Polar Covalent Bonds Atoms in bond are two different nonmetals Therefore, there is an unequal attraction for the e- 0.1 - 1.7 difference in electronegativity The higher the difference, the more polar the bond, and this correlates to the strength of the bond Electrons are attracted to one atom more than the other E- spend more time around nucleus of atom of higher electronegativity Can be single, double or triple bonds

Network Solids Special type of covalent bond Limited to substances containing C & Si C: Diamond, graphite, Buckyball Si: Silicon carbide, Silicon dioxide (sand)

Energy is involved in Bonding! Energy is RELEASED when bonds are formed. Why? Nature proceeds to achieve lower energy states. For ionic compounds, this is called LATTICE ENERGY. High Lattice Energy = high attractions of ions

Bond Forming Energy Cont’d For molecular compounds, this energy loss is represented as a drop in potential energy (PE)

Energy of breaking bonds Energy is ABSORBED when bonds are broken. Called Bond Energy Equals the Lattice Energy for ionic compounds. Bond strength correlates to bond energy.

Bond Type  Compound Properties Metallic Ionic Covalent Example Potassium Potassium Chloride (KCl) Chlorine Melting Pt. 63oC 770oC - 101oC Boiling Pt. 760oC 1500oC - 34.6oC Properties soft, silvery solid conductor as a solid crystalline white solid conductor when dissolved or molten - greenish, yellow gas - not a good conductor

Bonding Concept Map A “sea” of mobile electrons shared by all atoms made of Metallic Bonds hold Positively charged nuclei of metals together