Ch. 9.1The Covalent Bond

CHM Hon.

Ionic & Covalent Bonds…

Remember…

• Octet Rule = atoms lose, gain or share electrons in order to achieve a stable configuration of 8 valence electrons

Covalent Bond

• Chemical bond resultant from sharing electrons

• Generally occurs when elements are relatively close to each other on the periodic table

• Majority are formed between nonmetallic elements

Molecule

• Formed when 2 or more atoms bond covalently

Diatomic molecules

• Molecule made up of 2 atoms

• Some atoms are only found as diatomic molecules in nature due to stability– Hydrogen (H2), nitrogen (N2), oxygen (O2),

fluorine (F2), chlorine (Cl2), bromine (Br2), and iodine (I2)

Single Covalent Bonds

• All halogens from single covalent bonds– ex) F2

Sigma bonds

• Single covalent bond

• The merging of 2 valence atomic orbitals

• Atomic orbitals overlap end to end

Multiple Covalent Bonds

• Atoms may attain noble gas configuration by sharing more than one pair of electrons between 2 atoms

• C, N, O, S often form multiple bonds

• Double bond– O2

• Triple bond– N2

Pi bond

• Formed when parallel orbitals overlap to share electrons

• Multiple bonds consist of at least one sigma and one pi bond

• CH4

Strength of Covalent Bonds

• Strength depends on how much distance separates bonded nuclei

• Bond length = the distance between 2 bonding nuclei at the position of maximum attraction– Determined by size of atom and how many electron

pairs are shared– As # shared pairs increases bond length

decreases– The shorter the bond length the stronger the bond

• Triple bonds > double bonds > single bonds

An Energy change accompanies the forming or breaking of a bond

• Energy is released when a bond is formed

• Energy is (needed) added to break a bond

Bond dissociation Energy

• The amount of energy required to break a specific covalent bond– Always a (+) value– Indicates the strength of a chemical bond

• As 2 atoms are bonded closer together, greater amounts of bond energy are needed to separate them

Chemical reactions

• Bonds in reactant molecule are broken and new bonds formed as produce molecules form

• Endothermic reaction = occurs when a greater amount of energy is required to break the existing bonds in the reactants than is released when the new bonds form in product molecules

• Exothermic reactions = occur when more energy is released forming new bonds than is required to break bonds in the initial reactants