Chapter 9

COVALENT BONDING

Review….What is a chemical bond?

Force that holds two atoms togetherWhat is an ionic bond?

An electrostatic force that holds oppositely charged particles together in an ionic compound

Compounds formed from metal & nonmetal

Forms when….?What are atoms always trying to

achieve?StabilityComplete set of valence electrons…

OCTECT

What is a covalent bond?Chemical bond that results from sharing

of valence electronsOccurs b/w nonmetal & a nonmetalBalance b/w attractive and repulsive forces

2 Hydrogen Atoms

Sharing their 1 Ve-

MoleculesCompound made when 2 or more atoms are

bonded covalentlyDiatomic molecules

In nature, sometimes two atoms of the same element are more stable when they are covalently bonded than the individual atom alone…

BrINClHOF (pronounced “Brinkle Hoff”)Br2 I2 N2 Cl2 H2 O2 F2

Single Covalent BondsA single covalent bond –Atom shares 1 pair (2)

electrons.Shared pairs – both elements count the electron

pair to achieve octetLone pairs – pair of electrons that are not shared

b/w the atomsLewis structures- Use electron dot diagram to

show how atoms are arranged in a molecule.

H Cl

shared or Bond pair

Unshared orLone pair (LP)

. .

. .. .

In the Fluorine Molecule…..How many bonding pairs are there in

each?1

How many lone pairs are there each?3

Multiple Covalent Bonds

Double covalent bonds share two pairs of electrons. CO2 O=C=O

Triple covalent bonds share three pairs of electrons.N2 :N=N:

Covalent Bond Formation in Hydrogen

Increased overlap brings the electrons and nuclei closer together while simultaneously decreasing electron-electron repulsion.

However, if atoms get too close, the internuclear repulsion greatly raises the energy.

The attractive and repulsive forces in covalent bonding must be balanced.

– Bond Length - In general, the closer the electrons are held by the atoms, the shorter the bond length and the

higher the bond energy.

– Multiple bonds result in stronger, shorter bonds.

– Bond Energy - The amount of energy required to break a bond. The greater the energy, the stronger the bond.

– Bond breaking is an endothermic process, so bond breaking enthalpies are positive.

Comparing Bond Length and Bond Strength

Using the periodic table, but not Tables 9.2 and 9.3, rank the bonds in each set in order of decreasing bond length and bond strength:

(a) S F, S Br, S Cl

(b) C = O, C O, C O

Sigma () Bonds

Sigma bonds are characterized byHead-to-head overlap.Cylindrical symmetry of electron density

about the internuclear axis.

Pi () BondsPi bonds are

characterized bySide-to-side

overlap.Electron density

above and below the internuclear axis.

Single Bonds vs. Multiple bonds

Single bonds are always bonds, because overlap is greater, resulting in a stronger bond and more energy lowering.

In a multiple bond one of the bonds is a bond and the rest are bonds.

Orbital overlap

Lewis Dot Structures1. Determine the number of Valence e- for all

atoms in the moleculea. Divide the Ve- by 2 to get pairs (2 dots or 1 line)

2. Decide on central atom (least electronegative or furthest to the left).

a. Hydrogen & halogens are terminal atoms

3. Connect all atoms to the central atom by a bonding pair (single line)

4. Place remaining pairs around all atoms before moving on to central atom.

5. Check for octet (not H)a. If atom does not have an octet, move lone pairs

from a terminal atom to create a double or a triple bond (except grp 7).