Covalent Bonding

When atoms interact, the goal is to achieve a stableoctet.

When two nonmetals react together, they share theirvalence electrons to achieve a full valence shell.

The bond that forms as a result of sharing electrons iscalled a covalent bond.

Two or more atoms that are joined by covalent bonds iscalled a molecule.

Covalent Bonding

A shared pair of electrons (2 electrons) forms a singlecovalent bond.

Some molecules contain double (2 pairs of sharedelectrons) or triple (3 pairs of shared electrons) covalentbonds.

Molecules can be shown using Lewis structures.

The shared electrons in a Lewis structure is shown as adash, and the electrons not involved in bonding areshown as lone pairs.

READ the sample problems on pg.36 - 39.

COMPLETE the Practice Problems #1-4 on page 39

Electronegativity is a measure of an atom’s ability toattract a shared pair of electrons within a covalent bond.

In general, metals have lower electronegativities thannonmetals.

Electronegativity decreases as you go down a groupbecause the atomic radii increases (more energy shells)

And

Electronegativity increases as you go across theperiodic table because the atomic radii decreases(more protons in the nucleus)

Electronegativity

If an electron pair is shared equally, the difference inelectronegativity is between 0 - 0.4, then the bond is anonpolar covalent bond.

Ex. H-C EN of C = 2.5 and EN of H = 2.1

Therefore, ÎEN = 2.5 - 2.1

= 0.4

If an electron pair is not shared equally, the difference inelectronegativity is between 0.5 - 1.7, then the bond is apolar covalent bond.

Ex. C-O EN of C = 2.5 and EN of O = 3.5

Therefore, ÎEN = 3.5 - 2.5 = 1.0

Polar and Nonpolar CovalentBonds

Slightly negative Slightly positive

Higherelectronegativity

Lowerelectronegativity

Polar molecules are molecules that have a positivelycharged end and a negatively charged end.

Nonpolar molecules do not have charged ends.

The polarity of a molecule depends upon:

1. The presence of polar bonds

2. The overall 3-D shape

Polar and Nonpolar Molecules

There are five basic shapes used to determine if a moleculeis polar or nonpolar. They are:

Linear; symmetrical shape so forms anonpolar compound if both atoms arethe same regardless of the ªEN

Tetrahedral; symmetrical shape so formsa nonpolar compound if all four peripheralatoms are the same

Trigonal Planar; symmetrical shapeso forms a nonpolar compound ifall three peripheral atoms are thesame

Trigonal Pyramidal; asymmetricalshape so will form a polarcompound if the ªEN is 0.5 or above

Bent or V-shaped;asymmetrical shape so willform a polar compound ifthere are polar covalent bonds.

If the central atom was C, and theperipheral atoms were F - would thisbe a polar molecule?

No it is a nonpolar molecule. eventhough the ÎEN is 1.5 which arepolar bonds, it is a symmetricalshape.

Is ammonia, NH3 a polar ornonpolar compound?

Because of its shape, and thepresence of polar bonds, ammonia isa polar molecule.

Covalent and ionic bonds are strong bonds that hold theatoms of a molecule together. These are known asintramolecular bonds.

Molecules are attracted to other molecules by weakforces of attraction called intermolecular bonds.

There are three types of intermolecular forces:

1. London Dispersion forces (LDF)

2. Dipole-Dipole forces (DDF)

3. Hydrogen Bonding (HB)

Intermolecular Bonds

Results from a temporary imbalance in the position of anatoms electrons, leaving one side of the molecule slightly -ve charged and one side of the molecule slightly +vecharged.

The negatively charged end of one molecule is attractedto the positively charged end of another molecule.

This force occurs in all molecules and only occurs for afraction of a second at a time. It is the weakest of thethree types of intermolecular forces.

London Dispersion Forces

Forms between the slightly positive end of one polarmolecule and the slightly negative end of a neighbouringpolar molecule.

Dipole-Dipole Force

Hydrogen bonding is a special type of dipole-dipolebonding that occurs when H is bonded to N, O or F.

It is about 10x stronger than DDF.

H-bonding accounts for water’s special properties.

Hydrogen Bonding