Read Section 6.3 After Quiz

Lewis Structures & Molecular Geometries

Lewis Dot Diagrams• Represent the valence e- for an atom using dots.• Start at the top and place e- on each side going

clockwise. Once there are e- on each side, begin to pair them up.

Sodium 1 valence e-

Magnesium2 valence e-

Aluminum 3 valence e-

Silicon 4 valence e-

Phosphorus5 valence e-

Sulfur 6 valence e-

Chlorine 7 valence e-

Argon 8 valence e-

Elements in the same family have the same number of valence e- and the same Lewis dot arrangement

Bonding Using Lewis Dot Diagrams

and Share a pair of e- to form

So that each atom has a full outer energy level

A pair of shared e- can also be represented with a dashed line.

Exceptions to the Octet Rule• In covalent bonds, atoms always share e- to reach

a full valence shell of 8 valence e-…except…– Hydrogen only needs 2 e- in its outer energy level.– Boron only needs 6 e- in its outer energy level.– Some elements can form an expanded octet using

empty d-orbitals to form bonds and have more than 8 valence e-.

5 Steps for Drawing Lewis Structures1. Count the total number of valence electrons

for all atoms.2. Attach each atom to the central atom with a

single bond (single bond = 2 shared electrons)

3. Complete the octet for the attached atoms by adding pairs of non-bonding electrons.

4. Complete the octet for the central atom by adding pairs of non-bonding electrons

5. Count the total number of electrons in your structure and compare to step one. – If the number of e- is the same, it is correct.– If you used too many e-, add double

bond(s) and check your total again (usually add one double bond for each two electrons that you are over the total).– If there are extra electrons left over, add

them as non-bonding electrons on the central atom. This is called an expanded octet.

Valence Shell Electron Pair Repulsion Theory

• Abbreviated “VSEPR”• Pairs of e- around an atom repel each other

and will form an arrangement that minimizes this repulsion (i.e. spread as far apart from each other as possible). As a result, molecules tend to form predictable shapes.

• Lone pairs of non-bonding e- have greater repulsion than bonded pairs of e-.

Basic VSEPR Geometries Molecular Geometry ABE Notation Atoms bonded

to the central atom

Non-bonding pairs on the central

atom

Tetrahedral AB4 4 0

Trigonal Pyramid AB3E 3 1

Bent AB2E2 or AB2E 2 1 or 2

Linear AB2 2 0

Trigonal Planar AB3 3 0

Expanded Octet GeometriesMolecular Geometry ABE Notation Atoms bonded to

the central atomNon-bonding pairs

on the central atom

Octahedral AB6 6 0

Square Pyramid AB5E 5 1

Square Planar AB4E2 4 2

Trigonal Bipyramid

AB5 5 0

Seesaw AB4E 4 1

T-Shaped AB3E2 3 2

Linear AB2E4 2 4