Lecture 8.2- Lewis Dot Structures for Molecules

Bellwork- Ionic or covalent?

Are the following compounds ionic or covalent?a. NaClb. H2O

c. MgOd. AlCl3

e. SO2

f. C12H22O12

g. CO2

HOW DO YOU TELL?

Lecture 8.2- Lewis Dot Structures for molecules

In covalent bonds, electron sharing usually occurs so that atoms attain the electron configurations of noble gases.

Two atoms held together by sharing a pair of electrons are joined by a single covalent bond.

An electron dot structure such as H:H represents the covalent bond with two dots.

A structural formula such as H-H shows covalent bonds with dashes.

1. Draw dot structures for each atom2. Connect atoms so that H gets 2

and all others get 8.

ClH 2 dots 8 dots

The halogens form single covalent bonds in their diatomic molecules. Fluorine is one example.

A pair of valence electrons that is not shared between atoms is called an unshared pair, also known as a lone pair or a nonbonding pair.

H–F 3 lone pairs

1 bonding pair

In water the hydrogen and oxygen atoms attain noble-gas configurations by sharing electrons.

The ammonia molecule has one unshared pair of electrons.

Methane has no unshared pairs of electrons.

Practice

Atoms form double or triple covalent bonds by sharing two pairs or three pairs of electrons.

A bond that involves two shared pairs of electrons is a double covalent bond.

A bond formed by sharing three pairs of electrons is a triple covalent bond.

Each oxygen atom has two “single” electrons that can bond.

O O

Each oxygen atom can make two bonds.

A single bond still leaves two unpaired electrons.

O2 has a double bond.

Carbon dioxide gas is soluble in water and is used to carbonate many beverages. A carbon dioxide molecule has two carbon-oxygen double bonds.

Carbon dioxide is an example of a triatomic molecule.

O C O

Carbon can make four bonds Oxygen makes 2 bonds.

Usually, the unique atom is in the middle.

Simulation 6 Simulate the covalent bonding within molecules

A coordinate covalent bond is a covalent bond in which one atom contributes both bonding electrons.

In a structural formula, you can show coordinate covalent bonds as arrows that point from the atom donating the pair of electrons to the atom receiving them.

In a coordinate covalent bond, both of the shared electrons came from one atom.

ammonia + H+ ammonium ion

NH3 + H+ NH4+

Once formed, there are no identifiable differences between coordinate covalent

bonds and covalent bonds.

A polyatomic ion, such as NH4+,

is a covalently bound group of atoms that has a positive or negative charge and behaves as a unit.

Most plants need nitrogen that is already combined in a compound to grow.

Subtract an e- for each positive

charge. Add an e- for each negative charge.

The energy required to break a bond is called the bond dissociation energy.

A large bond dissociation energy corresponds to a strong covalent bond.

Ozone in the upper atmosphere blocks harmful ultraviolet radiation from the sun. At lower elevations, it contributes to smog.

A resonance structure is needed when two or more valid electron dot structures exist.

The structure of ozone is halfway in-between the two dot structures

The actual bonding of oxygen atoms in ozone is a hybrid, or mixture, of the extremes represented by the resonance forms.

1. In covalent bonding, atoms attain the configuration of noble gases by

a. losing electrons.

b. gaining electrons.

c. transferring electrons.

d. sharing electrons.

2. Electron dot diagrams are superior to molecular formulas in that they

a. show which electrons are shared.

b. indicate the number of each kind of atom in the molecule.

c. show the arrangement of atoms in the molecule.

d. are easier to write or draw.

3. Which of the following molecules would contain a bond formed when atoms share three pairs of electrons?

a. Se2

b. As2

c. Br2

d. Te2