Covalent Bonding Unit 5 - ESCS

Covalent Bond

Covalent Bonds

A covalent bond is a chemical bond in which 2 atoms share a pair of valence electrons.

A covalent bond occurs between 2 nonmetals.

The noble-gas elements (Group 18) are the exception because they usually do not form bonds.

Electrons want to be in pairs

Paired Electron Unpaired Electron

Sharing Creates a Bond

F F

Notice Only need to draw outer electrons from now on!

Outer Level Still only hold 8!!!!

Representing Covalent Bonds

Molecules of Elements

A molecule is a neutral group of atoms that are joined together by one or more covalent bonds.

The attractions between shared electrons and the protons in each nucleus hold the atoms together in a covalent bond.

Diatomic means “2 atoms”.

Many nonmetal elements exist as diatomic molecules.

Atoms of the same element will share bonding electrons equally.

Double Bonds: Carbon Dioxide (CO2)

Since each are sharing two pairs Double Bond!

C O O

Naming and Writing Formulas

Writing Formulas for Covalent

Write the symbol for the element.

The prefixes in the name will tell you the subscript of each element in the formula.

Example:

Carbon dioxide - CO2

the prefix di tells you there are 2 oxygen atoms

How do you know which element goes first?

The element with the lower EN (electronegativity) will go first. This means the element furthest from Fluorine goes first.

Naming Covalent Molecules

1. Confirm that it has 2 nonmetals.

2. Name the elements in the order listed in the formula.

3. Use prefixes to indicate the number of each kind of atom

(Omit mono- for one atom in the first element)

4. Suffix of the name on the second element is –ide

Mono- Di- Tri- Tetra- Penta- Hexa- Hepta- Octa- Nona- Deca-

1 2 3 4 5 6 7 8 9 10

Practice

Name the following molecules:

H2O

CO

Write the formula for the following molecules:

Trinitrogen pentafluoride

Nonasulfur dioxide

Activity - SPEED DATING! Put your name on a piece of paper and number it 1-15.

Drawing Lewis Structures

Lewis Structures for Compounds A straight line represents two electrons (one from each atom) involved in a covalent bond.

Bonding pair – any pair of electrons that results from the formation of a covalent bond.

Nonbonding pair – any pair of electrons that are in the electron-dot structure of the individual atom.

Drawing Lewis Structures

1. Total all valence electrons.

2. Draw the skeleton structure and add bonds between each atom.

3. Add electrons around each atom until every atom has 8 electrons (except H gets 2 e-)

4. Count all of your electrons on your structure.

5. Check your answer!

Example: CF4 STEP 1: Total all of the valence electrons.

Carbon has 4 valence electrons. Carbon has 1 atom in CF4

Fluorine has 7 valence electrons. Fluorine has 4 atoms in CF4

Carbon = 4 valence electrons x 1 atom = 4 electrons

Fluorine = 7 valence electrons x 4 atoms = 28 electrons

32 total electrons

Example: CF4

STEP 2: Draw the skeleton structure and add bonds between each atom.

Example: CF4

STEP 3: Add electrons around each atom until every atom has 8 electrons (except H gets 2 e-)

Example: CF4

STEP 4: Count all of your electrons on your structure.

There are 32 electrons. You are done!

Example: CF4

STEP 5: Check your answer!

If the total number of electrons in your structure (in #4) is the SAME as the total you calculated in #1 then your structure is completed.

Example

Draw the Lewis Structure for H2O

1. Total all valence electrons.

2. Draw the skeleton structure and add bonds between each atom.

3. Add electrons around each atom until every atom has 8 electrons (except H gets 2 e-)

4. Count all of your electrons on your structure.

5. Check your answer!

Draw the Lewis structure for CCl4

1. Total all valence electrons.

2. Draw the skeleton structure and add bonds between each atom.

3. Add electrons around each atom until every atom has 8 electrons (except H gets 2 e-)

4. Count all of your electrons on your structure.

5. Check your answer!

Worksheet

Lewis Structures for Multiple Bonds

Types of Covalent Bonds Weakest Strongest

When do you need double or triple bonds?

Sometimes when you count up your total for the valence electrons you do not get the number you originally calculated. You will need either a double bond or triple bond.

Example: CO2

Carbon = 4 × 1 = 4

Oxygen = 6 × 2 = 12

4 + 12 = 16 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 𝑡𝑜𝑡𝑎𝑙

We need to decrease the number of electrons.

Change one single bond into a double bond.

Now there is a total of 18 electrons. Remember we needed 16.

Change the other single bond into a double bond. Everything has a complete octet and a total of 16 electrons.

• •

Practice 1. Draw the Lewis Structure for P2.

2. Draw the Lewis Structure for S2.

3. Draw the Lewis Structure for Carbon Monoxide (CO).

White Board Activity!

Unequal sharing of electrons

Bond Polarity

Nonpolar Covalent Bond Equal sharing of electrons

This happens in 2 identical atoms bonded together.

Example: O2 and N2

Polar Covalent Bond Unequal sharing of electrons.

This happens when different atoms bond together.

Example: CO2 and CH4

Polar Covalent Bonds Sometimes the electron pair is not equally shared between the two atoms.

If an element has a higher electronegativity value then it will have a stronger pull on the electron pair. This is called a polar covalent bond.

When atoms form a polar covalent bond, the atom with the greater attraction for electrons has a partial negative charge. The other atom has a partial positive charge.

Hydrogen has an electronegativity value of 2.1

Chlorine has an electronegativity value of 3.0

Since Cl has a higher EN value, it will have a stronger pull on the electrons. It will be labeled with a δ-.

Since H has a lower EN value, it will have a weaker pull on the electrons. It will be labeled with a δ+

δ+

δ-

Determining the Type of Bond

Bonds are never completely ionic or covalent.

Difference in electronegativity will determine the type of bond.

Example Will the bonds in HF be polar or nonpolar?

1. We need to look up the EN value for Hydrogen and Fluorine. Use page 265.

2. Hydrogen = 2.1 & Fluorine = 4.0

3. Subtract the values to get a positive value: 4.0-2.1=1.9

4. 1.9 is very polar

Practice

Will the bonds in NO2 be polar or nonpolar?

1. N=3.0 & O=3.5

2. 3.5-3.0 = 0.5

3. 0.5 is moderately polar covalent

Polar & Nonpolar Molecules

Molecules are either polar or nonpolar.

The type of atoms in a molecule and its shape are factors that determine whether a molecule is polar or nonpolar.

Nonpolar Polar

Just because a molecule has a polar covalent bond doesn’t mean that it will be a polar covalent molecule.

Example: H2O and CCl4 both contain polar bonds (because of the electronegativity difference). However, only H2O is a polar molecule.

Let’s Watch

Video over Bond Polarity and Molecular Polarity

Practice Worksheet

Attraction Between Molecules

Forces with Molecules

There are forces that are within a molecule and there are forces between 2 molecules.

The forces within a molecule are called intramolecular forces. This includes ionic and covalent bonds.

The forces between 2 molecules are called intermolecular forces. These forces are much weaker than intramolecular forces.

Polar Molecules “Stickiness” Polar molecules electrically attract one another (the positive end of one molecule is attracted to the negative end of another).

It makes it relatively difficult to separate the molecules.

Polar molecules can be thought of as being “sticky”.

Attractions between polar molecules are stronger than attractions between nonpolar molecules.

Properties of Molecules

Substances composed of polar molecules have higher boiling points than substances composed of nonpolar molecules because of their “stickiness”.

It takes more energy to separate polar molecules into the gaseous phase

Ionic vs. Covalent Properties