Chapter 6: Chemical Bonds

Section 2: Covalent Bonds, Polar and Non Polar Molecules

Covalent - The other type of bond

Many elements do not easily donate or receive electrons. Instead they share them with another atom.

Covalent bonds are formed when elements share electrons between the elements (this is generally formed between non metals).

Covalently formed compounds are called molecules.

Covalent Bonds

It is easy to distinguish a covalent bond from an ionic bond as in a covalent bond all of the elements bonded are non metals (right of stair step plus hydrogen).

Water is a covalently bonded compound. Two hydrogen atoms and one oxygen atom share their electrons to form a stable configuration for both atoms (8 for Oxygen and 2 for Hydrogen)

Binary covalent bonds never include metals.

Covalent Bonds are in a Tug-of-War

Since the atoms in a covalent bond are not truly giving up their electrons, there is a constant pull on the shared electron between the atoms.

The larger atom has a greater pull and pulls the shared atom closer.

This results in “partial charges” with the larger mass elements taking on a partial negative charge and the smaller mass elements taking on a positive charge.

Polar Molecules

When atoms have a shape that gives them a slightly positive end and a slightly negative end, it is called a polar molecule.

Water (H2O) and ammonia (NH3)are polar molecules. Positive end

Negative end

The bent shape of the molecule gives them positive and negative poles

δ+

δ -

Non Polar Molecules

Some other covalent bonds have a symmetrical shape.

In these situations, electrons are shared equally and a nonpolar molecule is formed as there are no poles.

Oxygen molecule No pos.

and neg. poles

Carbon dioxide molecule

Diatomic molecules

Many non metals exist naturally in a covalent bond that forms a diatomic molecule. In this situation, two atoms share electrons to become stable. O2 is a common example of a diatomic molecule.

This diagram shows how two chlorine atoms share electrons to form the diatomic molecule Cl2

Here, two hydrogen atoms become a stable H2 molecule by sharing electrons

Naming Covalent Compounds

Writing the chemical formulas of these compounds is pretty straight forward, but the naming of them can get complicated.

For example N2O, NO, NO2 and N2O5 would all be read as nitrogen oxide under the naming rules for ionic compounds.

However, each of these covalent compounds is distinctly different. Covalent compound naming has one twist.

Greek prefixes

Add the following Greek prefixes to your periodic table (English meaning in parenthesis):

mono (1) hexa (6)di (2) septa (7)tri (3) octa (8)tetra (4) nona (9)penta (5) deca (10)

Naming covalent compounds includes Greek prefixes for numbersTo solve the naming problem, we turn to Greek

prefixes for numbers (now on your periodic table).

In covalent compounds, you have to include the numbers in the subscripts as part of the name.

N2O then becomes dinitrogen oxide

NO then becomes nitrogen monoxideNO2 then becomes nitrogen dioxide

N2O5 then becomes dinitrogen pentoxide