Chapter 3

Covalent Compounds

Where are the covalent compounds found?

Most of the human body is made of covalent molecules:
❑Water that makes up about 60–65% of total body mass
❑Proteins that do most of the work in cells
❑DNA that stores genetic information
❑Sugars that we burn for energy
❑Fats that build cell membranes
❑Hormones that carry signals
❑Most medications used in healthcare

How covalent compounds are formed?

Atoms “want” a stable outer shell of electrons—usually 8 electrons, called an octet (with
hydrogen as the main exception, since it is stable with 2).
❑In ionic compounds, atoms reach this stable state by giving up or taking electrons.
❑In covalent compounds, they reach the same goal by sharing electrons so that each
atom “feels” like it has a full outer shell.

Two kinds of electron pairs

Bonding pairs, Lone pairs

Bonding pairs

1. A pair of electrons between two atoms.
2. This pair is shared → it forms a covalent bond.

Lone pairs (non-bonding pairs)

1. A pair of electrons that belongs to one atom
   only.

2. It is drawn as a pair of dots on a single atom, not
   between atoms.

3. Lone pairs do not form bonds, but they still take
   up space around the atom

Polarity: Why some molecules “love water” and others don’t?

1. In many molecules, the sharing of electrons is not equal
   between the two atoms.

2. Some atoms pull on the shared electrons more strongly
   than others.

Electronegativity

1. Electronegativity is a measure of how strongly an atom attracts
   shared electrons.

2. An atom with a higher electronegativity pulls the shared
   electrons closer to itself.

3. This unequal sharing of electrons creates a polar covalent bond.

Nonpolar Covalent Bonds

1. In a nonpolar covalent bond the two atoms have similar electronegativities.

2. So, the shared pair of electrons is equally shared between the two atoms.

3. And because of the equal sharing, there is no significant “charge difference”
   along the bond.
   Examples: H2, O₂ and N2
   (In each case, the two atoms are the same (H–H, O–O, N–N), so they attract electrons equally)

Polar Covalent Bonds

1. In a polar covalent bond, one atom
   has a higher electronegativity than
   the other.

2. The shared electrons are pulled
   closer to the more electronegative
   atom.

3. This creates a slight charge
   separation.

4. The end where electrons spend
   more time is slightly negative, and
   the other end is slightly positive

Dipole

A slight positive and slight negative charge creates a dipole

Shape of the molecule affects the polarity

1. Symmetrical (linear CO₂) → dipoles cancel → non-polar molecule.

2. Non-symmetrical (bent H₂O) → dipoles don’t cancel → polar
   molecule

Special bonds between water molecules

1. The slightly positive hydrogen of one water
molecule is attracted to the slightly negative
oxygen of another.
2.This weak attraction is called a hydrogen
bond.
3. Hydrogen bonds are not as strong as
covalent bonds, but they are very important
when many of them act together

Water is a .

universal solvent

Cohesion

Water molceules stick together

Adhesion

Water molecules stick to other surfaces

Water has .

High surface tension