Chemical Bonds (Grade 11 NSC Matric Physical Sciences): Revision Notes

Worked Example: Hydrogen Bond Formation

As the atoms move closer together:

• From point A to point X: Attractive forces dominate, energy decreases
• At point X: Forces are balanced, energy is at minimum - this is the bond length
• Left of point X: Repulsive forces dominate, energy increases sharply

Result: Bond formation occurs when the energy minimum (point X) is low enough to keep the atoms together. For hydrogen, this energy minimum allows the formation of $\text{H}_2$ molecules where each hydrogen atom achieves a full outer shell through electron sharing.

Worked Example: Helium - No Bond Formation

For helium atoms:

• The energy minimum is very close to zero
• The atoms can approach and separate easily
• No stable bond forms because helium already has a complete outer shell
• This is why helium exists as single atoms ($\text{He}$), not molecules

Worked Example: Hydrogen chloride (HCl)

Step 1: Determine valence electrons

• Hydrogen: 1 valence electron (H•)
• Chlorine: 7 valence electrons (•Cl••••)

Step 2: Arrange electrons to fill outer shells

The shared pair between hydrogen and chlorine forms a covalent bond:

H•Cl•••• → H:Cl•••

Each atom now has a full outer shell through sharing.

Worked Example: Methane (CH₄)

Step 1: Determine valence electrons

• Carbon: 4 valence electrons
• Each hydrogen: 1 valence electron

Step 2: Arrange electrons

Carbon forms four single bonds with four hydrogen atoms:

Each hydrogen shares one electron with carbon, giving carbon a complete outer shell of 8 electrons.

Worked Example: Water (H₂O)

Step 1: Determine valence electrons

• Oxygen: 6 valence electrons
• Each hydrogen: 1 valence electron

Step 2: Arrange electrons

H:O:H with two lone pairs on oxygen

Oxygen forms two bonds with hydrogen atoms but keeps two lone pairs that don't participate in bonding.

Worked Example: Oxygen molecule (O₂)

Step 1: Each oxygen has 6 valence electrons

Step 2: To complete their outer shells, they share two pairs of electrons

O=O (double bond)

Worked Example: Hydrogen cyanide (HCN)

Step 1: Valence electrons

• Hydrogen: 1
• Carbon: 4
• Nitrogen: 5

Step 2: Arrangement

H-C≡N (triple bond between carbon and nitrogen)

Worked Example: Ammonium ion (NH₄⁺)

When ammonia ($\text{NH}_3$) bonds with a hydrogen ion ($\text{H}^+$):

• The hydrogen ion has no electrons
• Nitrogen in ammonia has a lone pair
• The lone pair forms a dative bond with $\text{H}^+$

$\text{NH}_3 + \text{H}^+ \rightarrow [\text{NH}_4]^+$

In structural formulas, dative bonds are often shown without a line, or with an arrow pointing from the donor atom.