Atoms bond because they want to achieve a stable electron arrangement — similar to the noble gases (helium, neon, argon) which have completely filled outermost shells.

• Helium has 2 electrons in its outer shell (stable)
• Neon, Argon and other noble gases have 8 electrons in their outer shells (stable)

Atoms with incomplete outer shells are unstable and will react to achieve stability by:

1. Losing electrons
2. Gaining electrons
3. Sharing electrons

Definition

Electrovalent bonding (also called ionic bonding) occurs when one atom transfers electrons to another atom, forming positively and negatively charged ions. These opposite charges attract each other.

How It Forms

Electrovalent bonding usually happens between:

• A metal (which loses electrons) and
• A non-metal (which gains electrons)

Worked Example: Formation of Sodium Chloride (NaCl)

Step 1: Write the electronic configurations:

• Sodium (Na): 2, 8, 1 → has 1 electron in outer shell
• Chlorine (Cl): 2, 8, 7 → has 7 electrons in outer shell

Step 2: Sodium loses 1 electron to achieve neon configuration (2, 8):

• Na → Na⁺ + e⁻

Step 3: Chlorine gains 1 electron to achieve argon configuration (2, 8, 8):

• Cl + e⁻ → Cl⁻

Step 4: The oppositely charged ions attract each other: $\text{Na}^+ + \text{Cl}^- \rightarrow \text{NaCl}$

Properties of Electrovalent (Ionic) Compounds

• Generally soluble in water
• Conduct electricity when dissolved or molten (not in solid form)
• Usually crystalline solids at room temperature
• Have high melting and boiling points

Definition

Covalent bonding occurs when atoms share electrons to achieve a stable electron configuration. This happens when atoms have similar ability to attract electrons (similar electronegativity).

How It Forms

Covalent bonding usually occurs between non-metal atoms.

Examples

1. Hydrogen molecule (H₂):

• Each hydrogen atom needs 1 more electron to achieve helium configuration (2 electrons)
• Two hydrogen atoms share their electrons: $H \cdot + \cdot H \rightarrow H:H$

2. Oxygen molecule (O₂):

• Each oxygen atom needs 2 more electrons
• Two oxygen atoms share 4 electrons (two pairs): $O::O \rightarrow O=O$

3. Hydrogen chloride (HCl):

• Hydrogen needs 1 electron, chlorine needs 1 electron
• They share 2 electrons: $H\cdot + \cdot \Cl{} \rightarrow H:Cl$

4. Water (H₂O):

• Oxygen needs 2 electrons, each hydrogen needs 1 electron
• Two hydrogen atoms share electrons with one oxygen atom: $2H\cdot + \cdot\O\cdot \rightarrow H_2O$

Properties of Covalent Compounds

• Usually have low melting and boiling points
• Often liquids or gases at room temperature
• Do not conduct electricity
• Generally insoluble in water but soluble in non-polar solvents like oil
• Can easily vaporise at room temperature

In everyday life in Tanzania, chemical bonding explains why table salt (sodium chloride) dissolves easily in cooking water but not in oil — this is because NaCl is an ionic compound soluble in polar solvents like water. Understanding chemical bonding also helps explain why cooking oil (a covalent compound) does not mix with water, which is important when preparing foods like ugali or fried fish where proper mixing of ingredients matters for good results.