How Ionic Bonds Form

An ionic bond forms when one or more electrons are transferred from a metal atom to a non-metal atom. The metal loses electrons to become a positively charged cation, while the non-metal gains electrons to become a negatively charged anion. The electrostatic attraction between these oppositely charged ions holds them together.

Sodium Chloride (NaCl) Formation

Na atom

11 electrons, 1 valence e^-

Config: [Ne] 3s¹

Cl atom

17 electrons, 7 valence e^-

Config: [Ne] 3s² 3p⁵

↓ electron transfer ↓

Na⁺ cation

10 electrons, stable octet

Config: [Ne]

Cl^- anion

18 electrons, stable octet

Config: [Ar]

Octet rule: Atoms tend to gain, lose, or share electrons to achieve a stable configuration of 8 electrons in their outer shell (like a noble gas).

Ionic Lattice Structure

Ionic compounds do not exist as individual molecules. Instead, they form a crystal lattice -- a three-dimensional, repeating arrangement of alternating cations and anions. Each ion is surrounded by ions of opposite charge, maximising the electrostatic attraction and minimising repulsion.

Lattice Structure (NaCl)

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

Blue = Na⁺ cations, Red = Cl^- anions (2D representation of a 3D lattice)

Lattice energy: The energy released when gaseous ions come together to form the lattice. Higher lattice energy means a more stable compound with higher melting point.

Properties of Ionic Compounds

The strong electrostatic forces in the ionic lattice give ionic compounds their characteristic properties.

High Melting/Boiling Points

Large amounts of energy are needed to overcome the strong electrostatic attractions between ions in the lattice. NaCl melts at 801 degrees C.

Conduct Electricity When Molten/Dissolved

In the solid state, ions are fixed in place and cannot move. When melted or dissolved in water, ions are free to move and carry charge.

Soluble in Water

Water molecules (polar) can surround and separate ions from the lattice through hydration. Not all ionic compounds dissolve easily.

Brittle

When a force shifts the lattice, like-charged ions align and repel each other, causing the crystal to shatter along cleavage planes.

Common Ionic Compounds

NaCl (sodium chloride) -- table salt, formed from Group 1 metal + Group 17 non-metal.

MgO (magnesium oxide) -- very high melting point (2852 degrees C) due to Mg²⁺ and O^2- (higher charges, stronger attraction).

CaCl₂ (calcium chloride) -- Ca loses 2 electrons to form Ca²⁺, requiring two Cl^- ions to balance charge.

Key Vocabulary

Cation

A positively charged ion formed when a metal atom loses one or more electrons. Example: Na⁺, Ca²⁺.

Anion

A negatively charged ion formed when a non-metal atom gains one or more electrons. Example: Cl^-, O^2-.

Crystal Lattice

A regular, three-dimensional arrangement of alternating positive and negative ions held together by electrostatic forces.

Electrostatic Attraction

The force of attraction between opposite charges. This is the force that holds ionic compounds together.

Worked Examples

1

Explain how magnesium oxide (MgO) forms.

Step 1: Mg (Group 2) has 2 valence electrons. O (Group 16) has 6 valence electrons.

Step 2: Mg loses 2 electrons to form Mg²⁺. O gains 2 electrons to form O^2-.

Step 3: Both ions now have stable noble gas configurations (Ne for Mg²⁺, Ne for O^2-).

Answer: The electrostatic attraction between Mg²⁺ and O^2- forms the ionic bond. The ratio is 1:1, giving the formula MgO.

2

Why does MgO have a higher melting point than NaCl?

Step 1: MgO contains Mg²⁺ and O^2- (both doubly charged). NaCl contains Na⁺ and Cl^- (singly charged).

Step 2: Higher charges produce stronger electrostatic attraction between ions.

Answer: MgO has stronger ionic bonds due to higher ionic charges, requiring more energy to break the lattice, so it has a higher melting point (2852 degrees C vs 801 degrees C).

3

Why does solid NaCl not conduct electricity, but molten NaCl does?

Step 1: In solid NaCl, ions are locked in fixed positions in the lattice and cannot move.

Step 2: Electrical conductivity requires mobile charge carriers.

Answer: When melted, the lattice breaks down and ions become free to move. These mobile ions can carry an electric current, so molten NaCl conducts electricity.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

An ionic bond forms between which types of elements?

Two metals Two non-metals Two noble gases A metal and a non-metal

Question 2

What charge does a sodium ion (Na⁺) have, and why?

+1, because it lost one electron +1, because it gained one electron -1, because it lost one electron +2, because it lost two electrons

Question 3

Why are ionic compounds brittle?

The bonds between ions are very weak When layers shift, like charges repel and the lattice shatters The electrons flow freely, weakening the structure The molecules separate easily

Question 4

What is the formula of the ionic compound formed between calcium (Ca) and fluorine (F)?

CaF Ca₂F CaF₂ Ca₂F₂

Question 5

Under which condition(s) does an ionic compound conduct electricity?

Only as a solid Only when heated but not melted Under all conditions When molten or dissolved in water

Ionic Bonding