The Nature of Ionic Bonding

Ionic Bonding

Define ionic bonding.

The strong electrostatic attraction between oppositely charged ions.

What is an ionic bond formed between?

Oppositely charged positive and negative ions.

How are positive ions (cations) formed?

By atoms losing one or more electrons.

How are negative ions (anions) formed?

By atoms gaining one or more electrons.

Define a cation.

A positively charged ion formed when an atom loses one or more electrons.

Define an anion.

A negatively charged ion formed when an atom gains one or more electrons.

How does a sodium atom form a sodium ion?

It loses one electron to form Na⁺.

Write the half-equation for the formation of a sodium ion.

Na → Na⁺ + e⁻

How does a chlorine atom form a chloride ion?

It gains one electron to form Cl⁻.

Write the half-equation for the formation of chloride ions from chlorine.

Cl₂ + 2e⁻ → 2Cl⁻

Write the equation for the formation of sodium chloride from sodium and chlorine.

2Na(s) + Cl₂(g) → 2NaCl(s)

What happens to electrons when sodium reacts with chlorine?

Electrons are transferred from sodium atoms to chlorine atoms.

Why does sodium form Na⁺?

It loses its one outer electron.

Why does chlorine form Cl⁻?

It gains one electron to complete its outer shell.

How does a magnesium atom form a magnesium ion?

It loses two electrons to form Mg²⁺.

Write the half-equation for the formation of a magnesium ion.

Mg → Mg²⁺ + 2e⁻

How does an oxygen atom form an oxide ion?

It gains two electrons to form O²⁻.

Write the half-equation for the formation of oxide ions from oxygen.

O₂ + 4e⁻ → 2O²⁻

Write the equation for the formation of magnesium oxide.

2Mg(s) + O₂(g) → 2MgO(s)

Why is the formula of magnesium oxide MgO?

Mg forms Mg²⁺ and oxygen forms O²⁻, so they combine in a 1:1 ratio to give an overall neutral compound.

What should a dot-and-cross diagram show?

The outer-shell electrons and the transfer of electrons between atoms.

Why are dots and crosses used in dot-and-cross diagrams?

To distinguish electrons originating from different atoms.

What should be shown around an ion in a dot-and-cross diagram?

Square brackets with the ion's charge outside the brackets.

Do you need to show all electrons in a dot-and-cross diagram?

No, normally only the outer-shell electrons need to be shown.

What structure do ionic solids have?

A giant ionic lattice.

Define a giant ionic lattice.

A regular three-dimensional repeating arrangement of oppositely charged ions.

How are ions arranged in an ionic lattice?

Positive and negative ions alternate so that oppositely charged ions are close together.

What holds a giant ionic lattice together?

Strong electrostatic attractions between oppositely charged ions.

In which directions does ionic bonding act in a giant ionic lattice?

In all directions throughout the lattice.

Why is ionic bonding described as strong?

There are strong electrostatic attractions between oppositely charged ions throughout the giant lattice.

What structure does solid sodium chloride have?

A giant ionic lattice of Na⁺ and Cl⁻ ions.

What two main factors affect the strength of ionic bonding?

The charges on the ions and the distance between the centres of the ions.

How does ionic charge affect the strength of ionic bonding?

The greater the charges on the ions, the stronger the electrostatic attraction.

How does ionic radius affect the strength of ionic bonding?

Smaller ions have a smaller distance between their centres, giving a stronger electrostatic attraction.

What happens to ionic attraction when the distance between ions increases?

The electrostatic attraction becomes weaker.

What happens to ionic attraction when the charges on the ions increase?

The electrostatic attraction becomes stronger.

Why is ionic bonding stronger in sodium fluoride than potassium fluoride?

Na⁺ is smaller than K⁺, so the distance between Na⁺ and F⁻ ions is smaller and the electrostatic attraction is stronger.

Why is ionic bonding much stronger in calcium oxide than potassium fluoride?

Ca²⁺ and O²⁻ have charges of 2+ and 2−, whereas K⁺ and F⁻ have charges of 1+ and 1−, producing much stronger electrostatic attraction.

Why is the ionic bonding in CaO approximately four times stronger than in KF based on charge?

The product of the charges in CaO is 2 × 2 = 4, compared with 1 × 1 = 1 in KF.

What experimental evidence supports the existence of ions?

Migration of ions during electrolysis and electron-density maps.

What happens when an electric current is passed through molten sodium chloride?

Na⁺ ions migrate towards the negative electrode and Cl⁻ ions migrate towards the positive electrode.

Which electrode do positive ions migrate towards?

The negative electrode.

Which electrode do negative ions migrate towards?

The positive electrode.

Why do Na⁺ ions move towards the negative electrode?

Opposite charges attract.

Why do Cl⁻ ions move towards the positive electrode?

Opposite charges attract.

What happens to Na⁺ ions at the negative electrode during electrolysis of molten NaCl?

They gain electrons and form sodium atoms.

Write the equation at the negative electrode during electrolysis of molten NaCl.

2Na⁺ + 2e⁻ → 2Na

What happens to Cl⁻ ions at the positive electrode during electrolysis of molten NaCl?

They lose electrons and form chlorine molecules.

Write the equation at the positive electrode during electrolysis of molten NaCl.

2Cl⁻ → Cl₂ + 2e⁻

Write the overall equation for electrolysis of molten sodium chloride.

2NaCl → 2Na + Cl₂

How can copper(II) chromate solution demonstrate the movement of ions?

The differently coloured Cu²⁺ and CrO₄²⁻ ions move towards opposite electrodes when a direct current is applied.

What colour are aqueous Cu²⁺ ions?

Blue.

What colour are aqueous chromate(VI), CrO₄²⁻, ions?

Yellow.

Where do Cu²⁺ ions migrate during the experiment?

Towards the negative electrode.

Where do CrO₄²⁻ ions migrate during the experiment?

Towards the positive electrode.

What does the migration of coloured ions towards opposite electrodes demonstrate?

That charged ions exist in solution and can move towards oppositely charged electrodes.

How do electron-density maps provide evidence for ions?

They show separate regions of electron density corresponding to individual positive and negative ions.

What does an electron-density map of sodium chloride show?

Separate Na⁺ and Cl⁻ ions rather than shared electron density between atoms.

What technique can be used to produce electron-density maps of ionic crystals?

X-ray diffraction.