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B3.4 · Compare formation of ionic and covalent bonds

Learn to compare formation of ionic and covalent bonds through clear examples and targeted practice.

Ontario Grade 11 Chemistry

Matter, Chemical Trends, and Chemical Bonding

Ontario Grade 11 Chemistry — Study topic B3.4

Some substances are described using ions, while others are described using molecules. A particle-level model helps explain the difference. In ionic bonding, electrons transfer between atoms and the resulting oppositely charged ions attract. In covalent bonding, atoms share pairs of electrons. This lesson focuses on comparing how these bonds form.

What you will learn

Explain how electron transfer leads to ionic bond formation.
Explain how sharing electron pairs leads to covalent bond formation.
Compare the particles and representations used for ionic and covalent bonding.

1. Start with atoms and valence electrons

Atoms contain positively charged protons and negatively charged electrons. In an atom with no overall charge, the number of protons equals the number of electrons. Electrons occupy regions around the nucleus; the outer occupied energy level is the one farthest from the nucleus that contains electrons.

Valence electrons are electrons in an atom’s outer occupied energy level. In the Grade 11 bonding model, these are the electrons involved in forming bonds. The periodic table can help you identify the number of valence electrons for many main-group elements. Atoms of different elements can have different numbers of valence electrons, though some different elements have the same number.

A bond is an attraction that holds particles together. To compare bond formation, track what happens to valence electrons. They can transfer from one atom to another, or atoms can share them.

Valence electrons are involved in the bonding models in this lesson.
A neutral atom has no overall charge.
Compare bonds by asking whether electrons transfer or are shared.

2. Ionic bonding: transfer creates ions

Table salt is a familiar example of an ionic substance. In the course-level model, sodium atoms transfer electrons to chlorine atoms. When an atom loses an electron, it has more protons than electrons and becomes a positive ion. When an atom gains an electron, it has more electrons than protons and becomes a negative ion.

An ion is an atom, or group of atoms, with an overall charge because it has gained or lost electrons. A sodium atom that loses one electron becomes a sodium ion. A chlorine atom that gains one electron becomes a chloride ion. The charge changes because electrons move; the number of protons does not change in this model.

The sodium and chloride ions have opposite charges, so they attract. This attraction between oppositely charged ions is an ionic bond. Keep the two parts of the explanation distinct: electron transfer forms the ions, and attraction between the ions is the bond.

Ionic substances are represented as repeating arrangements of ions, not as separate molecules. An ionic formula shows the simplest whole-number ratio of ions. For example, the formula for sodium chloride indicates a one-to-one ratio of sodium ions to chloride ions.

\mathrm{Na} \rightarrow \mathrm{Na^+} + e^-;\quad \mathrm{Cl} + e^- \rightarrow \mathrm{Cl^-}

Ionic bonding involves transfer of electrons and attraction between oppositely charged ions.
Losing electrons makes an ion positive; gaining electrons makes it negative.
An ionic formula represents an ion ratio, not a separate molecule.

3. Covalent bonding: atoms share electron pairs

A covalent bond forms when atoms share one or more pairs of valence electrons. Each shared pair is part of the bond between the atoms. In the course-level model, each atom counts the shared electrons as part of its outer level.

Covalent bonds commonly form between non-metal atoms. Unlike the ionic model, covalent bonding is not described as one atom transferring electrons to another to form oppositely charged ions. Instead, the atoms share electrons.

Two hydrogen atoms can form a hydrogen molecule by sharing one pair. Each atom contributes one electron to the shared pair. A line between the two symbols in a structural formula represents that shared pair. A structural formula is a representation that uses lines between atom symbols to show covalent bonds.

A Lewis diagram uses dots to represent valence electrons. A pair of dots between two atom symbols can represent a shared pair. The line in a structural formula and the pair of dots in a Lewis diagram are two ways to represent the same shared pair.

\mathrm{H} + \mathrm{H} \rightarrow \mathrm{H_2};\quad \mathrm{H-H}

Covalent bonding involves shared pairs of electrons.
A line between atom symbols can represent one shared pair.
In this model, the bonded particles are atoms in a molecule, not oppositely charged ions.

4. Compare the models and representations

Both models involve valence electrons, but the electrons behave differently in each description. In ionic bonding, electrons transfer from one atom to another. The resulting positive and negative ions attract. In covalent bonding, atoms share electron pairs.

The symbols also communicate different information. An ionic formula gives the simplest ratio of ions in the substance. A covalent molecular formula gives the kinds and numbers of atoms in a molecule. A structural formula can show which atoms are joined by shared pairs.

When asked to compare two bonds, explain the electron change and identify what holds the particles together. For an ionic bond, name the transfer and the attraction between ions. For a covalent bond, name the shared pair. These details explain bond formation more clearly than naming only the elements.

Ionic: electrons transfer; oppositely charged ions attract.
Covalent: atoms share one or more electron pairs.
The formula or diagram should match the particle model being described.

Comparing bond formation

Feature	Ionic bonding	Covalent bonding
Valence electrons	Transfer from one atom to another	Shared in pairs between atoms
Bonded particles in the model	Oppositely charged ions	Atoms in a molecule
What holds them together	Attraction between opposite charges	Shared electron pair between atoms
Common course-level pattern	Metal and non-metal	Non-metal atoms

Worked example

Compare sodium chloride and hydrogen bond formation

Use the course-level models to compare how sodium chloride and hydrogen form. State what happens to the electrons and what holds the particles together.

Track the transfer
A sodium atom transfers one electron to a chlorine atom. Sodium becomes a positive ion, and chlorine becomes a negative ion. The equations show the electron transfer for each atom.
$\mathrm{Na} \rightarrow \mathrm{Na^+} + e^-;\quad \mathrm{Cl} + e^- \rightarrow \mathrm{Cl^-}$
Identify the ionic bond
The sodium and chloride ions have opposite charges, so they attract. That attraction is the ionic bond. Their one-to-one ratio is represented by the formula for sodium chloride.
$\mathrm{Na^+ : Cl^-}$
Compare shared electrons
Two hydrogen atoms each contribute one electron to a shared pair. That shared pair forms a covalent bond. The line in the structural formula represents the pair; it does not represent electron transfer.
$\mathrm{H} + \mathrm{H} \rightarrow \mathrm{H_2};\quad \mathrm{H-H}$

Answer: Sodium chloride is described by electron transfer followed by attraction between oppositely charged ions. Hydrogen is described by two atoms sharing an electron pair. The ionic model focuses on ions and their attraction; the covalent model focuses on a shared pair between atoms.

Check: The comparison identifies both what happens to the electrons and what holds the particles together.

Common mistakes and how to avoid them

Calling electron transfer the ionic bond.

Correction: Transfer forms ions. The ionic bond is the attraction between the oppositely charged ions.

Saying that covalent bonding forms because one atom gives electrons to another.

Correction: In the covalent model, atoms share one or more pairs of electrons.

Treating an ionic formula as a separate molecule.

Correction: An ionic formula gives the simplest ratio of ions in the substance.

Assuming that every bond uses electrons in the same way.

Correction: Electrons transfer in the ionic model and are shared in the covalent model.

Lesson summary

Valence electrons are involved in both bonding models.
Ionic bond formation involves electron transfer and attraction between oppositely charged ions.
Covalent bond formation involves atoms sharing one or more pairs of electrons.
Ionic formulas show ion ratios; covalent formulas represent molecules, and structural lines can show shared pairs.

Check your understanding

Question 1

Which statement best compares ionic and covalent bond formation?

Ionic bonding involves electron transfer followed by attraction between oppositely charged ions; covalent bonding involves shared electron pairs.
Ionic bonding involves shared pairs; covalent bonding involves attraction between ions formed by electron transfer.
Both types form only when atoms transfer electrons and become ions.
Both types form only when atoms share electrons, but ionic formulas use lines instead of symbols.

Show answer and explanation

Ionic bonding involves electron transfer followed by attraction between oppositely charged ions; covalent bonding involves shared electron pairs.

The first choice describes electron transfer and ion attraction for ionic bonding, and shared pairs for covalent bonding.

Question 2

Two atoms each contribute one electron to a shared pair. Which bond does this model describe?

An ionic bond, because one atom receives both electrons.
A covalent bond, because the atoms share an electron pair.
An ionic bond, because every electron pair creates two ions.
Neither type, because valence electrons are not involved in bonding.

Show answer and explanation

A covalent bond, because the atoms share an electron pair.

A shared electron pair between atoms is the course-level model for a covalent bond.

Key terms

Valence electrons: Electrons in an atom’s outer occupied energy level that are involved in bonding in the course-level model.
Ion: An atom, or group of atoms, with an overall charge because it has gained or lost electrons.
Ionic bond: The attraction between oppositely charged ions.
Covalent bond: A bond formed when atoms share one or more pairs of electrons.
Lewis diagram: A diagram that uses dots to represent valence electrons.
Structural formula: A representation in which lines between atom symbols can show shared electron pairs.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation B3.4. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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