In simple terms
A friendly intro before the formal notes — no formulas yet.
The Ultimate Electron Sharing Agreement
Atoms form covalent bonds by sharing electrons to achieve a more stable arrangement, like a full outer shell. A coordinate bond is just a special type of covalent bond where one atom generously provides both electrons for the share.
Imagine two students preparing for an exam. A normal covalent bond is like both students bringing their own textbook and sharing them to study together. A coordinate bond is like one student, who has already made comprehensive notes, lending them to another student who has no materials, so they can both pass the exam.
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Covalent bonds form when atoms share electron pairs.
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Each covalent bond = one shared pair (single bond) or multiple pairs (double/triple).
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Dative covalent bonds: both electrons come from one atom (e.g. NH₄⁺).
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Lewis structures show all valence electrons — dots or crosses per atom.
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Full topic notes
Formal explanation with the rigour you need for the exam.
The Nature of Covalent Bonding
A covalent bond is fundamentally an electrostatic force. The positively charged nuclei of two atoms are attracted to the negatively charged pair of electrons shared between them. This mutual attraction holds the atoms together at a specific distance known as the bond length. This creates a strong, localised bond that requires a significant amount of energy to break, which is why substances with simple molecular structures can still have very strong bonds within their molecules.
Representing Covalent Bonds: Dot-and-Cross Diagrams
To visualise the sharing of electrons, we use 'dot-and-cross' diagrams, also known as Lewis structures. In this model, we only consider the valence (outer shell) electrons. The electrons from one atom are represented by dots (•) and the electrons from the other atom by crosses (×). This convention makes it easy to see how many electrons each atom contributes to the bond and to check if each atom has achieved a stable outer shell.
Multiple Covalent Bonds
Sometimes, atoms need to share more than one pair of electrons to achieve a stable octet. Sharing two pairs of electrons results in a double bond (e.g., in oxygen, ), and sharing three pairs results in a triple bond (e.g., in nitrogen, ). As the number of shared pairs (the bond order) increases, the electrostatic attraction between the nuclei and the electron-dense region becomes stronger. This pulls the atoms closer together, resulting in a shorter and stronger bond. Therefore, a triple bond is stronger and shorter than a double bond, which is in turn stronger and shorter than a single bond.
Coordinate (Dative Covalent) Bonding
A coordinate bond is a special type of covalent bond where one atom provides both of the electrons in the shared pair. This typically occurs when an atom with a lone pair of electrons (an electron-pair donor) interacts with an atom or ion that has a vacant orbital (an electron-pair acceptor). A classic example is the formation of the ammonium ion, , when ammonia () reacts with a hydrogen ion ().
The nitrogen atom in ammonia has a lone pair of electrons. The hydrogen ion, , has lost its only electron, leaving it with an empty 1s orbital. The nitrogen atom donates its lone pair to the ion, forming a new N-H bond. Once formed, this coordinate bond is identical in every way to the other three N-H covalent bonds in the ion.
When drawing a dot-and-cross diagram for a species containing a coordinate bond, like , ensure you show both electrons for that specific bond coming from the donor atom (e.g., both as crosses if nitrogen is the 'cross' atom). Crucially, always enclose the final ion in square brackets and write the overall charge in the top right corner. Forgetting the brackets and charge is a common way to lose marks.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Draw a 'dot-and-cross' diagram for a molecule of methane, . Show outer shell electrons only.
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Identify valence electrons: Carbon is in Group 14, so it has 4 valence electrons (we'll use ×). Hydrogen is in Group 1, so it has 1 valence electron (we'll use •).
A molecule of carbon dioxide, , contains two carbon-oxygen double bonds. Draw its 'dot-and-cross' diagram.
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Valence electrons: Carbon (Group 14) has 4 valence electrons (×). Oxygen (Group 16) has 6 valence electrons (•).
The reaction between boron trifluoride, , and ammonia, , forms an addition compound, . Explain how a coordinate bond is formed and draw the dot-and-cross diagram for the product.
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Identify donor and acceptor: Ammonia () has a lone pair on the nitrogen atom, so it can act as an electron-pair donor. Boron trifluoride () is electron deficient; the boron atom only has 6 electrons in its outer shell and has a vacant p-orbital. It can act as an electron-pair acceptor.
How it all connects
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Glossary
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Quick check
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Revision flashcards
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What is a covalent bond?
The strong electrostatic attraction between a shared pair of electrons and the nuclei of the two bonded atoms.
Key takeaways
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Describe a covalent bond as the strong electrostatic attraction between a shared pair of electrons and the nuclei of bonded atoms.
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Draw 'dot-and-cross' diagrams to represent the valence electrons in simple covalent molecules, including those with multiple bonds.
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Explain and represent coordinate (dative covalent) bonding, where one atom provides both electrons to a shared pair.
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Understand the relationship between the number of shared electron pairs (bond order), bond length, and bond strength.
Practice — then mark it
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Test Your Knowledge on Covalent Bonding
Test Your Knowledge on Covalent Bonding
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