What you'll learn
- What a single covalent bond is at the electron-pair level.
- How multiple bonds differ from single bonds.
- What makes a co-ordinate, or dative covalent, bond special.
- How to represent covalent bonds with lines and dative bonds with arrows.
Starting point: outer-shell electrons
Atoms contain electrons arranged in shells. The electrons in the outer shell are called outer-shell electrons or valence electrons. These are the electrons involved in bonding.
When non-metal atoms bond covalently, they usually share electrons so that each atom has a more stable outer-shell arrangement. At A-Level, it is important to focus on the electron-pair idea: covalent bonds are about shared pairs of electrons.
Valence electron
A valence electron is an electron in the outer shell of an atom. Valence electrons are the electrons that take part in chemical bonding.
Covalent bonding: sharing an electron pair
A covalent bond forms when two atoms share a pair of electrons. The shared pair is attracted to both positive nuclei, where a nucleus is the positively charged centre of an atom.
This attraction between the shared negative electrons and the two positive nuclei holds the atoms together.
Single covalent bond
A single covalent bond contains one shared pair of electrons between two atoms.
For example, in hydrogen, H2, each hydrogen atom contributes one electron. The two electrons form one shared pair, so there is one single covalent bond:
H–H
The line between the two atoms represents one shared pair of electrons.
Line means shared pair
In displayed formulae, one line represents one covalent bond, which means one shared pair of electrons.
Dot-and-cross diagrams and line diagrams
A dot-and-cross diagram shows the outer-shell electrons of atoms using dots for electrons from one atom and crosses for electrons from another. The dot and cross symbols help you see where the electrons originally came from.
A line diagram, or displayed formula, is quicker: each line represents one shared pair of electrons.
The diagram below compares a single covalent bond, a multiple covalent bond, and a dative covalent bond.
Converting shared pairs into lines
Show the covalent bonds in water, H2O, using lines.
- Oxygen is bonded to two hydrogen atoms, so the structure has two O–H bonds.
- Each O–H bond is one shared pair of electrons, so each shared pair is represented by one line.
- The displayed formula is H–O–H. The lone pairs on oxygen are usually not shown in a simple displayed formula unless the question asks for them.
Multiple covalent bonds
A multiple bond is a covalent bond containing more than one shared pair of electrons between the same two atoms.
- A double bond contains two shared pairs of electrons.
- A triple bond contains three shared pairs of electrons.
Multiple covalent bond
A multiple covalent bond contains multiple shared pairs of electrons between the same two atoms.
For example, oxygen molecules, O2, contain a double bond:
O=O
The double line represents two shared pairs of electrons.
Nitrogen molecules, N2, contain a triple bond:
N≡N
The triple line represents three shared pairs of electrons.
Counting lines
One line = one shared pair.
Two lines = two shared pairs.
Three lines = three shared pairs.
Identifying single, double, and triple bonds
A molecule contains two carbon atoms joined by three shared pairs of electrons. How should the bond between the carbon atoms be represented?
- The phrase “three shared pairs” tells you there are three covalent bonds between the same two atoms.
- Three shared pairs means a triple bond.
- The bond should be shown using three lines between the carbon atoms: C≡C.
Lone pairs: electron pairs not used in bonding
Not every pair of outer-shell electrons is shared in a covalent bond. Some electron pairs stay on one atom.
Lone pair
A lone pair is a pair of outer-shell electrons that is not shared in a covalent bond.
Lone pairs are especially important for dative covalent bonding, because one atom can use a lone pair to form a new bond.
For example, ammonia, NH3, has three N–H covalent bonds and one lone pair on the nitrogen atom.
Dative covalent bonding: one atom supplies both electrons
A co-ordinate bond, also called a dative covalent bond, is still a covalent bond because it contains a shared pair of electrons.
The special feature is how the shared pair forms: both electrons in the shared pair come from the same atom.
Co-ordinate or dative covalent bond
A co-ordinate bond, or dative covalent bond, contains a shared pair of electrons with both electrons supplied by one atom.
The atom that supplies both electrons must have a lone pair available. The atom or ion accepting the electron pair must be able to accept that pair.
A classic example is the reaction between ammonia, NH3, and a hydrogen ion, H+. The nitrogen atom in ammonia has a lone pair. The hydrogen ion has no electrons. Nitrogen donates both electrons from its lone pair to form a new N–H bond, producing NH4+.
Representing a dative bond with an arrow
A normal covalent bond is represented using a line:
N–H
A dative covalent bond is represented using an arrow:
N→H
The arrow starts at the atom that supplies both electrons and points towards the atom that accepts the electron pair.
Arrow direction
For a dative covalent bond, the arrow points from the electron-pair donor to the electron-pair acceptor.
Drawing the dative bond in ammonium
Show how NH3 forms NH4+ using a dative covalent bond.
- In NH3, nitrogen forms three ordinary covalent N–H bonds and has one lone pair remaining.
- H+ has no electrons, so it cannot contribute an electron to a normal shared pair.
- Nitrogen supplies both electrons from its lone pair to H+, forming a fourth N–H bond.
- Represent that newly formed bond as N→H, with the arrow pointing from nitrogen to hydrogen.
- Put the overall positive charge on the ion: NH4+.
Reversing the dative arrow
Do not draw H→N for the ammonium ion. The hydrogen ion does not supply the electron pair. Nitrogen supplies both electrons, so the arrow is N→H.
Is a dative bond different after it forms?
A dative covalent bond is different in its origin: both electrons came from one atom. However, after it has formed, it is still a covalent bond because the electron pair is shared between two atoms.
In many molecules or ions, the dative bond may behave just like the other covalent bonds. For example, in NH4+, the four N–H bonds are usually treated as equivalent once the ion has formed.
Thinking dative means ionic
A dative covalent bond is not an ionic bond. The electron pair is shared; the only unusual feature is that both electrons in the shared pair were supplied by one atom.
Another common example: H3O+
Water, H2O, has two lone pairs on oxygen. A hydrogen ion, H+, can accept one of these lone pairs.
This forms H3O+, often called the oxonium ion or hydronium ion. The dative bond can be shown as O→H.
Identifying the electron-pair donor in H3O+
In the formation of H3O+, identify the atom that donates the electron pair and state how to draw the dative bond.
- Oxygen in H2O has lone pairs, so it has an electron pair available to donate.
- H+ has no electrons, so it accepts the electron pair.
- The dative bond is drawn as O→H, because the arrow points from oxygen, the donor, to hydrogen, the acceptor.
Ordinary covalent vs dative covalent: the key comparison
Both ordinary covalent bonds and dative covalent bonds contain a shared pair of electrons. The difference is where the electrons came from when the bond formed.
- In an ordinary covalent bond, each atom usually contributes one electron to the shared pair.
- In a dative covalent bond, one atom contributes both electrons to the shared pair.
Best wording for definitions
For full marks, include both ideas: a dative bond is a shared pair of electrons, and both electrons are supplied by one atom.
Do not overuse arrows
Only use the arrow representation when you are specifically showing a co-ordinate/dative covalent bond. Ordinary covalent bonds should be shown with lines, not arrows.
In the exam
- For a covalent bond definition, write: “a shared pair of electrons” — not just “sharing electrons”.
- For a dative covalent bond, add: “both electrons are supplied by one atom”.
- When drawing a dative bond, make the arrow point from the atom with the lone pair to the atom or ion accepting the pair.
Check yourself
- What does one line between two atoms represent in a displayed formula?
- How many shared pairs are present in a double covalent bond?
- In NH4+, why is the dative bond drawn from nitrogen to hydrogen?
