Worked example 1
Draw the Lewis structure of N₂ and give the bond order.
Try it first: Count electrons and try single bonds first to see why they fail.
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Molecular StructureLewis structuresContent level: Core 20 min
Multiple Bonds and Polyatomic Ions
What you'll be able to do: Draw structures containing double and triple bonds, and handle the charge bookkeeping for polyatomic ions.
Best after: Drawing Lewis Structures Step by Step
Introduction
Nitrogen gas holds a triple bond and sulfate carries a 2- charge. Both are just the standard procedure with one extra habit.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Decide when a double or triple bond is required
- Relate bond order to bond length and bond strength
- Adjust the electron total for the charge on a polyatomic ion
- Draw a polyatomic ion with correct bracket and charge notation
Lesson
When a multiple bond is needed
After single bonds and terminal lone pairs are placed, a central atom that still has fewer than eight electrons needs a multiple bond. Bring in one lone pair from an outer atom for each pair of electrons the centre is missing: short by two means one double bond, short by four means either two double bonds or one triple bond.
Carbon, nitrogen and oxygen form multiple bonds readily. Halogens and hydrogen essentially never do in introductory work.
Bond order and its consequences
Bond order is the number of shared pairs between two atoms: 1 for a single bond, 2 for a double, 3 for a triple. Higher bond order pulls the nuclei closer, so the bond gets shorter and stronger. The C-C bond in ethane is longer and weaker than the C=C in ethene, which is longer and weaker again than the triple bond in ethyne.
Polyatomic ions
For an anion, add one electron per unit of negative charge before drawing. For a cation, remove one electron per unit of positive charge. Nitrate NO₃⁻ has 5 + 3(6) + 1 = 24 electrons, while ammonium NH₄⁺ has 5 + 4 - 1 = 8.
Always draw the finished ion inside square brackets with the charge as a superscript outside them. The charge belongs to the whole ion, not to one atom.
Checking a finished ion
Two checks catch nearly every error. First, the electrons drawn must equal the adjusted total. Second, the sum of the formal charges on all atoms must equal the overall charge on the ion, which is zero for a neutral molecule.
Key ideas
Definition
Bond order
The number of shared electron pairs between two bonded atoms.
Rule
Length and strength
As bond order increases, bond length decreases and bond energy increases.
Rule
Charge adjustment
Add electrons for negative charge, remove them for positive charge, before drawing.
Key concept
Nitrogen gas
N₂ has 10 valence electrons and needs a triple bond plus one lone pair on each nitrogen.
Equation
Charge check
sum of formal charges = overall charge on the species
Worked examples
Worked example 2
Draw the Lewis structure of the nitrate ion NO₃⁻.
Try it first: Adjust the electron total for the negative charge before you start.
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Common mistakes
Making a double bond to a terminal halogen or hydrogen.
Why it's wrong: Those atoms are complete with one single bond.
Check instead: Take the extra pair from oxygen, nitrogen or sulfur instead.
Writing the ionic charge on one atom instead of on the brackets.
Why it's wrong: The charge describes the whole ion, and individual atoms carry formal charges.
Check instead:
Forgetting that a triple bond and two double bonds both add four electrons to a centre.
Why it's wrong: Either can fix the same shortage, and other evidence decides which is correct.
Check instead: Use formal charges to choose between the options.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
A double bond shares two pairs and a triple bond shares three, so bond order rises while bond length falls and bond strength rises. Polyatomic ions follow the same drawing steps, with the charge added to or subtracted from the electron total and the finished structure enclosed in brackets with the charge written outside.
- Multiple bonds appear when the central atom is still short of an octet
- Bond order 1, 2 or 3 tracks with shorter and stronger bonds
- Adjust the electron total before drawing an ion
- Draw ions in brackets with the charge outside
- Formal charges must sum to the overall charge
Sources and further reading
- Chemistry 2e, Section 7.3: Lewis Symbols and StructuresOpenStax · Rice University · 7.3View source
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