C. Structure and Properties of Matter · Structure and Properties of Matter

VSEPR Theory: Expanded Octets with Five and Six Electron Groups

C2.3About 70 minutes

Learning goals

I can identify molecules with incomplete octets, odd electrons and expanded valence shells.
I can draw Lewis structures with more than eight electrons around a central atom.
I can predict the shapes of molecules and ions with five or six electron groups.
I can explain where lone pairs go in trigonal bipyramidal and octahedral arrangements.

Words to know

expanded valence shell
More than eight electrons around a central atom; possible only for atoms in period 3 and beyond.

axial / equatorial
The two positions along the axis / the three positions around the middle of a trigonal bipyramid.

trigonal bipyramidal
The arrangement of five electron groups: three at 120° in a plane, two above and below at 90°.

octahedral
The arrangement of six electron groups, all at 90° to their neighbours.

free radical
A molecule or atom with an unpaired electron, such as NO₂.

Breaking the octet rule

Sulfur hexafluoride, SF₆, is a heavy, unreactive gas used to insulate high-voltage electrical equipment. Its sulfur atom is bonded to six fluorine atoms, which means 12 electrons around sulfur. That breaks the Grade 11 "octet rule", and it is perfectly real.

Central atoms from period 3 and below (P, S, Cl, Br, I, Xe and others) can have more than eight electrons around them; this is called an expanded valence shell (or expanded octet). Period 2 atoms (C, N, O, F) never do, because level 2 has only four orbitals (2s and 2p). With five or six electron groups, VSEPR predicts some new and beautiful shapes.

Other octet exceptions

Incomplete octets: boron and beryllium compounds often have fewer than 8 electrons around the central atom: BF₃ (6 around B), BeCl₂ as a gas (4 around Be).
Odd-electron molecules: NO and NO₂ have an odd number of valence electrons, so one atom must have an unpaired electron. Such species are called free radicals and are usually very reactive. (NO₂ is the brown gas in smog.)

Drawing a Lewis structure with an expanded octet

Count the total valence electrons (adjust for charge).
Connect each outer atom to the central atom with a single bond.
Complete the octets of the outer atoms.
Put any remaining electrons on the central atom as lone pairs, even if that gives it more than 8.

Five electron groups: the trigonal bipyramid

Five groups point to the corners of a trigonal bipyramid: three equatorial positions in a triangle (120° apart) and two axial positions above and below (90° from the equator). The positions are not equivalent: an equatorial group has only two neighbours at 90°, while an axial group has three. Lone pairs need more room, so lone pairs always go equatorial.

Five electron groups form a trigonal bipyramid: three equatorial positions at 120° and two axial positions at 90° to them. Lone pairs always take equatorial positions, giving seesaw (one lone pair) and T-shaped (two lone pairs) molecules.

Six electron groups: the octahedron

Six groups point to the corners of an octahedron (two square pyramids base to base). All six positions are equivalent and 90° from their neighbours. The first lone pair can go anywhere; a second lone pair goes opposite the first (180° away) to keep them as far apart as possible.

Groups	Notation	Lone pairs	Molecular shape	Angles	Examples
5	AX₅	0	trigonal bipyramidal	90°, 120°	PCl₅, PF₅
5	AX₄E	1	seesaw	about 90° and less than 120°	SF₄, IF₄⁺
5	AX₃E₂	2	T-shaped	about 90°	ClF₃, BrF₃
5	AX₂E₃	3	linear	180°	XeF₂, I₃⁻
6	AX₆	0	octahedral	90°	SF₆
6	AX₅E	1	square pyramidal	about 90°	BrF₅, IF₅
6	AX₄E₂	2	square planar	90°	XeF₄, ICl₄⁻

Shapes for central atoms with five or six electron groups.

Six electron groups point to the corners of an octahedron, all 90° apart. One lone pair gives a square pyramid; two lone pairs sit opposite each other (180° apart), leaving the four atoms in a flat square.

Worked example

Example 1: Xenon tetrafluoride

Noble gases were once thought to form no compounds, but XeF₄ was made in 1962. Predict its shape.

Valence electrons: Xe 8 + 4 F × 7 = 36.
Four Xe–F bonds use 8 electrons; three lone pairs on each F use 24. Total so far: 32.
The remaining 4 electrons go on Xe as 2 lone pairs. Xe has 6 electron groups: AX₄E₂.
Octahedral arrangement; the two lone pairs are opposite each other.

XeF₄ is square planar, with F–Xe–F angles of 90°.

Worked example

Example 2: Triiodide ion

Predict the shape of I₃⁻ (found in the brown iodine solutions used to test for starch).

Valence electrons: 3 × 7 + 1 = 22.
Central I bonded to two outer I atoms: 4 electrons. Outer I atoms each get 3 lone pairs: 12. Total so far: 16.
6 electrons remain on the central I: 3 lone pairs. So 5 electron groups: AX₂E₃.
The three lone pairs take the three equatorial positions; the two I atoms are axial, 180° apart.

I₃⁻ is linear (180°).

Worked example

Example 3: Sulfur tetrafluoride

Predict the shape of SF₄ and explain where the lone pair goes.

Valence electrons: 6 + 4 × 7 = 34.
Four S–F bonds (8) + 12 lone pairs on F (24) = 32. 2 electrons remain: one lone pair on S. AX₄E.
Five groups → trigonal bipyramidal arrangement. In an equatorial position, the lone pair has only two neighbours at 90° (axial would give three), so it goes equatorial.

SF₄ is a seesaw shape, with the lone pair in an equatorial position.

Placing lone pairs

After you count electron groups, remember: 5 groups → lone pairs equatorial; 6 groups → second lone pair opposite the first. Then name the shape from the atoms only.

Just for fun

For decades, textbooks called the noble gases "inert". Then in 1962 Neil Bartlett, a chemist at the University of British Columbia, made the first noble-gas compound, a xenon compound. Within months, other labs had made XeF₄. Xenon had simply been waiting for someone to ask nicely.

Hands-on activity

Gumdrop and toothpick shapes

You need: about 30 gumdrops or mini marshmallows, or small balls of modelling clay, in two or three colours; a box of toothpicks; a protractor. Grown-up note: toothpicks are sharp; supervise younger siblings and keep them away from small children.

Use one colour for the central atom and another for outer atoms. Use a toothpick with a marshmallow of a third colour (or an empty toothpick) for a lone pair.
Build a trigonal bipyramid (PCl₅): three toothpicks in a flat triangle around the centre, one straight up, one straight down.
Replace one equatorial atom with a lone pair: that is SF₄ (seesaw). Now try replacing an axial atom instead and count how many neighbours are at 90° in each case.
Build an octahedron (SF₆): toothpicks along +x, −x, +y, −y, +z, −z.
Replace two opposite positions with lone pairs: XeF₄ (square planar). Measure an F–Xe–F angle with the protractor.

What to look for: in step 3, an equatorial lone pair has two neighbours at 90°; an axial one would have three, which is why lone pairs go equatorial. In step 5 the four remaining atoms lie flat in a square at 90°. Snap a photo of each model and label its AXₘEₙ notation.

Your turn

Which period 2 element could never have an expanded octet: P, S, N, Cl?

Predict the shape of PF₅.

Predict the shape of BrF₃.

Predict the shape of XeF₂.

Predict the shape of ICl₄⁻.

Why do lone pairs occupy equatorial positions in a trigonal bipyramid?

Predict the shape of BrF₅.

More practice: warm-up

How many electron groups give an octahedral arrangement?

What are the bond angles in an octahedral molecule?

How many electrons surround S in SF₆?

Name the shape of AX₄E₂.

Name the shape of AX₅.

More practice: on your own

Predict the shape of IF₅.

Predict the shape of the IF₄⁺ ion.

BeCl₂ (as a gas) has only 4 electrons around Be. Predict its shape.

Why is NO₂ called a free radical?

Explain why XeF₄ is square planar rather than seesaw-like.

Challenge

Predict the shape of the XeO₄ molecule, given that all four Xe–O bonds can be drawn as double bonds with no lone pairs on Xe.

ClF₃ is T-shaped. A student predicted "trigonal planar" because there are three F atoms. Explain the error.

Phosphorus forms PCl₅, but nitrogen (same group) only forms NCl₃, never NCl₅. Explain.

Check your understanding

Quiz

What is the shape of SF₆?

Which element can have an expanded valence shell?

What is the shape of XeF₂?

In a trigonal bipyramidal arrangement, where do lone pairs go?

Which AXₘEₙ type is square planar?

What is the shape of ClF₃?

For parents and teachers

Big idea (C2.3): VSEPR works for any number of electron groups. With five groups lone pairs go equatorial; with six, a second lone pair goes opposite the first. Expanded octets occur only for central atoms in period 3 or below.

How to help: use the gumdrop or clay models; 3-D shapes are much easier to see in your hands than on paper. Ask your teen to explain where the lone pairs went and why.

Common difficulties:

Forgetting to put leftover electrons on the central atom.
Counting atoms instead of electron groups (ClF₃ is not trigonal planar).
Trying to give period 2 atoms expanded octets.