JEE Mains Chemistry · Chemical Bonding and Molecular Structure

Shapes of Molecules and Ions

The shape of a molecule is named from its atoms only: fix the arrangement from the total electron pairs, then remove the lone pairs, so AX₃E is pyramidal and AX₂E₂ is bent.

All Chemical Bonding and Molecular Structure notes JEE Mains Chemistry strategy

Why this matters

Twenty-seven PYQs, nineteen of them multiple choice, and four from 2026. Seventeen match molecules and ions to their shapes or ask which species share a shape; ten count how many species in a list are linear, bent, pyramidal, square planar or some other shape. Two ideas cover the page.

Concept 1 of 2: Shapes from the AXE formula

Write the central atom as A, its bonded atoms as X and its lone pairs as E. The total X + E fixes how the pairs are arranged in space. The shape you name is what the atoms alone trace out, so the same arrangement gives different shapes as lone pairs replace atoms.

Definition

Find X (atoms bonded to the centre) and E (lone pairs on it); a double bond still counts as one X.
X + E = 2, 3, 4, 5, 6 gives a linear, trigonal planar, tetrahedral, trigonal bipyramidal or octahedral arrangement of pairs.
Isostructural species share a shape, whatever their formulas: $\mathrm{SO_4^{2-}}$ and $\mathrm{CrO_4^{2-}}$ (tetrahedral); $\mathrm{SiCl_4}$ and $\mathrm{TiCl_4}$ ; $\mathrm{SF_4}$ , $\mathrm{IF_4^+}$ and $\mathrm{XeO_2F_2}$ (see-saw).
$\mathrm{CH_4}$ , $\mathrm{NH_4^+}$ and $\mathrm{BH_4^-}$ are isoelectronic (10 electrons) and all tetrahedral.
Four-coordinate complexes: $\mathrm{[PtCl_4]^{2-}}$ , $\mathrm{[Ni(CN)_4]^{2-}}$ and $\mathrm{[Cu(NH_3)_4]^{2+}}$ are square planar; $\mathrm{[NiCl_4]^{2-}}$ , $\mathrm{[FeCl_4]^{2-}}$ and $\mathrm{Ni(CO)_4}$ are tetrahedral.
Aluminium dissolves in NaOH as the tetrahedral $\mathrm{[Al(OH)_4]^-}$ ion.

Type	Pairs (X + E)	Shape	Examples
AX₂	2	Linear	$\mathrm{BeCl_2}$ , $\mathrm{CO_2}$ , $\mathrm{NO_2^+}$ , $\mathrm{N_3^-}$ , $\mathrm{HC{\equiv}C^-}$
AX₃	3	Trigonal planar	$\mathrm{BF_3}$ , $\mathrm{SO_3}$ , $\mathrm{NO_3^-}$ , $\mathrm{CO_3^{2-}}$
AX₂E	3	Bent	$\mathrm{SO_2}$ , $\mathrm{O_3}$ , $\mathrm{NO_2^-}$
AX₄	4	Tetrahedral	$\mathrm{CH_4}$ , $\mathrm{NH_4^+}$ , $\mathrm{SO_4^{2-}}$ , $\mathrm{SO_2Cl_2}$
AX₃E	4	Trigonal pyramidal	$\mathrm{NH_3}$ , $\mathrm{H_3O^+}$ , $\mathrm{SO_3^{2-}}$ , $\mathrm{ClO_3^-}$ , $\mathrm{BrO_3^-}$ , $\mathrm{XeO_3}$
AX₂E₂	4	Bent	$\mathrm{H_2O}$ , $\mathrm{OF_2}$ , $\mathrm{ClO_2^-}$ , $\mathrm{BrF_2^+}$
AX₅	5	Trigonal bipyramidal	$\mathrm{PCl_5}$ , $\mathrm{PF_5}$ , $\mathrm{Fe(CO)_5}$
AX₄E	5	See-saw	$\mathrm{SF_4}$ , $\mathrm{SeF_4}$ , $\mathrm{IF_4^+}$ , $\mathrm{XeO_2F_2}$
AX₃E₂	5	T-shaped	$\mathrm{ClF_3}$ , $\mathrm{BrF_3}$ , $\mathrm{IF_3}$
AX₂E₃	5	Linear	$\mathrm{XeF_2}$ , $\mathrm{I_3^-}$ , $\mathrm{IBr_2^-}$
AX₆	6	Octahedral	$\mathrm{SF_6}$ , $\mathrm{[CrF_6]^{3-}}$
AX₅E	6	Square pyramidal	$\mathrm{BrF_5}$ , $\mathrm{IF_5}$ , $\mathrm{XeOF_4}$
AX₄E₂	6	Square planar	$\mathrm{XeF_4}$ , $\mathrm{ICl_4^-}$ , $\mathrm{BrF_4^-}$
AX₇	7	Pentagonal bipyramidal	$\mathrm{IF_7}$
AX₆E	7	Distorted octahedral	$\mathrm{XeF_6}$

Same total of pairs, same arrangement; the lone pairs decide the name of the shape.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 28 Jan 2026 Shift 2 · Q37Moderate

Example 1 · Chemical Bonding and Molecular Structure · Shapes of Molecules and Ions

Match List-I with List-II according to shape. List-I List-II (A)

{XeO}_{3}

(I)

{BrF}_{5}

(B)

{XeF}_{2}

(II)

{NH}_{3}

(C)

{XeO}_{2}{\text{ }F}_{2}

(III)

\left\lbrack I_{3} \right\rbrack^{-}

(D)

{XeOF}_{4}

(IV)

\ {SF}_{4}

Choose the correct answer from the options given below:

Name the shape from the atoms, not the pairs

\mathrm{NH_3}

has a tetrahedral arrangement of pairs, but its shape is trigonal pyramidal.

\mathrm{XeF_4}

has an octahedral arrangement, but it is square planar. Options often offer the arrangement as a decoy.

Same formula type, different shape

\mathrm{SF_4}

(see-saw) and

\mathrm{XeF_4}

(square planar) are both AF₄, and

\mathrm{PCl_5}

(trigonal bipyramidal) and

\mathrm{BrF_5}

(square pyramidal) are both AX₅ formulas. Count the lone pairs before naming the shape.

Drill 16 more on shapes from the axe formula

Concept 2 of 2: Counting species of one shape in a list

A list question is the AXE table used many times. Work through the list one species at a time, write X and E for each, and tick only those that match. Do not guess from the formula: the charge and the lone pairs change everything.

Definition

For each species: count the atoms bonded to the centre (X), find the lone pairs (E) from the steric number, name the shape.
Linear: AX₂ ( $\mathrm{BeCl_2}$ , $\mathrm{CO_2}$ , $\mathrm{C_3O_2}$ , $\mathrm{N_3^-}$ , $\mathrm{NO_2^+}$ ) or AX₂E₃ ( $\mathrm{XeF_2}$ , $\mathrm{I_3^-}$ ). Not linear: $\mathrm{I_3^+}$ (AX₂E₂), $\mathrm{BCl_2^-}$ (AX₂E), $\mathrm{SO_2}$ , $\mathrm{O_3}$ , $\mathrm{NO_2}$ , $\mathrm{F_2O}$ .
Planar species: any three-atom species, AX₃, AX₃E₂ (T-shaped) and AX₄E₂ (square planar). Pyramidal, tetrahedral, see-saw and bigger shapes are non-planar. $\mathrm{H_2O_2}$ is not planar: its two O–H bonds lie in different planes.
Trigonal bipyramidal needs AX₅ with no lone pair: $\mathrm{PF_5}$ , $\mathrm{PCl_5}$ , $\mathrm{Fe(CO)_5}$ , not $\mathrm{BrF_5}$ .

Worked example

How many of these are pyramidal:

\mathrm{NH_3}

\mathrm{BF_3}

\mathrm{SO_3^{2-}}

\mathrm{ClO_3^-}

\mathrm{NO_3^-}

\mathrm{PCl_3}

\mathrm{XeO_3}

\mathrm{SO_3}

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 22 Jan 2025 · Q48Moderate

Example 2 · Chemical Bonding and Molecular Structure · Shapes of Molecules and Ions

The number of molecules/ions that show linear geometry among the following is

\_\_\_\_\_

SO_{2},BeCl_{2},CO_{2},{\text{ }N}_{3}^{-},NO_{2},{\text{ }F}_{2}O,XeF_{2},NO_{2}^{+},I_{3}^{-},O_{3}

Charge changes the shape

\mathrm{I_3^-}

is linear (three lone pairs) but

\mathrm{I_3^+}

is bent (two lone pairs).

\mathrm{NO_2^+}

is linear but

\mathrm{NO_2^-}

is bent. Always put the charge into the steric number.

Two nickel complexes, two shapes

Strong-field

\mathrm{CN^-}

pairs up nickel's d electrons, so

\mathrm{[Ni(CN)_4]^{2-}}

dsp^2

and square planar. Weak-field

\mathrm{Cl^-}

does not, so

\mathrm{[NiCl_4]^{2-}}

sp^3

and tetrahedral.

Drill 9 more on counting species of one shape in a list

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (1)

Shapes from the AXE formula15 rows

Type	Pairs (X + E)	Shape	Examples
AX₂	2	Linear	$\mathrm{BeCl_2}$ , $\mathrm{CO_2}$ , $\mathrm{NO_2^+}$ , $\mathrm{N_3^-}$ , $\mathrm{HC{\equiv}C^-}$
AX₃	3	Trigonal planar	$\mathrm{BF_3}$ , $\mathrm{SO_3}$ , $\mathrm{NO_3^-}$ , $\mathrm{CO_3^{2-}}$
AX₂E	3	Bent	$\mathrm{SO_2}$ , $\mathrm{O_3}$ , $\mathrm{NO_2^-}$
AX₄	4	Tetrahedral	$\mathrm{CH_4}$ , $\mathrm{NH_4^+}$ , $\mathrm{SO_4^{2-}}$ , $\mathrm{SO_2Cl_2}$
AX₃E	4	Trigonal pyramidal	$\mathrm{NH_3}$ , $\mathrm{H_3O^+}$ , $\mathrm{SO_3^{2-}}$ , $\mathrm{ClO_3^-}$ , $\mathrm{BrO_3^-}$ , $\mathrm{XeO_3}$
AX₂E₂	4	Bent	$\mathrm{H_2O}$ , $\mathrm{OF_2}$ , $\mathrm{ClO_2^-}$ , $\mathrm{BrF_2^+}$
AX₅	5	Trigonal bipyramidal	$\mathrm{PCl_5}$ , $\mathrm{PF_5}$ , $\mathrm{Fe(CO)_5}$
AX₄E	5	See-saw	$\mathrm{SF_4}$ , $\mathrm{SeF_4}$ , $\mathrm{IF_4^+}$ , $\mathrm{XeO_2F_2}$
AX₃E₂	5	T-shaped	$\mathrm{ClF_3}$ , $\mathrm{BrF_3}$ , $\mathrm{IF_3}$
AX₂E₃	5	Linear	$\mathrm{XeF_2}$ , $\mathrm{I_3^-}$ , $\mathrm{IBr_2^-}$
AX₆	6	Octahedral	$\mathrm{SF_6}$ , $\mathrm{[CrF_6]^{3-}}$
AX₅E	6	Square pyramidal	$\mathrm{BrF_5}$ , $\mathrm{IF_5}$ , $\mathrm{XeOF_4}$
AX₄E₂	6	Square planar	$\mathrm{XeF_4}$ , $\mathrm{ICl_4^-}$ , $\mathrm{BrF_4^-}$
AX₇	7	Pentagonal bipyramidal	$\mathrm{IF_7}$
AX₆E	7	Distorted octahedral	$\mathrm{XeF_6}$

Same total of pairs, same arrangement; the lone pairs decide the name of the shape.

Watch out for (4)

Name the shape from the atoms, not the pairs
→ Shapes from the AXE formula
Same formula type, different shape
→ Shapes from the AXE formula
Charge changes the shape
→ Counting species of one shape in a list
Two nickel complexes, two shapes
→ Counting species of one shape in a list

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