Molecular Geometry

📖 Learn

Molecular geometry (or molecular shape) describes the three-dimensional arrangement of atoms in a molecule. The shape of a molecule affects its physical and chemical properties, including polarity, reactivity, and biological function.

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs around a central atom repel each other and arrange themselves to minimize repulsion. This arrangement determines the molecular geometry.

Key principles of VSEPR theory:

Electron pairs (bonding and lone pairs) repel each other
Lone pairs repel more strongly than bonding pairs
Atoms arrange to maximize the distance between electron pairs
The number of electron domains determines the electron geometry

Electron Domain

An electron domain is a region around the central atom where electrons are likely to be found. Each single bond, double bond, triple bond, or lone pair counts as one electron domain.

Electron Domain Geometries

# Electron Domains	Electron Geometry	Bond Angles
2	Linear	180 degrees
3	Trigonal Planar	120 degrees
4	Tetrahedral	109.5 degrees
5	Trigonal Bipyramidal	90 and 120 degrees
6	Octahedral	90 degrees

Common Molecular Geometries

The molecular geometry depends on both the number of bonding pairs and lone pairs around the central atom.

Electron Domains	Bonding Pairs	Lone Pairs	Molecular Geometry	Example
2	2	0	Linear	CO2, BeCl2
3	3	0	Trigonal Planar	BF3, SO3
3	2	1	Bent (Angular)	SO2, O3
4	4	0	Tetrahedral	CH4, CCl4
4	3	1	Trigonal Pyramidal	NH3, PH3
4	2	2	Bent (Angular)	H2O, H2S

Effect of Lone Pairs on Bond Angles

Lone pairs occupy more space than bonding pairs because they are attracted only to one nucleus. This causes them to repel bonding pairs more strongly, reducing bond angles:

Tetrahedral (no lone pairs): 109.5 degrees
Trigonal pyramidal (1 lone pair): about 107 degrees
Bent (2 lone pairs): about 104.5 degrees

Steps to Determine Molecular Geometry

Draw the Lewis structure for the molecule
Count the total number of electron domains around the central atom (bonds + lone pairs)
Determine the electron geometry based on the number of domains
Identify the molecular geometry based on the arrangement of atoms only (ignore lone pairs)
Predict bond angles, adjusting for lone pair effects

Molecular Polarity

Molecular geometry affects whether a molecule is polar or nonpolar:

Polar Molecule

A polar molecule has an uneven distribution of electron density, resulting in a net dipole moment. This occurs when polar bonds are arranged asymmetrically.

Nonpolar Molecule

A nonpolar molecule has an even distribution of electron density with no net dipole moment. This can occur with nonpolar bonds OR when polar bonds are arranged symmetrically so that dipoles cancel.

Molecule	Geometry	Bond Polarity	Molecular Polarity	Reason
CO2	Linear	Polar C=O bonds	Nonpolar	Symmetrical; dipoles cancel
H2O	Bent	Polar O-H bonds	Polar	Asymmetrical; dipoles don't cancel
CCl4	Tetrahedral	Polar C-Cl bonds	Nonpolar	Symmetrical; dipoles cancel
NH3	Trigonal pyramidal	Polar N-H bonds	Polar	Asymmetrical; dipoles don't cancel

SAT/ACT Connection

Science sections may present molecular diagrams and ask you to predict properties like polarity or bond angles. Remember that molecular shape directly affects properties: polar molecules dissolve in polar solvents, and molecular geometry determines how molecules interact with each other.

💡 Examples

Example 1: Determining Geometry of Methane (CH4)

Problem: Determine the molecular geometry of methane (CH4).

Solution:

Step 1: Draw Lewis structure. Carbon has 4 valence electrons, each hydrogen has 1. Total = 4 + 4(1) = 8 electrons.

Step 2: Carbon forms 4 single bonds to hydrogen atoms (no lone pairs).

Step 3: Count electron domains: 4 bonding pairs, 0 lone pairs = 4 domains.

Step 4: With 4 electron domains, the electron geometry is tetrahedral.

Step 5: With all 4 domains being bonding pairs, molecular geometry is also tetrahedral.

Answer: Methane has a tetrahedral geometry with bond angles of 109.5 degrees.

Example 2: Determining Geometry of Water (H2O)

Problem: Determine the molecular geometry of water (H2O).

Solution:

Step 1: Draw Lewis structure. O has 6 valence electrons, each H has 1. Total = 8 electrons.

Step 2: Oxygen forms 2 bonds to hydrogen atoms and has 2 lone pairs.

Step 3: Count electron domains: 2 bonding pairs + 2 lone pairs = 4 domains.

Step 4: With 4 electron domains, electron geometry is tetrahedral.

Step 5: With 2 bonding pairs and 2 lone pairs, molecular geometry is bent.

Step 6: Lone pairs compress the bond angle from 109.5 degrees to about 104.5 degrees.

Answer: Water has a bent (angular) molecular geometry with a bond angle of about 104.5 degrees.

Example 3: Determining Geometry of Ammonia (NH3)

Problem: What is the molecular geometry of ammonia (NH3)?

Solution:

Step 1: Draw Lewis structure. N has 5 valence electrons, each H has 1. Total = 8 electrons.

Step 2: Nitrogen forms 3 bonds to hydrogen atoms and has 1 lone pair.

Step 3: Count electron domains: 3 bonding pairs + 1 lone pair = 4 domains.

Step 4: With 4 electron domains, electron geometry is tetrahedral.

Step 5: With 3 bonding pairs and 1 lone pair, molecular geometry is trigonal pyramidal.

Step 6: The lone pair compresses bond angles from 109.5 degrees to about 107 degrees.

Answer: Ammonia has a trigonal pyramidal geometry with bond angles of about 107 degrees.

Example 4: Comparing CO2 and H2O Polarity

Problem: Explain why CO2 is nonpolar but H2O is polar, even though both contain polar bonds.

Solution:

CO2:

Has 2 electron domains (2 double bonds, no lone pairs)
Linear geometry (180 degrees)
The two C=O dipoles point in opposite directions
Dipoles cancel out, resulting in no net dipole moment

H2O:

Has 4 electron domains (2 bonds + 2 lone pairs)
Bent geometry (about 104.5 degrees)
The two O-H dipoles point in different directions at an angle
Dipoles don't cancel; there is a net dipole moment

Answer: CO2 is nonpolar because its linear shape causes polar bond dipoles to cancel. H2O is polar because its bent shape prevents dipole cancellation.

Example 5: Predicting Geometry of BF3

Problem: Determine the electron geometry, molecular geometry, and polarity of BF3.

Solution:

Step 1: Lewis structure: B has 3 valence electrons, each F has 7. Total = 24 electrons.

Step 2: Boron forms 3 single bonds to fluorine (B is an exception to the octet rule with only 6 electrons).

Step 3: Electron domains: 3 bonding pairs, 0 lone pairs = 3 domains.

Step 4: Electron geometry: Trigonal planar

Step 5: Molecular geometry: Trigonal planar (same, since no lone pairs)

Step 6: Bond angles: 120 degrees

Step 7: Polarity: Each B-F bond is polar, but the symmetrical arrangement cancels dipoles.

Answer: BF3 has trigonal planar geometry with 120-degree bond angles and is nonpolar due to its symmetry.

✏️ Practice

1. What is the molecular geometry of a molecule with 4 electron domains and 2 lone pairs?

A) Tetrahedral

B) Trigonal pyramidal

C) Bent

D) Linear

2. According to VSEPR theory, electron pairs arrange themselves to:

A) Maximize attraction

B) Minimize repulsion

C) Form the strongest bonds

D) Share electrons equally

3. What is the bond angle in a tetrahedral molecule?

A) 90 degrees

B) 104.5 degrees

C) 109.5 degrees

D) 120 degrees

4. Which molecule has a trigonal pyramidal shape?

A) CH4

B) NH3

C) H2O

D) CO2

5. Why does water have a bond angle of about 104.5 degrees instead of 109.5 degrees?

A) The O-H bonds are weak

B) Lone pairs repel more strongly than bonding pairs

C) Hydrogen atoms are very small

D) Water is a polar molecule

6. Which of the following molecules is polar?

A) CO2 (linear)

B) CCl4 (tetrahedral)

C) NH3 (trigonal pyramidal)

D) BF3 (trigonal planar)

7. A molecule with 3 electron domains and no lone pairs has what geometry?

A) Linear

B) Bent

C) Trigonal planar

D) Tetrahedral

8. How many electron domains does a triple bond count as?

A) 1

B) 2

C) 3

D) 6

9. What is the electron geometry of a molecule with 2 bonding pairs and 2 lone pairs?

A) Linear

B) Bent

C) Tetrahedral

D) Trigonal planar

10. Which factor does NOT affect the shape of a molecule?

A) Number of bonding pairs

B) Number of lone pairs

C) Atomic mass of atoms

D) Repulsion between electron pairs

Click to reveal answers

C - 4 domains with 2 bonds and 2 lone pairs gives a bent molecular geometry.
B - VSEPR theory is based on electron pairs arranging to minimize repulsion.
C - Tetrahedral geometry has bond angles of 109.5 degrees.
B - NH3 has 4 electron domains (3 bonds + 1 lone pair), giving trigonal pyramidal shape.
B - Lone pairs occupy more space and repel bonding pairs more strongly, compressing bond angles.
C - NH3 is polar because its pyramidal shape creates an asymmetric charge distribution.
C - 3 electron domains with no lone pairs results in trigonal planar geometry.
A - Any bond (single, double, or triple) counts as one electron domain.
C - The electron geometry is determined by total domains (4), which is tetrahedral. The molecular geometry (what atoms look like) would be bent.
C - Atomic mass does not affect molecular geometry; electron pair arrangements do.

✅ Check Your Understanding

Question 1: Explain the difference between electron geometry and molecular geometry. Why can they be different?

Reveal Answer

Electron geometry describes the arrangement of all electron domains (both bonding pairs and lone pairs) around the central atom. Molecular geometry describes only the arrangement of atoms (ignoring lone pairs). They differ when lone pairs are present because lone pairs take up space and affect the electron geometry but are "invisible" in the molecular geometry. For example, water (H2O) has tetrahedral electron geometry (4 domains) but bent molecular geometry because only the 2 hydrogen atoms are visible in the shape.

Question 2: Both CH4 and NH3 have 4 electron domains around the central atom. Explain why they have different molecular geometries and different bond angles.

Reveal Answer

Both molecules have tetrahedral electron geometry (4 domains), but their molecular geometries differ due to lone pairs. CH4 has 4 bonding pairs and 0 lone pairs, giving it tetrahedral molecular geometry with 109.5-degree bond angles. NH3 has 3 bonding pairs and 1 lone pair, giving it trigonal pyramidal molecular geometry. The lone pair on nitrogen repels the bonding pairs more strongly than they repel each other, compressing the H-N-H bond angles to about 107 degrees.

Question 3: A student predicts that because PCl5 has 5 bonding pairs and no lone pairs, all the bond angles must be equal. Is this correct? Explain.

Reveal Answer

The student is incorrect. PCl5 has trigonal bipyramidal geometry, which actually has two different types of positions: equatorial (3 positions in a plane) and axial (2 positions above and below the plane). Equatorial bond angles are 120 degrees (between equatorial positions) and 90 degrees (between equatorial and axial positions). The axial positions are 180 degrees apart. So not all bonds are equivalent - there are two distinct environments in a trigonal bipyramidal geometry.

Question 4: Carbon tetrachloride (CCl4) has polar C-Cl bonds, yet the molecule is nonpolar. Carbon monoxide (CO) has only one bond, and the molecule is polar. Using your understanding of molecular geometry, explain these observations.

Reveal Answer

Molecular polarity depends on both bond polarity AND molecular geometry. CCl4 has a tetrahedral geometry with 4 equivalent polar C-Cl bonds pointing to the corners of a tetrahedron. Because of this symmetry, the bond dipoles cancel out completely (they pull in opposite directions), resulting in no net dipole moment - the molecule is nonpolar. CO has only one bond, so there's no possibility of cancellation. The electronegativity difference between C and O creates a permanent dipole, making CO polar. This demonstrates that symmetric molecules with polar bonds can be nonpolar overall, while asymmetric molecules or those with only one bond will be polar if the bond is polar.

🚀 Next Steps

Review any concepts that felt challenging
Move on to the next lesson when ready
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