How Molecular Geometry Works

Quick Answer

Molecular geometry is the arrangement of bonded atoms around a central atom, assigned by VSEPR from the steric number. Steric number is bonding pairs plus lone pairs. That integer picks the electron geometry: 2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral. Molecular shape then ignores the lone pairs, so water is tetrahedral in its electrons and bent in its atoms at 104.5°. Set the two counts here and watch the two names come apart.

What Is Molecular Geometry?

It is the shape the bonded atoms make around a central atom. The formula does not print this. You count the groups on the central atom and read a table. Water looks bent, methane looks tetrahedral, carbon dioxide looks linear, and those three facts decide polarity, boiling point, and a lot of exam marks. The calculator on this page is that table, drawn as a molecule you can turn.

• The shape is of the atoms, not of every electron group.
• A polar bond does not make a polar molecule if the shape cancels the dipoles, which is the CO2 case.

What Does VSEPR Actually Say?

Valence shell electron pair repulsion says electron groups push each other as far apart as they can. Two groups sit 180° apart, three sit 120° apart in a plane, four sit at 109.5° as a tetrahedron, five as a trigonal bipyramid, six as an octahedron. That is the electron geometry. VSEPR theory is that list, plus the rule that lone pairs occupy sites but do not count as atoms when you name the shape.

• Used as a VSEPR calculator, this page is steric number in, both names out.
• The five electron geometries cover steric numbers 2 through 6, which is the whole first-course table.

How Do You Calculate Steric Number?

Add the bonding pairs to the lone pairs on the central atom. Carbon dioxide has two bonds and no lone pairs, so 2. Water has two bonds and two lone pairs, so 4. Sulfur hexafluoride has six bonds and no lone pairs, so 6. Double and triple bonds still count as one group. That is the whole calculation, and it is the step a steric number calculator exists to stop people skipping.

• Bonding pairs are X in AXE notation. Lone pairs are E. Steric number is X plus E.
• For example, ammonia is AX3E: three bonds, one lone pair, steric number 4.

Electron Geometry vs Molecular Geometry

They are two names for two counts. Electron geometry vs molecular geometry is the comparison this page is built around. Electron geometry includes lone pairs. Molecular geometry, also called molecular shape, does not. Methane is tetrahedral both ways. Water shares that electron geometry and does not share that shape: the atoms are bent. A static chart that prints one word for water is picking a side and not telling you.

• Electron geometry vs molecular shape is the same split under a slightly different name.
• XeF4 is the inorganic version: octahedral electrons, square planar atoms.
• When the two names match, this page stays quiet. When they split, it says so.

How Do You Find The Bond Angle From VSEPR?

Start from the ideal angle of the electron geometry, then compress it if lone pairs are present. Linear is 180°, trigonal planar 120°, tetrahedral 109.5°, trigonal bipyramidal and octahedral 90° (with 120° in the TBP equator). On a tetrahedral centre each lone pair knocks about 2.5° off, so ammonia is 107° and water is 104.5°. A bond angle calculator that always returns the ideal is returning the electron geometry.

• Methane really is 109.5°, because it has no lone pair to compress the angle.
• SO2, bent with one lone pair on a trigonal-planar centre, sits just under 120°.

Why Is Water Bent And Carbon Dioxide Linear?

Both have two bonds from the central atom. Carbon has no lone pairs, so two groups sit 180° apart. Oxygen has two lone pairs as well, so four groups sit tetrahedral and the hydrogens occupy two of those sites. Same count of bonds, different count of groups, opposite shapes. That is also why CO2 has no dipole and water does: two polar bonds cancel in a line and add in a bent molecule.

• Open Water, bent and CO2, linear as a pair. The bonding-pair slider does not move. The lone-pair slider does.
• Ammonia is the in-between case: one lone pair, trigonal pyramidal, 107°.

Where Does The Shape Get Used?

Polarity is the everyday answer. A molecule with polar bonds is polar only if the shape does not cancel the dipoles. Work out the bonds first on the chemical bond calculator, then come here for the shape. The same split decides why methane is a gas with no dipole, why water is a bent liquid that hydrogen-bonds, and why xenon tetrafluoride is square planar rather than tetrahedral.

• Exam questions that ask for molecular geometry want the atom shape, not the electron geometry.
• If a bond dipole can cancel, you cannot stop at the bond. You need this page as well.

Common Mistakes