Chemistry · Reference Sheet

Molecular Geometry

Electrons in bonds and lone pairs repel and push apart. The shape is the arrangement that gives them the most room.

Draw the Lewis structure. Find the central atom.

Count surrounding atoms on the central atom. Count an attached atom once, even with a double or triple bond.

Count lone pairs on the central atom only. Lone pairs on surrounding atoms do not count.

Match your two counts to a row below.

Molecular geometry	Surrounding atoms	Lone pairs on the central atom	Examples
Linear 180° The bonds push to opposite sides, making a line.	2	0	CO₂carbon dioxide HCNhydrogen cyanide
Trigonal planar 120° · flat Three bonds push apart evenly, making a flat triangle.	3	0	HCHOformaldehyde COCl₂carbonyl chloride
Tetrahedral 109.5° · 3D Four bonds need more room, so the bonds push into 3D.	4	0	CH₄methane CCl₄carbon tetrachloride
Trigonal pyramidal 107° · 3D The lone pair pushes the three bonds down, making a pyramid.	3	1	NH₃ammonia PBr₃phosphorus tribromide
Bent 104.5° Two lone pairs push on the two bonds, leaving a bent shape.	2	2	H₂Owater H₂Shydrogen sulfide

Lone pairs push too. Bonds contain electrons, and lone pairs also contain electrons. Trigonal pyramidal and bent start from the same four directions as tetrahedral, but lone pairs take some of the spots and push the bonds closer together. lone pairs on the central atom lone pairs on surrounding atoms (do not count)