molecular geometry
A molecule is not a flat scribble on paper — it is a real three-dimensional object, like a tiny piece of sculpture. Molecular geometry is the description of that shape: where each atom sits in space relative to the others, and the angles between the bonds.
Precisely, molecular geometry is the arrangement of atoms in a molecule in three dimensions, captured by its bond lengths and bond angles. It is set by how the electron groups around each atom spread out to avoid one another, and is commonly named by shape: linear, trigonal planar, bent, tetrahedral, trigonal pyramidal, and so on. The geometry of the atoms can differ from the arrangement of all the electron groups because lone pairs take up room without adding a visible atom.
Geometry matters because shape is destiny in chemistry: it decides whether a molecule is polar, how it packs and flows, what it smells like, and whether it fits the lock of a biological receptor. The honest caveat is that real molecules are not rigid statues — bonds stretch and bend constantly with thermal motion, so a stated geometry is the average, equilibrium shape, not a frozen one.
Methane (CH₄) is tetrahedral: the four hydrogens sit at the corners of a tetrahedron around the central carbon, with H–C–H angles of 109.5°. Carbon dioxide (CO₂) is linear, with its three atoms in a straight line. Same kinds of atoms, very different shapes — and very different behavior.
Same atom types, different geometries — and different chemistry.
Electron-group geometry and molecular geometry are not always the same. The first counts all electron groups including lone pairs; the second names only the positions of the atoms. Water has tetrahedral electron-group geometry but a bent molecular geometry, because its two lone pairs are invisible in the atom-only shape.