🧬 Teoria VSEPR — przewidywanie geometrii molekularnej

VSEPR (Valence Shell Electron Pair Repulsion) theory predicts the 3D arrangement of atoms around a central atom in a molecule. Its core principle is that all electron pairs surrounding a central atom — whether they are shared in a bond or sitting alone as a lone pair — repel each other and arrange themselves in space to be as far apart as possible, minimizing that repulsion.

Electron-pair geometry describes the arrangement of all electron pairs (bonding and lone) around the central atom. Molecular geometry describes only the positions of the bonded atoms, ignoring the lone pairs' own positions even though their repulsion still shapes the arrangement. When lone pairs are present, the two geometries diverge — for example, four total pairs with one lone pair still have tetrahedral electron-pair geometry, but the molecular geometry is trigonal pyramidal, like ammonia (NH₃).

A lone pair's electron density is held close to a single nucleus, while a bonding pair is shared and pulled between two nuclei, spreading it out more. That makes lone pairs more spatially diffuse and more repulsive toward neighboring electron pairs. The resulting order of repulsion strength is lone pair–lone pair > lone pair–bond pair > bond pair–bond pair, which is why each additional lone pair squeezes the remaining bond angles a little tighter.

Hybridization is the mixing of a central atom's atomic orbitals into a new set of equivalent hybrid orbitals that match the number of electron-pair domains predicted by VSEPR. Four electron pairs correspond to sp³ hybridization (tetrahedral electron-pair geometry), five pairs to sp³d (trigonal bipyramidal), and six pairs to sp³d² (octahedral), and so on.

Symmetric geometries with identical substituents and no lone pairs on the central atom — like the linear CO₂ or tetrahedral CH₄ — have their individual bond dipoles cancel out, making the molecule nonpolar overall. Asymmetric geometries, or ones where lone pairs break the symmetry of the arrangement, usually leave a net dipole and are polar, as in bent H₂O or trigonal pyramidal NH₃.

Water's oxygen atom has four electron pairs — two bonding pairs to hydrogen and two lone pairs — giving it tetrahedral electron-pair geometry. Naming only the positions of the two hydrogen atoms gives a bent molecular geometry, and the two lone pairs' extra repulsion pushes the remaining H-O-H bond angle down from the idealized 109.5° to roughly 104.5°.

Both nitrogen in NH₃ and carbon in CH₄ have four electron pairs and tetrahedral electron-pair geometry. But methane's four pairs are all bonding pairs, so its bond angles stay at the ideal 109.5°. Ammonia has one lone pair among its four pairs, and that lone pair's stronger repulsion compresses the three N-H bond angles down to about 107°.

No — VSEPR theory counts a double bond or triple bond to one neighboring atom as a single electron-pair domain, in the same way it counts a single bond, because all the electron density of a multiple bond points in one overall direction from the central atom. This simulator focuses on the standard case of single bonding pairs and lone pairs, which covers the great majority of introductory VSEPR examples.

A molecule's 3D shape governs how it interacts with other molecules — enzymes and drug receptors recognize target molecules largely through shape complementarity, so predicting geometry from a Lewis structure is a foundational skill in chemistry and biochemistry. It's also why water's bent, polar shape gives it its unusually strong hydrogen bonding and solvent properties, a topic covered in other simulations on this site.