Bonding and shapes
Type a molecule. See its shape.
Why this shape
- O has 6 valence electrons. It uses two in two single bonds to H, so four are left as two lone pairs.
- Steric number = two sigma bonds + two lone pairs = 4, so O is sp³ hybridised.
- Four electron pairs point to the corners of a tetrahedron (tetrahedral electron geometry). With two lone pairs in place of atoms, the atoms make a bent shape.
- Repulsion falls in the order lone pair to lone pair > lone pair to bond pair > bond pair to bond pair. The lone pairs push the bond pairs closer, so the angle is 104.5°, less than the ideal 109.5°.
- The bond dipoles do not cancel in the bent shape, so H₂O is polar.
Lewis structure
8 valence electronsFormal charges: none
Shape explorer
i
Electron pairs are the bond pairs plus the lone pairs on the central atom (the steric number). A double or triple bond counts as one pair.
Pick a molecule
sp³ 27 species
Lewis structure of
Bond order and MO diagrams
VSEPR theory and shapes
Same subject: organic conversions, name reactions, electron configuration, oxidation numbers and redox, balance equations, mole calculator.
Hybridisation, VSEPR and bond order
How do I find the hybridisation of the central atom?
Count the sigma bonds the central atom makes and the lone pairs it keeps. Their sum is the steric number: 2 is sp, 3 is sp2, 4 is sp3, 5 is sp3d, 6 is sp3d2 and 7 is sp3d3. A double or triple bond counts once, because only its sigma bond uses a hybrid orbital.
Why is the bond angle in water less than 109.5 degrees?
Oxygen has two bond pairs and two lone pairs, which point to the corners of a tetrahedron. Lone pairs repel more strongly than bond pairs (lone pair to lone pair > lone pair to bond pair > bond pair to bond pair), so they squeeze the two O-H bonds together to 104.5 degrees. In ammonia one lone pair closes the angle to 107 degrees.
What is the difference between electron geometry and shape?
The electron geometry counts every electron pair around the central atom, bonded or not. The shape describes only where the atoms are. With no lone pairs the two are the same; with lone pairs the shape gets a different name, such as bent for water and trigonal pyramidal for ammonia.
How is bond order found from molecular orbital theory?
Fill the electrons into the molecular orbitals in energy order, then bond order = (Nb - Na) / 2, where Nb is the number of electrons in bonding orbitals and Na in antibonding ones. A higher bond order means a stronger, shorter bond, and a bond order of zero means the molecule does not exist. Unpaired electrons make it paramagnetic.
Are the values the ones in the NCERT book?
Yes, where the book gives them: the shapes, the hybridisation and the angles such as 104.5 degrees for water, 107 for ammonia and 120 and 90 for PCl5 follow NCERT Class 11 Chapter 4. Angles the book does not give are measured values, shown with "about".