Hybridization and Steric Numbers

Hybridization Some atoms form a different number of covalent bonds than the electron configurations of those atoms might suggest they should form.

Hybridization In general, when we look at the electron configuration for an atom, we can infer the type of bonding that it will undergo. However, certain atoms like carbon, beryllium, and boron, sometimes react differently than is expected.

Hybridization Carbon has an electron configuration of 1s 2 2s 2 2p 2. That indicates that there are 4 electrons in its valence shell. The 2s orbital is full, and there are 2 half-filled p-orbitals.

Hybridization The two half-filled p-orbitals would suggest that carbon would form two covalent bonds. However, scientific research proves otherwise.

Hybridization In reality, carbon will “promote” one of the electrons in the 2s orbital into the p orbital, leaving it with 4 half- filled orbitals. This gives carbon the ability to form 4 covalent bonds.

Hybridization hybridization Chemists describe this unusual behavior as hybridization. Hybridization is the rearrangement of electrons within the valence orbitals of atoms during a chemical reaction.

Hybridization sp 3 hybrid orbital In carbon, the four half-filled orbitals all have identical energies, and each is referred to as an sp 3 hybrid orbital. There are a total of 4 sp 3 hybrid orbitals, as is illustrated by adding the superscripts (a 1 is assumed for the s).

Hybridization Once hybridization has taken place, any one of the four is equivalent to any other. All four orbitals have new bond characteristics that are different from the characteristics of the s and p orbitals.

Hybridization Hybridization also occurs in compounds of beryllium. The electron configuration if Be is 1s 2 2s 2. It would appear to have no half-filled orbitals with which to form covalent bonds.

Hybridization However, Be can form 2 covalent bonds by “promoting” one of the electrons in the 2s orbital into the 2p orbital, giving it 2 half-filled orbitals.

Hybridization sp hybrid orbitals The new half-filled orbitals formed by Be are referred to as sp hybrid orbitals. There are 2 sp hybrid orbitals.

Hybridization Boron also undergoes a type of hybridization. The electron configuration of B is 1s 2 2s 2 2p 1. Boron will promote one of the 2s electrons, giving it 3 half- filled orbitals in its valence shell, allowing it to form 3 covalent bonds.

Hybridization sp 2 hybridization This kind of hybridization is referred to as sp 2 hybridization.

Hybridization Another approach to this problem is to consider the steric number. The steric number is the number of atoms bonded to the central atom AND the number of lone PAIRS of electrons.

Steric Numbers Steric numbers can be used to determine the geometry and the bond angles that would be formed with the atoms and the central atom.

Steric Numbers The steric number is determined by counting the number of bonded groups attached to the central atom and the number of lone pairs of electrons attached to the central atom. Note that steric numbers are determined by first drawing the Lewis dot structure of the compound or ion.

Steric Numbers A steric number of four results in a tetrahedral arrangement in three dimensions with bond angles of 109.5°. A steric number of three results in a trigonal planar arrangement in three dimensions with bond angles of 120°.

Steric Numbers A steric number of two results in a linear arrangement in three dimensions with bond angles of 180°.