How to Determine Hybrid Orbitals

Quick review: Remember that electrons in atoms exhibit what's called wave-particle duality, meaning they have properties that resemble those of waves and properties that resemble those of particles. That's why we can never be 100 percent certain about both an electron's location and its momentum at the same time. Consequently, we describe the behavior of electrons in atoms using a wavelike function called a wavefunction. The square of the wavefunction gives us the probability the electron can be found in any one place at any one given time. If we were to graph or map the probability density in 3-D -- maybe coloring points with a higher probability a dark color and points with a lower probability a light color -- and only include points with a high probability, we would get a region of space where we are very likely to find an electron with that quantum state. This region of space is called an atomic orbital. For a quick reminder on what different orbitals look like, check the link in Resources; it has animations of all the orbitals up through the d-set. You will never need to use f-orbitals in an organic chemistry or biochemistry class; the s, p and d orbitals are the ones you should remember.

Note that in valence bond theory, we imagine that the atom can form hybrid orbitals from a combination of valence atomic orbitals (atomic orbitals in its outermost shell). Let's say the element is carbon, so we have a 2s valence orbital and three 2p valence orbitals (again, see the Resources for pictures of what these look like). We could combine some or all of these valence orbitals to form hybrid orbitals. Any time we combine a given number of atomic orbitals, we get the same number of hybrid orbitals. Consequently, if we combine all four of carbon's valence orbitals, we get four hybrid orbitals. This configuration is called sp3-hybridization. Here are the other common configurations you'll encounter: sp2-hybridized: 2 p orbitals and an s orbital combine to form three hybridized orbitals. sp-hybridized: a p orbital and an s orbital combine to form two hybridized orbitals. sp3-d-hybridized: 3 p orbitals, 1 s orbital and 1 d orbital combine to form five hybridized orbitals. In sp3 configuration, each hybrid orbital points towards the corners of a tetrahedron. In sp2 configuration, each hybridized orbital points towards the corners of an equilateral triangle. In sp configuration, the two hybridized orbitals are linear and point in opposite directions. Finally, in sp3-d configuration, three hybrid orbitals point towards the corners of an equilateral triangle in the same plane, while the other two hybridized orbitals point straight up and down (at right angles to the triangle).

Draw the structural formula for the molecule you want to analyze. If you're working this problem as part of an organic chemistry assignment, this information will typically be given to you.

Determine how many regions of electron density are present around each atom in your structural formula. A bond (regardless of whether it's single, double or triple) counts as one region of electron density, while a lone pair (two electrons not participating in a bond -- generally represented as two dots) is another.

Determine the hybridization. If your atom has four regions of electron density, it's sp3-hybridized. If it has five regions of electron density, it's sp3-d hybridized. If it has three regions of electron density, it's sp2 hybridized, and if it has two regions of electron density, it's sp-hybridized.