2.2 Atomic Orbitals

Atomic orbitals

A hydrogen atom (atomic number 1) has only one electron surrounding one positive proton in the nucleus. This lone electron is in a round orbital called a 1s orbital and is in the first and only shell of electrons of the hydrogen atom. The outermost electrons of an atom are called its valence electrons. Since this first shell is also the outermost shell of electrons, this 1s electron is the only valence electron for hydrogen.

Hydrogen atom, one shell, 1s orbital
Hydrogen atom, one shell, 1s orbital

The first shell is small and can only hold two electrons in it. Helium (atomic number 2) fills up this first shell with its two electrons, both in the 1s orbital. Another shell of electrons outside of this first shell is needed for the other important organic chemistry atoms like carbon, nitrogen and oxygen because they contain more than two electrons. Therefore, these other important organic chemistry atoms have two shells of electrons.

The two electron shells of common organic atoms
The two electron shells of common organic atoms

For these atoms, the first, innermost shell of electrons is made up of two electrons in a round orbital called a 1s orbital. These two electrons are not very interesting because they are tucked inside the atom and don’t interact with other atoms. The second shell is farther away from the positively charged nucleus. It is more interesting because it contains the outermost electrons of the atom. These electrons are the ones that will bump into other atoms to form bonds. This outermost electron shell is called the valence shell and its electrons are the valence electrons. It is also made up of two electrons in a round orbital called a 2s orbital. It is the same shape as the 1s orbital, but just a little bigger as the electrons are a little farther from the nucleus in the second shell.

1s and 2s orbitals

The round 2s orbital is not the only type of electron orbital in the 2nd shell. There are also three double-lobed 2p orbitals in this valence shell. These three p orbitals are all rotated 90° from each other.

2p orbitals
The two electron shells of common organic atoms

The round 2s orbital is not the only type of electron orbital in the 2nd shell. There are also three double-lobed 2p orbitals in this valence shell. These three p orbitals are all rotated 90° from each other.

2nd valence shell with 2s and 2p orbitals
Showing how 2s and 2p orbitals make the valence (2nd) shell

This is what the atomic orbitals of carbon atom look like. To make bonds between atoms, the outermost valence electrons of one atom must overlap with the outermost valence electrons of another atom to make bonds. If the atomic orbitals described above were the only electron orbitals that atoms contain, we would expect that we could only bond atoms with a carbon atom at 90° and 180° angles. This is not the case. We often see bond angles of 180°, 120°, and 109.5°, but not 90°. Let’s look at methane (CH4) if only atomic orbitals can overlap compared to its true, tetrahedral shape.

Methane if only atomic orbitals overlap vs. the true tetrahedral shape of methane
CH4 if only atomic orbitals overlap vs. its true shape

Something else must be going on to get these other bond angles. Because all electron orbitals are waves, we can get these other bonding angles when the s and p atomic orbitals that are on the same atom mix together to form new oblong electron orbitals that are called hybrid orbitals. A hybrid is the offspring of two different species. So, hybrid orbitals are the offspring of two different types of orbitals on the same atom. It is important that you realize we are mixing together electron waves on the same atom because mixing together orbitals that are on different atoms is something else entirely, something we call making bonds.