The HOMO (highest occupied molecular orbital) of the fluoride anion, F⁻, is the 2p orbital. Because F⁻ is a single atom rather than a molecule, this is more precisely called the highest occupied atomic orbital, but the concept is identical: it’s the highest-energy orbital that contains electrons.
Why the 2p Orbital Is the HOMO
A neutral fluorine atom has 9 electrons. When it gains one electron to become F⁻, it holds 10 electrons total, giving it the ground-state electron configuration 1s² 2s² 2p⁶. That’s the same electron configuration as neon, making F⁻ isoelectronic with a noble gas.
The orbitals fill in order of increasing energy: 1s is lowest, then 2s, then 2p. Since the 2p subshell is the last one to receive electrons, and it’s now completely filled with six electrons, it sits at the highest energy level among the occupied orbitals. That makes 2p the HOMO.
What the Full 2p Subshell Means
The 2p subshell consists of three individual orbitals (2pₓ, 2pᵧ, and 2p_z), each holding two electrons. In F⁻, all three are completely filled. This full-shell configuration is exceptionally stable, which is a major reason fluorine so readily accepts an extra electron in the first place. It also explains why F⁻ has a very high ionization energy for an anion and is extremely reluctant to lose that extra electron once gained.
Because the 2p orbitals are completely occupied, the LUMO (lowest unoccupied molecular orbital) of F⁻ jumps up to the 3s orbital, which is significantly higher in energy. That large HOMO-LUMO gap reinforces the chemical stability of the fluoride ion and is why F⁻ behaves as a hard base in acid-base chemistry: its electron density is tightly held in compact, low-energy orbitals close to the nucleus.
How This Affects Reactivity
In reactions where F⁻ donates electrons (acting as a nucleophile or a base), the electrons come from the 2p HOMO. The 2p orbitals on fluoride are relatively low in energy and compact compared to, say, the HOMO of iodide (I⁻), which sits in a much larger 5p orbital. This is why fluoride is a poor nucleophile in polar protic solvents but a strong base. Its electron-donating orbital is small and energy-dense, favoring proton abstraction over attack on bulky carbon centers.
If you’re comparing fluoride to other halide anions, the trend is straightforward. Cl⁻ has its HOMO in the 3p orbital, Br⁻ in 4p, and I⁻ in 5p. As you move down the group, the HOMO rises in energy and becomes more diffuse, making the heavier halides better nucleophiles but weaker bases.