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The smaller and lighter charged particles that revolve around the nucleus are called electrons. They have a negative charge.

The arrangement of electrons in energy shells that surrounds the nucleus is referred to as electron configuration.

The electron shells surrounding the nucleus are arranged in a hierarchy, with the shells furthest from the nucleus having the highest energy. Beginning with K, the shells or energy levels are numbered or lettered as follows: K is one, L is two, M is three, etc.

There is just one sub-shell in the first shell, and that sub-shell has only one orbital, the s orbital. The s-orbital can hold two electrons. As a result, the first shell K may accommodate two electrons.

The second energy level has one s orbital and three orbital in the p sub-shell that may house a total of six electrons. As a result, the second L shell can house up to eight electrons in total. The last shell to be filled is referred to as the valence shell.

Fluorine has nine electrons; thus, its electronic configuration is 1s²2s²2p⁵. The electronic configuration of F exhibits that F occupies the first two shells, the K(1) and L(2) shells. The K shell has an s-orbital, which contains two electrons, and the L shell has s- and p-orbitals, so it contains two and five electrons, respectively.

Thus, F has two electron shells in which three orbitals, which are 1s, 2s, and 2p, are occupied by electrons. As F has five electrons in 2p orbital (the valence shell), it needs one more electron to fill the valence shell.