Fluorine at a glance
- Atomic number
- 9
- Symbol
- F
- Atomic mass
- 18.998 u
- Category
- Halogen
- Group / period
- 17 / 2
- Electron configuration
- [He] 2s 2 2p 5
- State at 25 °C
- Gas
- Melting point
- -219.6 °C
- Boiling point
- -188.1 °C
- Density
- 1.7 g/L
- Electronegativity
- 3.98
- 1st ionization energy
- 1681 kJ/mol
- Oxidation states
- −1
- Discovered
- 1886
Atomic structure
A neutral fluorine atom has 9 protons and 9 electrons, and its most common isotope carries about 10 neutrons. One atom weighs about 3.15 × 10⁻²⁶ kg, so a mole of fluorine (6.022 × 10²³ atoms) weighs 18.998 g. That nucleus holds 1.11 neutrons for every proton. The electrons fill 2 shells (K 2, L 7), which gives the configuration [He] 2s 2 2p 5. With 7 electrons in its outer shell, it tends to gain or share electrons to complete its outer shell. It has only one stable isotope, F-19. No other element has a stable isotope with mass number 19, so F-19 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) fluorine is a gas. It melts at -219.6 °C and boils at -188.1 °C, so it stays liquid across a 31.5-degree range. To turn liquid it has to be cooled below -188.1 °C, colder than dry ice (−78 °C). Only 3 elements have a lower melting point. As a gas it weighs 1.7 g per litre, 1.4 times heavier than air. Its atoms have a radius of about 50 pm (Oxygen next door is 60 pm). In the solid, its atoms pack into a monoclinic lattice.
Chemical behaviour
Its electronegativity is 3.98 on the Pauling scale, the 1st highest of all elements, so it pulls hard on shared electrons. Pulling off its outermost electron takes 1681 kJ/mol. A bond between fluorine and chlorine would be polar covalent, with an electronegativity difference of 0.82. It releases 328 kJ/mol when it gains an extra electron, one of the largest electron affinities known. Its common oxidation states are −1, so in compounds you will usually meet it as F⁻. Group 17 also holds Chlorine, Bromine, Iodine, Astatine and Tennessine, which share its outer-shell pattern. Its Lewis symbol is F surrounded by 7 dots. Handle with care: as a pure element it is toxic, oxidiser and corrosive.
Where it comes from
It makes up 585 mg/kg of Earth's crust, the 13th most abundant element there. In the human body it accounts for about 0.0037 % of body mass. It was first identified in 1886, making it the 73rd element to be discovered (counting the 11 known since antiquity together).