Fluorine

Fluorine is atom number 9 — every Fluorine atom has exactly 9 protons in its center, and that number is what makes it Fluorine and nothing else. Its symbol is F. At room temperature it is a gas.

Fluorine is a chemical element with symbol F and atomic number 9. It is the lightest halogen and exists as a highly toxic pale yellow diatomic gas at standard conditions. As the most electronegative element, it is extremely reactive:almost all other elements, including some noble gases, form compounds with fluorine.

Summary from the source dataset, derived from Wikipedia (CC BY-SA 3.0) · original article

Fluorine sits in period 2, group 17 of the p-block. Its 9 electrons fill shells of 2, 7 — written as [He] 2s2 2p5. Its electronegativity is 3.98 on the Pauling scale, and pulling the first electron off takes 1681 kJ/mol. The outermost shell holds 7 electrons — the ones that do all the chemistry.

Full configuration: 1s2 2s2 2p5
Ionization energies (kJ/mol): 1681, 3374.2, 6050.4, 8407.7, 11022.7, 15164.1, 17868, 92038.1 … (9 published)
Electron affinity: 328.1649 kJ/mol · Electronegativity (Pauling): 3.98
Density: 1.696 g/L · melts 53.48 K · boils 85.03 K

Values exactly as published — see where the data comes from.