S: (n) fluorine, F, atomic number 9 (a nonmetallic univalent element belonging to the halogens; usually a yellow irritating toxic flammable gas; a powerful oxidizing agent; recovered from fluorite or cryolite or fluorapatite)
S: (n) farad, F (the capacitance of a capacitor that has an equal and opposite charge of 1 coulomb on each plate and a voltage difference of 1 volt between the plates)
S: (n) F, f (the 6th letter of the Roman alphabet)