How do you work out the difference in electronegativity?
Easy! Subtraction. Fluorine is 4.0 Carbon is 2.5 the electronegativity difference is 1.5. What does this mean? If carbon bonds to fluorine the electrons will tend to spend more time in the orbit of the more electronegative atom in this case fluorine, the electron cloud would look a bit like a teardrop. You can find the electronegativity values on some of the periodic tables.
