Electronegativity is the term used to describe the ability of a nucleus of an atom in being able to attract electrons from other atoms towards it.
The trend within the periodic table is that as you go across the groups from one to seven, (not eight or zero, as these are inert elements), the electronegativity of the elements increases. Also, as you descend the periods, the electronegativity decreases. Therefore, due to this trend, Fluorine is the most electronegative element.
This trend is apparent as you can work out the electronegativity of a given element by knowing the amount of inner electrons that cause shielding from the nucleus and the total amount of electrons. For example, with Fluorine, it has a total of nine electrons and two of those cause shielding from the nucleus. Therefore, it's overall electronegativity is notated as 7+. Clearly, this can also be defined by the number of outer electrons an element has and the group it is in (as Fluorine has seven outer electrons and is a member of group seven).
Though this method of working out electronegativity would imply that all elements of the same group have the same electronegativity, size is a determining factor. This is because with smaller sized atoms, there are fewer electrons shielding potential bonds from the nucleus. Therefore, electrons from other atoms are more easily attracted. Whereas, with larger atoms, there are many more electrons shielding the nucleus, making it more difficult to attract other electrons and therefore less electronegative.
This electronegativity plays a part in polar molecules. This occurs within covalent compounds where the given compound might show properties of an ionic compound.
An example of a polar compound would be HF. This is because Fluorine is more electronegative than Hydrogen and therefore pulls the electrons in the covalent bond closer towards it. This means that the Fluorine atom is slightly negative, while the Hydrogen atom is slightly positive.
Positive and negative charges are what occurs in ionic bonding when, generally, metals either lose or gain electrons (thus making them positive or negative). Therefore, though the charges in HF are not as significant as you would find when an atom becomes an ion, the slight charge creates some ionic properties.
However, this does not happen with all covalent compounds. For example, in F2, there is no polarisation because each atom present is equally electronegative. Also in BF3, there is no polarisation as, even though Fluorine is more electronegative than Boron, each of the three Fluorine atoms is equally pulling the electrons towards them, meaning that the distribution is not uneven.