We said previously that in a covalent bond, electrons are shared between two atoms. In a polar covalent bond, electrons are shared unequally, so that the electrons spend more time near one atom than the other. This can create a dipole moment, which is the separation of the center of positive and negative charge in a molecule. If we look at HF, for example, the fluorine and hydrogen atoms are sharing two electrons in a single bond, but the two electrons spend more time near the fluorine than the hydrogen atom. We can see this by looking at the electrostatic potential map for HF. Areas that are blue have relatively low electron density in the molecule, while areas that are red have relatively high electron density. The hydrogen side of the molecule is blue, and the fluorine side of the molecule is red. This sets up a partial negative charge on fluorine (which is indicated using a delta negative) and a partial positive charge on hydrogen (which is indicated using a delta positive). We can also represent the separation of charge using an arrow pointing from the partial positive charge to the partial negative charge. The tail of the arrow has a plus sign, and the head points to the partial negative. There are many reasons why bond polarity and dipole moments are important. They play a central role in many of the physical properties of matter. For example, a polar molecule would respond to an electric field. If we subjected gaseous HF molecules to an electric field, the partial negative on fluorine is attracted to the positive of the applied electric field, and the partial positive on hydrogen is attracted to the negative of the applied electric field. Electronegativity is an atom's ability to attract electrons to itself in a chemical bond. Electronegativity generally increases going up and to the right in the periodic table, with fluorine having the highest electronegativity of 4.0 and the other elements scaling down from there. The difference in electronegativity between two atoms of a bond determines the type of bond. If there is no electronegativity difference between two atoms, the electrons are shared equally, and the bond is called purely covalent or nonpolar covalent. This is the case for a chlorine molecule, Cl2. If there is an intermediate difference in electronegativity, we'll have a polar covalent bond, where there is a partial negative on the more electronegative atom and a partial positive on the less electronegative atom. This is the case for HCl, where there is an electronegativity difference of 0.9 between hydrogen and chlorine. If there is a large electronegativity difference, as is the case when we have a metal and a nonmetal, electrons are transferred, and we have an ionic bond. Although a particular bond will fall on a continuum from sharing electrons equally to transferring electrons, we can use general categories based on the electronegativity differences between the two atoms in a bond. If two atoms have a small electronegativity difference of about 0.4 or less, we'll categorize them as covalent bonds. This means the electrons are shared more or less equally. If we have an electronegativity difference greater than 0.4 but less than 2.0, we'll categorize the bond as being polar covalent. And if there is a larger electronegativity difference than about 2.0, we'll categorize it as an ionic bond.