Animation of a model of a molecule of water (H2O) showing its polarity. Polarity is a property of a molecule that has an uneven distribution of electric charge around it, giving it two or more poles of opposite charge. Here, red areas have a slight negative charge, blue ones a positive charge. This arises as the central oxygen atom has two lone pairs of negative electrons that are not involved in bonding, and also attracts the bonding electrons away from the hydrogen atoms as well. The hydrogen atoms are left with a small positive charge. The measure of an atom's ability to attract electrons is called its electronegativity, and is related to the number of protons it has in its nucleus, and the number of electron shells around the nucleus. Polarity plays an important part of determining the properties of a substance. With an asymmetrically-charged molecule like water, the negative side of one molecule can attract and form weak temporary bonds with the positive side of a nearby molecule. At a macroscopic level means the substance has a higher melting and boiling point that may be expected, and a higher viscosity. It also means that the substance can dissolve other polar or charged substances, such as ionic compounds like salt.