Recall that when atoms form covalent bonds, they do not always share their electrons equally. This is because the atoms differ in their electronegativity, or their ability to attract electrons. If the electronegativity for an atom is high, then it strongly attracts electrons.
If the electronegativity for an atom is low, it does not pull at the electrons as much. We can look at this portion of the periodic table to help visualize the electronegativity of the first several elements. Notice the gradient of light blue to dark blue that indicates the increasing electronegativity of the elements. Look closely at the symbols for carbon and hydrogen.
Notice that they have similar shading and close electronegativity values. This means that when carbon and hydrogen are bonded together, as shown in this methane molecule, both atoms have a similar ability to attract electrons. Neither element pulls more than the other. This type of covalent bond is called a nonpolar bond, since the electrons are more or less equally shared between the two atoms. The electronegativities of hydrogen and oxygen are very different.
As you can see from the chart, the electronegativity of oxygen is higher than the electronegativity of hydrogen. Because of this, when oxygen and hydrogen are in contact, the electrons are more or less equally shared. are bonded together, as shown in this water molecule, oxygen pulls at the electrons more than hydrogen does.
This causes the covalent bond between the oxygen and hydrogen atoms to become polar. bond, one of the atoms, in this case oxygen, has a slightly negative charge since it has a greater affinity for electrons, while the other atom, hydrogen, has a slightly positive charge because its electrons are spending more time around the oxygen atom.