A chemical bond is the force that holds atoms together in a compound. Chemical bonds form because they lower the potential energy of the charged particles that compose atoms. Ionic bonds are the electrostatic forces that hold ionic compounds together, occurring between cations (which are positively charged) and anions (which are negatively charged). Since we have ions in an ionic compound, ionic bonds occur when electrons have been transferred. Ionic compounds are generally formed from a metal and a nonmetal because metals tend to give away electrons and nonmetals tend to accept electrons. An example is sodium chloride, NaCl. Each sodium has given one electron to each chlorine, creating a crystal lattice of cations and anions. On the other hand, covalent bonds involve sharing electrons. Covalent bonds generally occur between two nonmetals. Nonmetals want additional electrons to stabilize their electron configuration, so when a nonmetal bonds with another nonmetal, they generally share electrons. An example is H2O: the oxygen and hydrogen atoms are sharing electrons. The relatively simple act of transferring an electron from sodium to chlorine to produce sodium chloride has a profound effect on the properties of matter. Sodium is a highly reactive metal: if it's put in water, it can react explosively. But it's soft enough to be cut with a knife. And chlorine is a diatomic gas at room temperature. If you inhaled chlorine gas, it would damage your lungs, and at concentrations of just 400 chlorine molecules per million, it could be fatal. But the act of transferring an electron from sodium to chlorine, producing the ionic compound sodium chloride, completely changes its properties. Sodium chloride has a very high melting point at about 800°C or 1475°F. If you put it in water, it can dissolve, and without sodium chloride, your body couldn't function. A chemical formula represents the elements present in a compound and the relative numbers of atoms or ions. There are three types of chemical formulas we'll discuss: the empirical formula, the molecular formula, and the structural formula. The molecular formula indicates the actual number of atoms of each element in one molecule of a compound. For example, glucose is C6H12O6, so one glucose molecule has six carbon atoms, 12 hydrogen atoms, and six oxygen atoms. Hydrogen peroxide is H2O2, and carbon dioxide is CO2. The empirical formula is the smallest whole number ratio of atoms of each element in a compound. For example, the empirical formula of glucose is CH2O. The empirical formula and molecular formula could be the same for a given compound; this is the case for CO2. There can also be multiple compounds with the same empirical formula. The structural formula represents covalent bonds as lines and shows what atoms are bonded together. The structural formulas for CO2 and hydrogen peroxide are shown. There are additional molecular models that can show the three-dimensional arrangement of atoms. The ball and stick molecular model represents bonds as sticks and atoms as spheres. However, the bonds are accentuated so that the distances are much larger than are actually present in the molecule. The space-filling molecular model, on the other hand, shows the three-dimensional placement of atoms as they would occur in the actual molecule. Each one of these ways to represent a compound gives unique information about the compound. As you study chemistry, you'll become more familiar with how they are interrelated.