The Lewis model is a two-dimensional model for representing atoms or compounds. It represents valence electrons as dots and shows how atoms are bonded together in the structure of the molecule. It is a localized electron model, which means it assumes valence electrons are either localized on a particular atom as a lone pair of electrons or shared between two atoms as a bond. We could draw the Lewis structure for an oxygen atom by surrounding the chemical symbol for oxygen with six dots to represent oxygen's six valence electrons. For main group elements, the number of valence electrons is equal to the group number. For example, lithium is in group 1A, so it has one valence electron; nitrogen is in group 5A, so it has five valence electrons. When we discussed electron configurations, we said that elements that do not have the electron configuration of the noble gas tend to either transfer or share electrons to obtain the electron configuration of their nearest noble gas. All noble gases have eight valence electrons, with the exception of helium. This gives rise to the octet rule: nonmetals will tend to either accept electrons or share electrons to obtain eight valence electrons. Metals generally try to give away electrons to have only core electrons--that is, the same electron configuration as their nearest noble gas. A notable exception to the octet rule is for hydrogen. Since hydrogen's nearest noble gas is helium, hydrogen follows the duet rule: it wants two valence electrons to have the same electron configuration as helium. We represent an ionic compound using its formula unit, which is the smallest whole-number, electrically-neutral ratio of ions. Formula units are not molecules. We would rarely see just one sodium and one chloride ion together; instead, sodium chloride exists as a repeating pattern of cations and anions in a crystal lattice. We can use the Lewis model to predict the ratio of cations and anions that produce an electrically neutral formula unit. For example, if we want to know the formula unit for the ionic compound formed between potassium and chlorine, we can see from the Lewis structure for potassium that it wants to give away one valence electron. We can also see that chlorine has seven valence electrons to begin with, so it wants just one more valence electron. Chlorine follows the octet rule since it is a nonmetal. So the smallest whole-number ratio of ions that are electrically neutral would be one K+ and one Cl-. Or we can look at the formula unit for the ionic compound formed between sodium and sulfur. Sodium has one valence electron it has to give away, but sulfur wants two additional valence electrons to satisfy the octet rule. So we need two sodium atoms for every one sulfur atom; thus, sodium sulfide is Na2S. When creating the formula for an ionic compound, we should keep a few things in mind. First, ionic compounds always contain both cations and anions because they must be electrically neutral. This means the sum of the charges of the cations and anions in the formula unit should equal zero, and the formula unit is the smallest whole-number ratio of ions that produces an electrically neutral ionic compound. Let's practice by predicting the formula unit for the ionic compound that forms between a metal and a nonmetal. We'll start with aluminum and bromine. From the periodic table, aluminum forms a 3+ ion because it needs to give away 3 electrons to have the same electron configuration as neon. Bromine forms a 1- ion because it needs one additional electron to have the same electron configuration as krypton. To make an electrically neutral compound, we can use the charge on the anion to give us the number of cations we need. In this case, bromide has a one minus charge, so we need one aluminum three plus. Then we can use the charge on the cation to give us the number of anions we need. The charge of aluminum is three plus, so we need three bromide anions. This gives us the formula unit of AlBr3; this is the smallest whole-number electrically neutral ratio of ions. Now let's try magnesium and oxygen. Magnesium forms a 2+ ion because it is an alkaline earth metal. Oxygen forms a 2- ion because it is in main group 6A. We can use the charge on oxygen to determine the number of magnesium ions, and then we can use the charge on magnesium to determine the number of oxide ions. For the formula unit, we want the smallest whole-number ratio, so we'll reduce Mg2O2 to just MgO. We can also use the formula to determine the charge on a metal that has more than one possible charge. Let's find the charge on manganese in Mn2O3. We know that Mn2O3 is electrically neutral. So if we add together two times the charge on manganese with three times the charge on oxygen, it needs to equal zero. From the periodic table, we know oxygen produces an ion with a two minus charge. Now we can solve for the charge on manganese. First, we'll multiply three times -2, and then we can add six to both sides. And finally, we divide by two. So we see our charge on manganese must be +3 for our ionic compound to be electrically neutral.