Since they were introduced by Gilbert N. Lewis in 1916, chemists have been using Lewis dot diagrams to represent the bonding of covalent molecules and coordination complexes. You represent valence electrons as dots and arrange them in such a way that the outer shells of the elements in the compound have a filled shell of either eight or twelve electrons, depending on the element. Hydrogen, the exception, needs only two electrons to fill its outer shell. To construct a Lewis diagram, you have to start with a central atom around which all the other atoms congregate. The central atom is the one with the lowest electronegativity, and you can compare electronegativity by looking at the periodic table. You can also use one or both of two other methods to determine the central atom.
Method 1: Compare Electronegativity
The electronegativity of an element is its propensity to attract electrons, and the element in a compound with the lowest electronegativity is usually the central one. The exception to this rule is hydrogen, which is never the central atom except in the H2 molecule.
Comparing electronegativity is the most reliable way to determine the central atom. You can determine relative electronegativity by looking at the periodic table. Allowing for a few exceptions, electronegativity increases as you move up and toward the right. Francium, element number 87 at the bottom of the first period, has a very low electronegativity while fluorine, element number 9 at the top of period 17, has a very high one. The noble gases, which form the last column in the table, do not form compounds.
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Method 2: Find the Least Numerous Element
As a rule, the element that occurs the least number of times in the compound is the central one. This is an easy method to use, because it allows you to determine the central atom simply by looking at the chemical formula. For example, oxygen is the central atom in H2O (water), and carbon is the central atom in CO2 (carbon dioxide). Unfortunately, this method leaves you completely in the dark when it comes to compounds that contain elements that occur in equal numbers, such as HCN (hydrogen cyanide).
Method 3: Memorize a List
A short list of elements, arranged in priority order, can make determining the central atom very easy, and when combined with method 2, eliminates the need to consult the periodic table in the majority of cases. The list is C, Si, N, P, S and O. If you have a compound that contains one or more of these elements, the one that occurs first on the list is the central atom. For example, in the carbon phosphate molecule (C3O16P4), carbon is the central atom because it occurs first on the list. You can also tell it's the central atom because it's the least numerous one.