How to Determine Which Atom to Use As the Central Atom

By Jack Brubaker

Chemists use Lewis dot structures to represent the bonding schemes in molecules. The concept of Lewis structures primarily relies on the octet rule, which states that most atoms prefer eight electrons in their outermost shells. The electrons surrounding each atom can either reside on the atom itself, which chemists refer to as a “lone pair,” or be shared between two atoms. Lewis dot structure provides a remarkably simple and effective method of visualizing bonding schemes, but constructing a proper Lewis structure starts with the identification of the central atom, or the atom to which most of the other atoms bond.

Step 1

Write down the molecular formula for the compound under investigation. For purposes of demonstration, consider the molecule chloroform, or CHCl3.

Step 2

Refer to a periodic table of the elements, such as that provided in the Resources section, to determine the relative electronegativities of the elements in the compound. Electronegativity reflects an atom’s ability to attract electrons to itself, and it increases from left to right and bottom to top in the periodic table. In the case of CHCl3, the electronegativities of the atoms increase as H < C < Cl. If in doubt about the electronegativities of two elements, refer to a periodic table that gives numerical values for each element.

Step 3

Choose the element with the lowest electronegativity -- other than hydrogen -- as the central atom for the Lewis structure. Hydrogen can never form more than one bond, which precludes it from serving as a central atom. Therefore, in the case of CHCl3, carbon represents the central atom.