The central atom of a molecule is the atom to which all other atoms in the structure are chemically bonded. Identifying this atom is the foundational step required before sketching a Lewis structure or predicting the molecule’s three-dimensional geometry. The entire arrangement of electrons, bonds, and lone pairs hinges on correctly positioning this central component. Without accurately determining the core atom, any subsequent analysis of bonding and structure will be incorrect.
Electronegativity and Bonding Capacity
The chemical principle guiding central atom selection involves an atom’s inherent tendency to attract electrons, a property known as electronegativity. Atoms with lower electronegativity values are chosen as the central atom because they are less competitive for electrons and are better suited to share electrons with multiple surrounding atoms. This willingness to share electrons allows the atom to form more bonds, distributing electron density more evenly across the molecule. For instance, in carbon dioxide ($\text{CO}_2$), carbon is less electronegative than oxygen, making it the choice for the central position.
Hand-in-hand with electronegativity is an atom’s maximum bonding capacity, which is also a strong predictor of centrality. Atoms that belong to the third period or higher, such as sulfur or phosphorus, possess accessible d orbitals that allow them to expand beyond the typical octet rule. This ability means they can form four, five, or even six bonds, naturally positioning them as the hub for numerous surrounding atoms. Consequently, an atom capable of forming the most bonds will usually occupy the central position in a molecular structure.
Mandatory Exclusions and Structural Exceptions
Certain atoms are structurally incapable of acting as a central atom. Hydrogen (H) is the most notable exclusion because it possesses only one valence electron and can form only a single chemical bond. Since a central atom must be bonded to at least two other atoms, hydrogen is always relegated to a terminal, or outer, position in a molecule.
Halogen atoms—fluorine (F), chlorine (Cl), bromine (Br), and iodine (I)—are also generally excluded from the central position because they typically form only one bond to achieve a complete octet. Fluorine, being the most electronegative element, is almost always terminal. However, the heavier halogens (Cl, Br, I) can act as central atoms in specific interhalogen compounds or polyatomic ions, such as the chlorate ion ($\text{ClO}_3^-$). In these cases, they utilize d orbitals to expand their valence shell and accommodate multiple bonds. In polyatomic ions like sulfate ($\text{SO}_4^{2-}$), the central atom is usually the single, non-metal atom present, which is sulfur in this example.
Systematic Determination Process
The systematic process for identifying the central atom begins by eliminating all atoms that are structurally incapable of being central. This initial step requires immediately disregarding every hydrogen atom present in the compound, as well as considering the high probability that halogens will also be terminal atoms. Once the non-central candidates have been excluded, the remaining atoms are evaluated using the core chemical principles of bonding and electron affinity.
Between the remaining atoms, the one with the lowest electronegativity value should be selected as the central atom candidate. This atom is the least competitive for electrons and is the most likely to share its valence electrons to form the maximum number of bonds. For example, in the molecule $\text{OF}_2$, oxygen has a significantly lower electronegativity than fluorine, confirming oxygen’s placement at the center.
In cases where multiple atoms share similar low electronegativity values, the tie is broken by assessing the maximum number of bonds each atom can form. Atoms from the third period and beyond, like phosphorus in $\text{PCl}_5$ or sulfur in $\text{SF}_6$, can expand their octets and form five or six bonds, respectively. This ability makes them the default central atom over elements like nitrogen or oxygen. The atom that is present only once in the molecular formula, such as sulfur in $\text{SO}_3$, is also a strong indicator of centrality.
Verification Through Formal Charges
Once the central atom has been selected and the Lewis structure is constructed, the accuracy of that initial choice can be verified by calculating the formal charge on every atom. The formal charge is a theoretical charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally. It is calculated by taking the number of valence electrons an atom normally possesses and subtracting the number of non-bonding electrons and half the number of bonding electrons.
The most stable molecular structure is the one that minimizes the formal charge on all atoms, ideally resulting in a formal charge of zero for every component. If a molecule can be drawn with two possible central atoms, the one that results in the lowest magnitude of formal charges across the entire structure confirms the correct placement. The optimal central atom arrangement ensures that any negative formal charges reside on the most electronegative atoms, further confirming the choice was chemically sound.