Central Atom
Definition and meaning of Central Atom in chemistry.
A central atom is the atom in a molecule or polyatomic ion that sits at the structural core, directly bonded to two or more other atoms or ligands that surround it.
In more detail
When drawing a Lewis structure, the central atom is usually the least electronegative element present (excluding hydrogen, which can form only one bond and so is never central) and is often the atom written first in the chemical formula. The arrangement of bonding pairs and lone pairs around the central atom, as described by VSEPR theory, determines the molecule's overall geometry. In coordination chemistry, the central atom is typically a transition metal cation surrounded by ligands that donate electron pairs to form coordinate covalent bonds.
Key facts
| Field | General Chemistry |
|---|---|
| Role | Core atom bonded to surrounding atoms/ligands |
| Determines | Molecular geometry via VSEPR theory |
| Example formula | H2O (O is the central atom) |
In water, H2O, oxygen is the central atom bonded to two hydrogen atoms; its two lone pairs compress the H-O-H bond angle to about 104.5°, giving the molecule its bent shape.
Frequently asked questions
Can hydrogen ever be a central atom?
No. Hydrogen can form only one covalent bond, so it can only occupy a terminal position, never the central one.
How do you pick the central atom when drawing a Lewis structure?
Choose the atom with the greatest bonding capacity and lowest electronegativity (excluding hydrogen), which is often the single unique atom in the formula, such as carbon in CO2 or sulfur in SO3.