When you draw a chemical molecule on paper, you must decide which atom goes in the middle. This central atom is usually the element that has the lowest electronegativity. Electronegativity is simply how strongly an atom pulls on shared electrons.
Because the central atom pulls less strongly, it willingly shares its electrons with multiple partners. Hydrogen can never act as a central atom. A hydrogen atom can only form one single chemical bond.
A central atom must connect to at least two other atoms. Therefore, hydrogen must always sit on the outside edge of the molecule. The central atom controls the exact shape of the entire molecule.
It holds onto both bonding electron pairs and non-bonding lone pairs. These pairs of negative electrons push away from each other. They arrange themselves to be as far apart as possible around the central core.
This pushing creates specific bond angles that give the molecule its three-dimensional shape. In more complex chemistry, transition metals often act as central atoms. These metal ions sit in the middle of large structures. Surrounding molecules, called ligands, donate pairs of electrons to stick to the central metal.