Final answer:
Changing the number of atoms in a molecule alters the electron group arrangement around the central atom and thus changes the molecule's shape. Adding or removing an atom can switch a molecule from one geometric form to another, such as from linear to bent or vice versa, according to VSEPR theory.
Step-by-step explanation:
The shape of a molecule is determined by the valence shell electron pair repulsion (VSEPR) theory, which states that electron pairs repel each other and thus arrange themselves as far apart as possible. This arrangement defines the molecule's geometry. If an atom is added to or removed from a molecule, the number of electron groups around the central atom changes. Consequently, the arrangement of electron pairs, and therefore the overall molecular geometry, will change. For example, adding an atom might change a linear molecule into a bent one, while removing an atom from a tetrahedral molecule might result in a pyramidal shape.
An illustrative case: With two electron groups, the molecule assumes a linear shape because electron groups are 180° apart. If a third atom is added, forming three electron groups, the new electron group geometry might become trigonal planar, changing the molecular shape to bent.