A molecule with the formula AB(3) has a trigonal planar geometry.

How many electron groups are on the central atom (A)?

To determine the number of electron groups on the central atom (A) in a molecule with the formula AB(3) and a trigonal planar geometry, you need to consider the number of atoms bonded to the central atom (A) and the lone pairs of electrons around the central atom.

In a trigonal planar geometry, the central atom (A) is surrounded by three atoms bonded to it and no lone pairs. The three atoms bonded to the central atom and the absence of lone pairs give a total of three electron groups.

Therefore, a molecule with the formula AB(3) and a trigonal planar geometry has three electron groups on the central atom (A).