Here's how to figure out the maximum number of atoms in the 3rd shell of an atom with 4 shells:

* Electron Shells: Atoms have shells (energy levels) where electrons reside. The shells are numbered starting from 1 (closest to the nucleus) and going outward.

* Shell Capacity: Each shell has a maximum number of electrons it can hold:

* Shell 1: 2 electrons

* Shell 2: 8 electrons

* Shell 3: 18 electrons

* Shell 4: 32 electrons

Therefore, the 3rd shell of an atom with 4 shells can hold a maximum of 18 electrons.