A lattice model of the atom is used to study the excitonic transition in dense fluids. With the use of a simple Hamiltonian for both atomic and interatomic interactions, simulations were made by allowing nuclei to move and by using hard-sphere configurations for the nuclei. The hard-sphere simulations are compared with several analytical theories. Rehybridization of atomic orbitals is found to occur in the hard-sphere systems, while clustering is present in the full simulations.