Which is the most important factor in determining the state of a substance?
the size of the atoms in a substance
the number of molecules in a substance
the position of the electrons on the outer valence shells
the balance between intermolecular forces and kinetic energy

The balance between intermolecular forces and kinetic energy

Explanation:

If the kinetic energies of the particles are not strong enough to overcome the intermolecular forces, the particles will be locked in place in a crystal lattice. The substance will be a solid.

If the kinetic energies of the particles are slightly stronger than the intermolecular forces, the particles will be able to slide past each other. The substance will be a liquid.

If the kinetic energies of the particles are much stronger than the intermolecular forces, the particles will be escape completely from the attractions of their neighbours. The substance will be a gas.