Particle and Molecule, collectively known as X-Robots, are the bosses of stage 7 from Streets of Rage 2, they are joined by Isotope and Uranium on harder difficulties. Palette swaps named Oxygen, Hydrogen, and Mercury appear as mini-bosses at the beginning of the final stage.
These robots are incapable of walking; moving around the level by hopping and jumping. The heads contain laser cannons that can shoot for long range attacks. When using the laser attack, the robots will kneel down to aim. Mid and close range attacks consist of launching spiked balls on a tether towards the player. They also have a area attack where they spin the spiked balls around themselves; forcing the player to keep their distance.
If the player moves in and grabs one of the robots, they will send an electric charge through it's body; electrocuting the player. Upon defeat, the robots self-destruct. These explosions can hurt the player.
- It is not explained why they only hop despite being capable of walking (going by their joint design).
- They are one of the few enemies without an HD sprite in Streets of Rage 4.
- The fact that there are only a few of them suggests that they are expensive to produce (likely the reason why the P-Robots are developed as a cheaper but less powerful alternative to them).
- A drawing of it appears in one of the stained glass windows on the last stage from Streets of Rage 4.