Controls: Click on nodes to place particles in them. The left panel tells you which particle you are currently placing. You can use the arrow keys or the arrow buttons at each side of the particle icon to cycle through particles. You may replace a particle by clicking on it while another particle is active, and you may remove a particle by clicking on it while the same type of particle is active. Hover your mouse over a particle to see which constraints are unmet. The goal is to form a stable molecule by placing the particles such that all constraints are met.