Learn the charges of common ions as you play classic Asteroids. When an ion appears on the screen, shoot it with its charge. For example, if the aluminum ion appears, shoot it with a plus three bullet. The 1, 2 and 3 keys shoot a charge with their numeric values. The polarity is either plus or minus. To change the polarity, tap the space bar.
Each time all ions are cleared, a new batch will appear in greater number and moving faster.