Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Strong Force

Protons and neutrons in the nuclei of atoms are held together by the strong force. This force must overcome the electromagnetic force of repulsion the protons in a nucleus exert on one another. The strong force that occurs between protons alone, however, is not enough to hold them together. Other particles that add to the strong force, but not to the electromagnetic force, must be present to make a nucleus stable. The particles that provide this additional force are neutrons. Neutrons add to the strong force of attraction but have no electric charge and so do not increase the electromagnetic repulsion.