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.


Neutron, electrically neutral elementary particle that is part of the nucleus of the atom. Elementary particles are the smallest parts of matter that scientists can isolate. The neutron is about 10-13 cm in diameter and weighs 1.6749 x 10-27 kg. See also Atom.

Neutrons and protons bind tightly together to create atomic nuclei. The number of protons an atom contains determines which chemical element it is, ranging from 1 proton for hydrogen to 92 for uranium, the largest naturally occurring element. Each atom usually contains about as many neutrons as protons, but different atoms of the same element may have different numbers of neutrons.

Atoms that differ only in the number of neutrons are called isotopes. For example, most atoms of the simplest element, hydrogen, have a nucleus containing only a single proton. In natural hydrogen, however, 0.015 percent of the atoms have a neutron in addition to the proton. This isotope is called heavy hydrogen or deuterium. An element usually has several isotopes, all nearly identical in the way they react chemically with other elements and each other. Scientists can distinguish different isotopes of an element by examining properties of the element’s nuclei, such as the mass of the nucleus.


The neutron is slightly heavier than a proton and 1,838 times as heavy as the electron. It is affected by all the four fundamental forces of nature. Because it has mass, it is affected by gravitation, the force of attraction between all objects in the universe. Although the neutron has no electrical charge, it is slightly magnetic, so it is affected by the electromagnetic force, the force of attraction or repulsion between electrically charged or magnetic objects. The neutron is affected by the strong nuclear force, an attraction that binds the neutron to protons and other neutrons in the nucleus. The neutron is also affected by the weak nuclear force, an interaction among the building blocks of the neutron that causes the neutron to decay, or break apart. Isolated from nuclear matter, a free neutron decays into a positively charged proton and a negatively charged electron, releasing energy in the process (see Nuclear Energy). The average lifetime of a free neutron is just under 15 minutes.