Mass Defect | Research & Encyclopedia Articles

Mass Defect

All matter in the universe is composed of atoms. Atoms, in turn, are comprised of differing combinations of subatomic particles. The subatomic composition of an atom determines which element it is, and therefore what physical and chemical properties it has. The subatomic construction of an atom is comprised of three particles. Protons are the positively charged particles that reside in the nucleus (the massive center of an atom). Neutrons carry no charge and also are found in the nucleus. Electrons are small, negatively charged particles, having almost no mass and moving with great velocity within shells that surround the nucleus of the atom. The individual masses of these three subatomic particles have been deduced by scientists and are standard constants. Together, the combination of protons, neutrons, and electrons constitute the entire mass of an atom, the bulk of which is found in the nucleus.

Because the total number of each of the three subatomic particles of a given atom of any element is known, and the individual mass of each particle is also known, the total mass of the atom can be calculated. For example, the element helium (He) has two protons and two neutrons in its nucleus, with two orbiting electrons. If the masses of all six particles are added together, the total should equal the entire mass of a single atom of helium (its atomic mass). However, when the mass of a single helium atom is actually measured, it is always less than the combined weights of its component particles. This loss of mass, called the mass defect, is observed for all atoms of all the elements. How is this possible?

One of the most startling scientific advances of the twentieth century was Albert Einstein's now famous equation E=mc². This revolutionary formula demonstrates the equivalence of matter and energy. That is, energy can be transformed into matter, and matter into energy. Einstein's most memorable contribution to physics explains why an atom of helium weighs less than its component particles, i.e. it explains the origin of the mass defect.

The combination of protons and neutrons to form the nucleus of an atom requires energy to hold the nucleus together. The mass defect arises when protons and neutrons combine to form the nucleus and a bit of their mass is converted into this nuclear binding energy, which stabilizes the nucleus and holds it together. Therefore, the nuclear binding energy can also be defined as the amount of energy required to break apart a nucleus into its component subatomic particles. From the mass defect for the atom, Einstein's equation can be used to calculate the nuclear binding energy for that atom.

Mass defect also works in reverse. If a nucleus is cleaved (in a process called fission), the resulting subatomic particles have slightly less mass than they did while in the nucleus. The lost mass is converted into energy, which is released in the nuclear reaction. The nuclear energy that is released per atom is quite large, so when large numbers of atoms are involved in a fission reaction, the energy release is huge. This is the reaction that produces nuclear power and atomic bombs. In nuclear reactors, atoms are cleaved allowing the release of their subatomic protons and neutrons in controlled chain reactions. These controlled reactions produce an enormous quantity of heat that is used to generate electricity. The mass defect that occurs during the fission reactions of an atomic bomb explosion, however, is the product of uncontrolled chain reactions.

Even the Sun in our solar system experiences mass defect. The massive amounts of light energy and heat energy generated by the Sun are the result of the fusion of hydrogen atoms to form helium atoms. When hydrogen protons combine to form the nucleus of a helium atom, the resulting helium atom has less mass than the combined hydrogen protons had originally. The loss of mass in the Sun, or mass defect, occurs as matter is transformed into energy, and this energy warms the Earth with rays of sunshine.