Notes

atomic mass The total number of protons plus neutrons in an atomic nucleus.
isotope Atom with the standard atomic number and chemical properties but different atomic mass and physical properties.
meson A particle one-eighth the mass of protons; charge varies. Symbolized by K, π, or μ.
neutrino Neutral particle of nearly zero mass, produced by decaying nuclei and symbolized by ν.
positron Nuclear particle of the mass of an electron but with a positive charge, symbolized by .
Vocab Arcade
NUCLEAR COMPOSITION

You already know that atoms contain protons, neutrons, and electrons. These particles contain even smaller parts. In this lesson, you will learn about seven particles and one ray.

Protons - ρ. Protons are positively charged particles and are given the mass number of 1.007582. All atoms have at least one proton, and each element is known by the number of protons in its nucleus. All atoms of the same element have the same number of protons. The number of protons contained in an atom is its atomic number.

Click on the picture to view isotopes of common elements.


Neutrons - η. Neutrons are particles with no electrical charge and a mass number of 1.00897, slightly greater than that of a proton. Together with the protons, they make up the atomic mass of atoms. We know that the nuclei of the same elements have the same number of protons, but the atoms of an element can have differing numbers of neutrons. Therefore, the atomic mass of the atom in an element can vary, depending upon the number of neutrons it contains. The members of each element that have differing atomic masses are called isotopes. The atomic mass, or mass number, is equal to the protons (ρ) plus neutrons (η) for each element. Two elements can have the same mass number but never the same atomic number. We can figure out the number of neutrons in each case by subtracting the atomic number from the atomic mass:


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number of neutrons = atomic mass - atomic number

The formula for oxygen isotope 16 is this:

number of neutrons = 16 - 8 = 8 neutrons

For oxygen isotope 18, the formula would be:

number of neutrons = 18 - 8 = 10 neutrons

The isotopes are usually written in symbol form to make them easier to read. Oxygen isotope 16 could be written ; and isotope 18 as .


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Each element has its atomic mass listed on the Periodic Table. This number represents the average sum of protons plus neutrons of the element as it occurs naturally in our universe.

Beta particles - β. Beta particles are negative particles with a negative charge. They are almost identical to the electrons that are in the outer paths of each atom. The only difference is that beta particles are high in energy and occur as a result of a nuclear decay. These particles can travel at near the speed of light (301,320 km per second, or 186,000 miles per second); but because they have such a small mass, they cannot penetrate very far into any substance. For example, beta particles will only penetrate several millimeters into human tissue. Beta particles are symbolized by the Greek letter β. The mass of an electron is negligible compared to the total mass of an atom. Therefore the symbol for an electron or a beta particle often shows zero mass.

Alpha particles - α. Sometimes in a nuclear decay, both protons and neutrons are emitted together in one particle. This particle is called an alpha particle and is made up of two protons and two neutrons. An alpha particle is just like a helium nucleus (He) except that it is emitted from the nucleus of some atom rather than being made from a helium atom. The alpha particles (the Greek letter α) are very energetic, but relatively slow. They travel at speeds up to 21,000 km per second (13,000 miles per second). This speed covers a distance about half the circumference of the earth in one second, but it is only a fraction of the speed of light. Alpha particles are easily stopped and only penetrate the first few layers of skin.

Gamma rays - γ. Gamma rays have no charge. They are simply bundles of energy that are released from nuclei at the speed of light as the nuclei decay. Because of their high energy, they are able to pass through or penetrate solids, including the ability to pass through a person. Several feet of concrete or lead are necessary to stop these rays. The symbol for gamma rays is the Greek letter γ.

Positron - . In 1932, Carl Anderson conducted a series of experiments with a device called a cloud chamber. This chamber contains a cloud of vapor similar to a cloud in the sky. Whenever a particle passes through the cloud chamber, it leaves a track in the cloud. By studying the track, the scientist can determine the particle's speed, mass, and charge. As a result of such a study, Anderson found evidence that a particle just like the electron existed, except this new particle had an opposite charge. Therefore, the new discovery was called a positron because it was just like an electron, but positive in charge. It is represented by .



Neutrino - ν. Since we live in an orderly universe, apparent contradictions to natural laws are reason enough to investigate. An example of such a contradiction is the "disappearance" of small amounts of energy during nuclear reactions. This apparent destruction of energy is contrary to the Law of Conservation of Energy. To reconcile the apparent contradiction, scientists hypothesized the existence of a subatomic particle with certain characteristics and called it a neutrino. The neutrino must be uncharged and of very small mass. Evidence for the neutrino's existence is indirect, being proposed as a way to balance the natural laws governing the universe.

Neutrinos are very prevalent and more or less invisible. It is a very difficult particle to study because it is so elusive. It has no electrical charge, and it affects matter only in a very weak manner. Trillions of neutrinos are zooming through you every second, yet you are not affected. As a matter of fact, about one neutrino per year will interact with you. With this level of interaction with nature, you can see the difficulty in studying it.

Meson - π. Nuclear particles with a mass of about one-eighth the mass of protons are known as mesons. They have a charge of +, -, or 0 and are very unstable. The Table of Atomic Particles below summarizes seven of the particles that make up a nucleus. The bottom number indicates the charge of a particle.



NUCLEAR STRUCTURE

It may seem contradictory that the nucleus of an atom sticks together when we know that like charges repel. Why don't the protons repel each other? It does not seem to fit the predicted pattern.

One theory suggests a model that arranges the protons and neutrons in a series of shells, or energy levels, like the electrons around the nucleus. The nucleus is held together by mesons, positrons, and neutrinos. Gamma rays are emitted when a proton or neutron drops to a lower energy level. Some nuclei are unstable. They have such a delicate balance of forces holding them together that the slightest disturbance will cause them to fly apart. This results in radioactive decay.

Stable nuclei are determined by the ratio of neutrons to protons. The greater the atomic number, the greater the number of neutrons necessary to keep the nucleus stable. The neutrons tend to act as a buffer among the protons, keeping them from repelling each other with such great force. Figure 9 shows the ratio n:p necessary to ensure a stable isotope.
Notice that as the atomic number increases, the number of neutrons must get larger to offset the increase in nuclear charge. Ratios that deviate very much from the curved line are unstable ratios and indicate radioactive isotopes of that particular element. No completely stable nuclei are in elements 83 and above. All isotopes of elements beyond bismuth (83) are unstable and radioactive.