Naturally occurring radioactive elementsbelong to one of three decay series. Each series ends with a stable (nonradioactive) isotope of lead. In the diagram below, the diagonal arrows represent the emission of an alpha particle. This results in a reduction in atomic mass of four units (horizontal scale) and a reduction in atomic number of two (vertical scale). Beta decay increases atomic number by one, but does not affect atomic mass. It is represented by the vertical arrows. There is also a fourth decay series involving artificial radioactive isotopes.
Natural radioactive decay series
Uranium-235 series ends at lead-207
The elements from actinium (Ac) to lawren-cium (Lr) are known as the actinides. They form a series that is an offshoot of Group 3B of the periodic table. These highly radioactive elements have many physical and chemical characteristics in common. The elements beyond uranium are also known as the transuranium elements. Their atomic numbers are higher than 92, the atomic number of uranium. These transuranium elements rarely occur naturally. Only minute amounts of elements 93 and 94— neptunium and plutonium—have been found in nature. All the elements beyond these have been made only synthetically.
Beyond lawrencium, the last of the actinides, six further elements have been identified. Each is very short-lived and highly radioactive. Only a few atoms of each have been observed. In theory, the number of elements and the periodic classification could extend indefinitely. As such, there is a continuous search for new elements. But as the elements get larger, they are likely to get more unstable, undergoing radioactive decay very rapidly. It has been calculated that the element with atomic number 110 would have a half-life of about one ten-thousand millionth of a second. Radioactivity and half-life are explained later in this section.
All of the actinides and the elements beyond lawrencium are very radioactive. This means that the nuclei (centers) of these atoms are unstable. All nuclei (with the exception of the hydrogen nucleus) consist of protons (particles carrying one unit of positive electricity each) and neutrons (particles that are electrically neutral). In a stable (nonradioactive) atom,
the protons are balanced by the neutrons. In a radioactive atom, the neutrons are unable to counteract the electrostatic forces between the protons. As a result, the nucleus of a radioactive atom throws out excess protons and neutrons, trying to reach a stable condition. These excess protons and neutrons are emitted as energy in the form of alpha particles, beta particles, or gamma rays. By emitting these particles, the nucleus of an atom of one element changes and becomes an atom of another element.
If the immediate product of radioactive decay is itself radioactive, it also decays to form another element. This process continues until the particular atom has become an atom of a stable element. For example, one of the isotopes of uranium decays spontaneously through 16 elemental changes, before becoming a stable isotope of lead.
All the elements from actinium onward are continually undergoing such changes at different rates, producing different isotopes with different half-lives. A half-life is the length of time it takes for half the atoms of a radioactive substance to break down or decay. For the elements with higher atomic numbers, these half-lives are short and the radioactive decay is rapid. For some of the elements with lower atomic numbers, however, such as thorium and protactinium, the half-lives are much longer—in the range of many thousands or millions of years. For example, a particular isotope of thorium, which occurs naturally, has a half-life of 13,900 million years.
Thorium-232 series ends at lead-208
All the actinides are metals, showing physical and chemical metallic characteristics. Ac-
Major groups of elements: The actinides and beyond 61
tinium is a silvery-white substance that glows in the dark because of its radioactivity. Otherwise, its chemistry is very similar to that of lanthanum, which is just above it in the periodic table. Thorium is again a white metal, but it tarnishes rapidly in the air. It can be machined and forged. It loses electrons very easily and catches fire in air if it is in finely divided form. Protactinium is an unreactive metal that tarnishes in air.
Uranium tarnishes first to a yellow and then to a black film that does not protect the bulk of the metal. It is extracted in large quantities for use as a fuel in nuclear reactors. Neptunium and plutonium are formed as a result of the radioactive decay of this uranium fuel and are recovered from it. They are both silvery metals. Plutonium is also a very toxic substance, used as a fuel for nuclear reactors and in the manufacture of nuclear weapons.
Scientific experimentsleft on the Moon by Apollo astronauts were powered by a thermoelectric generator. It made use of the heat produced by the radioactive decay of an isotope of plutonium.
Otto Hahn, Lise Meitner, Frederick Soddy, and John Cranston
E.M. McMillan, and P.H. Abelson
G.T. Seaborg, J.W. Kennedy, E.M. McMillan, and A.C. Wahl
G.T. Seaborg, R.A. James, L.O. Morgan, and A. Ghiorso
G.T. Seaborg, R.A. James, and A. Ghiorso
G.T. Seaborg, S.G. Thompson, and A. Ghiorso
G.T. Seaborg, S.G. Thompson, A. Ghiorso, and K. Street, Jr.
Argonne, Los Alamos, and University of California
Argonne, Los Alamos, and University of California
G.T. Seaborg, A. Ghiorso, B. Harvey, G.R. Choppin, and S. G. Thompson
A. Ghiorso, G.T. Seaborg, T. Sikkeland, and J.R. Walton
'A number in parentheses
indicates the atomic
weight of the most stable
A. Ghiorso, T. Sikkeland, A.E. Larsh, and R. M. Latimer
Actiniumwas discovered in 1899 by the French scientist Andre Debierne. Its name is derived from the Creek aktis, meaning a ray of light. At. no. 89; at. mass 227.028; m.p. 81T C; b.p. 2470" C.
Actinidesare listed in the above table, which gives brief details about the atomic number and weight and the discovery of each of them.
RadioactivityThe German physicist Roentgen first observed radioactivity scientifically in 1895. He noticed that a cathode-ray tube emitted rays that could penetrate substances that visible radiation could not. A year later, the French physicist Henri Becquerel found that uranium compounds af-
fected a photographic plate. This happened even if it was covered with paper or a thin sheet of metal. He found that the intensity of the rays, which he called radioactivity, was proportional to the amount of uranium present in the compound. Marie and Pierre Curie investigated the effects of the ra-
dioactivity of uranium. They discovered the elements polonium and radium, from which Friedrich Dorn discovered radon.
The ornamental horses' heads(far right) date from the Bronze Age. They were made in Denmark in about 1000 B.C. Bronze was one of the first alloys made and used by humans. It was considerably harder than copper and other metals previously known.
Powdered iron,magnified nearly 2,500 times (right), consists of a random collection of minute spheres. In a bulk metal, the atoms are ar ranged in a regular crystal lattice. One form is called hexagonal close-packed (below), in which the atoms lie in staggered layers. Each atom is surrounded by six neighbors.
Metals and alloys
Metals are characterized by their density, strength, ability to transmit heat and electricity (thermal and electrical conductivity), and the ease with which they can be formed into different shapes. In many applications, however, not all of these properties are needed—or wanted—at the same time. Sometimes hard metals are required, sometimes soft ones. Often, they need to show great strength or resistance to chemical attack. There are times when no single metal has exactly the required combination of properties.
Each atom is surrounded by six neighbors
To provide materials to suit such a wide range of applications, metallurgists (experts in the science of metals) have developed alloys. An alloy consists of two or more elements, at least one of which is a metal. An alloy is formed by deliberately mixing the so-called base metal with an alloying element or elements to give the required degree of strength, malleability, and so on. Alloys have many applications, from high-strength and high-resistance steels to light, magnesium-based alloys used in the construction of aircraft. In practice, very few metals are used in the pure state.