The actinides and beyond

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.

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.

Fact entries

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.

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.

Date: 2015-12-11; view: 1681