Actinides
Actinides
Physical and chemical properties
Actinides, or actinoids, is a generic term that refers to a series of 15 radioactive chemical elements with atomic numbers 89 (actinium) through 103 (lawrencium). Denoted by the generic symbol An, these elements are all radioactive heavy metals, positioned in the seventh period and elaborated upon at the bottom of the periodic table. In order of increasing atomic number, the actinide series members are: actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, and lawrencium. (Note: Lawrencium is sometimes not grouped with the actinides but with the transition metals.) The actinide elements share the following properties: radioactive; easily tarnished in air; highly electropositive; very dense metals with unique structures; ability to combine directly with most non-metals; and ability to react with dilute acids or boiling water with the subsequent release of hydrogen gas.
Occurrence
Only actinium (atomic symbol, Ac), thorium (Th), protactinium (Pa), and uranium (U) are extracted from deposits in nature. In Canada, the United States, South Africa, and Namibia, thorium and protactinium are available in large quantities. All other actinides are synthetic, or artificially made.
To understand the physical and chemical properties of actinides, a basic foundation of atomic structure, radioactivity, and the periodic table is required. The atomic structure can be pictured like a solar system. In the middle of the atom is the nucleus, composed of neutrons (no charge) and protons (positively charged). Around the nucleus, electrons (negatively charged) are rotating on their own axis, as well as circulating in definite energy levels. Each energy level (or shell) is designated by a principal quantum number (n) as K, L, M, N, O, etc. or 1, 2, 3, 4, 5, etc., respectively. Each shell has sub-shells, or orbitals. The first energy level consists of one orbital (s); the second level consists of two orbitals (s and p); the third level consists of three orbitals (s, p, and d), and from the fourth level on up there are four orbitals (s, p, d, and f). The orbitals closer to the nucleus are lower in energy level than the orbitals further away from the nucleus.
The electrons are distributed according to Pauli’s exclusion principle. In any atom, the number of protons is equal to the number of electrons, thus bringing neutrality in charge. These stable and abundant atoms exist in nature only. In the unstable and less abundant atoms, the number of neutrons is more than the number of electrons (one element with the same atomic number but with a different atomic mass). These unstable atoms are known as isotopes, some of which are radioactive.
Radioactive isotopes become nonradioactive by the decaying process. The decaying process may involve an emission of: (1) electrons or negative beta particles; (2) helium nuclei or alpha particles; (3) gamma rays or very high frequency electromagnetic waves; or (4) positrons or positively charged electrons or positive beta particles.
The decaying process may also be due to K-capture (an orbital electron of a radioactive atom that may be captured by the nucleus and taken into it). Each of the above mentioned decay processes results in an isotope of a different element (an element with a different atomic number). The emission of alpha particles also results in elements with different atomic weights.
The most important decay process in actinides is K-capture, followed by the splitting, or fission, of the nucleus. This fission results in enormous amounts of energy and two or more extra neutrons. These newly formed neutrons can further start K-capture, with the subsequent reactions going on like a chain reaction. Atomic reactors and atomic bombs depend on the chain reactions.
A scheme of the classification of all known (both discovered and artificially-made) elements is represented in the modern periodic table. The periodic table is divided into vertical columns and horizontal rows representative of the periods with increasing atomic numbers. Each box contains one element and is represented by its symbol, a single or double letter, with the atomic number as a superscript, and the atomic weight as a subscript. Note that in the sixth and seventh periods, there are breaks between atomic numbers 57 and 72 (lanthanides) and 89 and 104 (actinides). Fourteen elements are present between the atomic numbers 89 and 104; these are elaborated upon at the bottom of the periodic table. These 14 elements, plus actinium, are known as actinides.
General preparation
All actinide metals are prepared on a scale by the reduction of AnF3 or AnF4 with vapors of lithium (Li), magnesium (Mg), calcium (Ca), or barium (Ba) at 2, 102–2, 552°F (1, 150–1, 400°C); sometimes chlorides or oxides are used. The Van Arkel-de Boer process is a special preparation method used for thorium and protactinium.
Physical and chemical properties
A common feature of actinides is the possession of multiple oxidation states. The term oxidation state refers to the number of electron(s) that are involved or that can possibly become involved in the formation of chemical bond(s) in that compound, when one element combines with another element during a chemical reaction. The oxidation state is designated by a plus sign (when an electron is donated or electro-positive) or by a minus sign (when the electron is accepted or electronegative). An element can have more than one oxidation state. An electron configuration can provide the information about the oxidation state of that element. The most predominant oxidation state among actinides is +3, which is similar to lanthanides. The crystal structure (geometry), solubility property, and the formation of chemical compounds are based on the oxidation state of the given element.
Actinides ions in an aqueous solution are colorful, containing colors such as red purple (U3+), purple (Np3+), pink (Am3+), green (U4+), yellow green (Np4+), and pink red (Am4+). Actinides ions U, Np, Pu, and Am undergo hydrolysis, disproportionation, or formation of polymeric ions in aqueous solutions with a low pH.
All actinides are characterized by partially filled 5f, 6d, and 7s orbitals. Actinides form complexes easily with certain ligands as well as with halide, sulfate, and other ions. Organometallic compounds (compounds with a sign bond between the metal and carbon atom of organic moiety) of uranium and thorium have been prepared and are useful in organic synthesis. Several alloys of protactinium with uranium have been prepared.
Uses of actinides
Even though hazards are associated with radioactivity of actinides, many beneficial applications exist as well. Radioactive nuclides are used in cancer therapy, analytical chemistry, and in basic research in the study of chemical structures and mechanisms. The explosive power of uranium and plutonium are exploited in making atom bombs. In fact, the uranium-enriched atom bomb that exploded over Japan was the first uranium bomb released. Nuclear reactions of uranium-235 and plutonium-239 are currently utilized in atomic energy power plants to generate electric power. Thorium is economically useful for the reason that fissionable uranium-233 can be produced from thorium-232. Plutonium-238 is used in implants in the human body to power the heart pacemaker, which does not need to be replaced for at least 10 years. Curium-244 and plutonium-238 emit heat at 2.9 watts and 0.57 watts per gram, respectively. Therefore, curium and plutonium are used as power sources on the Moon and on spacecraft to provide electrical energy for transmitting messages to Earth.
Sutharchanadevi Murugen