2 minute read



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 man-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 the 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; (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 a 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 man-made) elements is represented in the modern periodic table. The periodical 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, and are elaborated upon at the bottom of the periodic table. These 14 elements, plus actinium, are known as actinides.

Additional topics

Science EncyclopediaScience & Philosophy: 1,2-dibromoethane to AdrenergicActinides - Occurrence, Physical And Chemical Properties, Uses Of Actinides - General preparation