Element, Families of
Element, Families of
The search for patterns among the elements Johann
Hydrogen: The elemental orphan
The coinage metals and the platinum metals
A family of chemical elements usually consists of elements in the same group (the same column) on the periodic table. The term is also applied to certain closely related elements within the same period (row). Just as the individual members in a human family are different but have common characteristics, such as hair color, the elements in a chemical family have certain properties in common, and others that make them unique.
The search for patterns among the elements Johann
Döbereiner (1780–1849) made one of the earliest attempts to organize the elements into families in 1829, when he observed that for certain groups of three elements, called triads, the properties of one element were approximately midway between those of the other two. However, because the number of elements known to Döbereiner was far fewer than it is today, the number of triads that he was able to find was very limited.
In 1864 John Newlands (1837–1898) noticed that when the known elements were arranged in order of increasing atomic weight, every eighth element showed similar properties. This observation, which was at first dismissed by the chemical community as being purely coincidental, is readily explicable using the modern periodic table and the concept of families of elements.
In 1869, after organizing the known elements so that those with similar properties were grouped together, Dmitri Mendeleèev (1834-1907) predicted the existence and properties of several as-yet undiscovered elements. The subsequent discovery of these elements, and the accuracy of many of Mendeleév’s predictions, fully justified the notion that the elements could be organized into families. Today, we recognize that the basis for this classification is the similarity in the electronic configurations of the atoms concerned.
Main-group families
For those families of elements found among the main group elements, that is, elements in groups 1 and 2, and 13 through 18 of the periodic table, each member of a given family has the same number of valence electrons. A detailed examination of the electron configurations of the elements in these families reveals that each family has its own characteristic arrangement of electrons.
For example, each element in group 1, the alkali metals, has its valence electron in an s sublevel. As a result, all the elements in this family have an electron configuration which, when written in linear form, terminates with ns 1, where n is an integer representing the principal quantum number of the valence shell. Thus, the electron configuration of lithium is 1s 22s 1, that of sodium is 1s 22s 22p 63s 1, potassium is 1s 22s 22p 63s 23p 64 s 1, and so on.
In a similar way, the elements in group 2, the alkaline earth metals, each have two valence electrons and electron configurations that terminate in ns 2. For example, beryllium is 1s 22 s 2, magnesium is 1s 22 s 22 p 63 s 2, calcium is 1s 22 s 22p 63 s 23p 64s 2.
Because the s sublevel can only accommodate a maximum of 2 electrons, the members of group 13, which have 3 valence electrons, all have electron configurations terminating in ns2;np1; ; for example, aluminum is 1s 22s 22p 6 3s 23p 1. The remaining main-group families, group 14 (the carbon family), group 15 (the pnicogens), group 16 (the chalcogens), group 17 (the halogens), and group 18 (the rare gases) have 4, 5, 6, 7, and 8 valence electrons, respectively. Of these valence electrons, two occupy an s sublevel and the remainder occupy the p sublevel having the same principal quantum number.
The similarity in electron configurations within a given main group family results in the members of the family having similar properties. For example, the alkali metals are all soft, highly reactive elements with a silvery appearance. None of these elements is found uncombined in nature, and they are all willing to give up their single valence electron in order to form an ion with a charge of +1. Each alkali metal will react with water to give hydrogen gas and a solution of the metal hydroxide.
Characteristic patterns of behavior can also be identified for other main-group families; for example, the members of the carbon family all form chlorides of the type ECl4 and hydrides of the type EH4, and have a tendency towards catenation, that is, for identical atoms to join together to form long chains or rings. Similarly, although little is known about the heaviest, radioactive halogen, astatine, its congeners all normally exist as diatomic molecules, X2, and show a remarkable similarity and predictability in their properties. All the members of this family are quite reactive-fluorine, the most reactive, combines directly with all the known elements except helium, neon and argon-and they all readily form ions having a charge of -1.
The family of elements at the far right of the periodic table, the rare gases, consists of a group of colorless, odorless gases that are noted for their lack of reactivity. Also known as noble gasses, the first compounds of these elements were not prepared until 1962. Even today there are only a limited number of krypton compounds known and still no known compounds of helium, neon, or argon.
Hydrogen: The elemental orphan
When the elements are organized into families, hydrogen presents a problem. In some of its properties, hydrogen resembles the alkali metals, but it also shows some similarities to the halogens. Many periodic tables include hydrogen in group 1; others show it in groups 1 and 17. An alternative approach is to recognize hydrogen as being unique and not to assign it to a family.
Other families of elements
In addition to the main-group families, other families of elements can be identified among the remaining elements of the periodic table.
The transition metals
The elements in groups 3 through 12, the transition metals or d -block elements, could be considered as one large family. Their characteristic feature, with some exceptions, is the presence of an incomplete d sublevel in their electron configurations. As with any large family, transition metals show considerable diversity in their behavior, although there are some unifying features, such as their ability to form ions with a charge of +2. Another similarity between these elements is that most of their compounds are colored.
The coinage metals and the platinum metals
At least two small family units can be identified within the larger transition-metal family. One of these small families, the coinage metals, consists of copper, silver and gold, the three elements in group 11. The other family, the platinum metals, includes elements from three groups: ruthenium and osmium from group 8; rhodium and iridium from group 9; and palladium and platinum from group 10.
The coinage metals are resistant to oxidation, hence their traditional use in making coins. Unlike the majority of the transition metals, the coinage metals each have a full d sublevel and one electron in an s sublevel, that is, an electron configuration that terminates in (n-1) d 10 n s 1. One result of this electron configuration is that each of these metals will form an ion of the type M+, although it is only for silver that this ion is relatively stable.
The platinum metals occur together in the same ores, are difficult to separate from one another, and are relatively unreactive.
The lanthanides and actinides
The lanthanides (or rare-earth elements) and actinides are two families that are related because they both result from electrons being added into an f sub-level. Both families have 14 members, the lanthanides consisting of the elements with atomic numbers 58 through 71, and the actinides including the elements with atomic numbers 90 through 103. However, it is sometimes convenient to consider lanthanum (atomic number 57) as an honorary member of the lanthanide family and to treat actinium (atomic number 89) in a similar manner with respect to the actinides.
The lanthanides are usually found together in the same ores and despite their alternative name of the rare-earth elements, they are not particularly rare. In contrast, only two of the actinides, thorium and uranium, occur in nature, the remainder having been synthesized by nuclear scientists. Members of both families form ions with a charge of +3, although other ions are also formed, particularly by the actinides.
See also Element, chemical; Element, transuranium.
KEY TERMS
Catenation— The ability of identical atoms to bond together to form long chains or rings.
Congeners— Elements in the same group of the periodic table.
Diatomic molecule— A molecule consisting of two atoms.
Electron configuration— The arrangement of electrons in the occupied electron energy levels or sub-levels of an atom.
Main-group elements— Those elements in groups 1, 2 and 13 through 18 of the periodic table.
Principal quantum number— An integer used to identify the energy levels of an atom.
Triad— A group of three elements displaying a certain regularity in their properties.
Valence electrons— The electrons in the outermost shell of an atom that determine an element’s chemical properties.
Resources
BOOKS
Emsley, John. Nature’s Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press, 2002.
Norman, Nicholas C. Periodicity and the s- and p-Block Elements. Oxford Chemistry Primers, no. 51. New York: Oxford Univ. Press, 1997.
Silberberg, Martin. Chemistry: The Molecular Nature of Matter and Change. St. Louis: Mosby, 1996.
OTHER
Rice School. “Exploring the Periodic Table and Families of Elements: Some Important Facts” <http://www.ruf.rice.edu/sandyb/Lessons/chem.html> (accessed November 21, 2006).
Arthur M. Last