The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Together with hydrogen they constitute group 1,which lies in the s-block of the periodic table. All alkali metals have their outermost electron in an s-orbital: this shared electron configuration results in their having very similar characteristic properties.[note 4] Indeed, the alkali metals provide the best example of group trends in properties in the periodic table, with elements exhibiting well-characterised homologous behaviour. This family of elements is also known as the lithium family after its leading element.
The alkali metals are all shiny, soft, highly reactive metals at standard temperature and pressure and readily lose their outermost electron to form cations with charge +1. They can all be cut easily with a knife due to their softness, exposing a shiny surface that tarnishes rapidly in air due to oxidation by atmospheric moisture and oxygen (and in the case of lithium, nitrogen). Because of their high reactivity, they must be stored under oil to prevent reaction with air, and are found naturally only in salts and never as the free elements. Caesium, the fifth alkali metal, is the most reactive of all the metals. All the alkali metals react with water, with the heavier alkali metals reacting more vigorously than the lighter ones.
All of the discovered alkali metals occur in nature as their compounds: in order of abundance, sodium is the most abundant, followed by potassium, lithium, rubidium, caesium, and finally francium, which is very rare due to its extremely high radioactivity; francium occurs only in minute traces in nature as an intermediate step in some obscure side branches of the natural decay chains. Experiments have been conducted to attempt the synthesis of ununennium (Uue), which is likely to be the next member of the group; none was successful. However, ununennium may not be an alkali metal due to relativistic effects, which are predicted to have a large influence on the chemical properties of superheavy elements; even if it does turn out to be an alkali metal, it is predicted to have some differences in physical and chemical properties from its lighter homologues.
Most alkali metals have many different applications. One of the best-known applications of the pure elements is the use of rubidium and caesium in atomic clocks, of which caesium atomic clocks form the basis of the second. A common application of the compounds of sodium is the sodium-vapour lamp, which emits light very efficiently. Table salt, or sodium chloride, has been used since antiquity. Lithium finds use as a psychiatric medication and as an anode in lithium batteries. Sodium and potassium are also essential elements, having major biological roles as electrolytes, and although the other alkali metals are not essential, they also have various effects on the body, both beneficial and harmful.
Alkali metal salts were known to the ancients. The Old Testament refers to a salt called neter (sodium carbonate), which was extracted from the ash of vegetable matter. Saltpetre (potassium nitrate) was used in gunpowder, which was invented in China about the 9th century AD and had been introduced into Europe by the 13th century.
In October 1807 the English chemist Sir Humphry Davy isolated potassium and then sodium. The name sodium is derived from the Italian soda, a term applied in the Middle Ages to all alkalies; potassium comes from the French potasse, a name used for the residue left in the evaporation of aqueous solutions derived from wood ashes.
Lithium was discovered by the Swedish chemist Johan August Arfwedson in 1817 while analyzing the mineral petalite. The name lithium is derived from lithos, the Greek word for “stony.” The element was not isolated in pure form until Davy produced a minute quantity by the electrolysis of lithium chloride.
While the German chemists Robert Bunsen and Gustav Kirchhoff were investigating the mineral waters in the Palatinate in 1860, they obtained a filtrate that was characterized by two lines in the blue region of its spectrum (the light emitted when the sample was inserted into a flame). They suggested the presence of a new alkali element and called it cesium, derived from the Latin caesius, used to designate the blue of the sky. The same researchers, on extracting the alkalies from the mineral lepidolite, separated another solution, which yielded two spectral lines of red colour. They proposed the name rubidium for the element in this solution from the Latin rubidus, which was used for the darkest red colour. Francium was not discovered until 1939 by Marguerite Perey of the Radium Institute in Paris.
In the 19th century the only use for the alkali metals was the employment of sodium as a reagent in the manufacture of aluminum. When the electrolytic process for aluminum purification was established, it appeared that large-scale use of sodium would cease. Subsequent improvements in the electrolytic production of sodium, however, reduced the cost of this element to such an extent that it can be employed economically to manufacture gasoline additives, reagents for chemical industry, herbicides, insecticides, nylon, pharmaceuticals, and reagents for metal refining. The continuous electrolysis of sodium hydroxide, a technique called the Castner process, was replaced in 1926 by the Downs cell process. This process, in which a molten sodium chloride–calcium chloride mixture (to reduce the melting point) is electrolyzed, produces both sodium metal and chlorine.
General properties of the group
The alkali metals have the high thermal and electrical conductivity, lustre, ductility, and malleability that are characteristic of metals. Each alkali metal atom has a single electron in its outermost shell. This valence electron is much more weakly bound than those in inner shells. As a result, the alkali metals tend to form singly charged positive ions (cations) when they react with nonmetals. The compounds that result have high melting points and are hard crystals that are held together by ionic bonds (resulting from mutually attractive forces that exist between positive and negative electrical charges). In the metallic state, either pure or in alloys with other alkali metals, the valence electrons become delocalized and mobile as they interact to form a half-filled valence band. As with other metals, such a partially filled valence band is a conduction band and is responsible for the valence properties typical of metals. In passing from lithium to francium, the single electron tends to be less strongly held. Generally, the energy necessary to remove the outermost electron from the atoms of an element, the ionization energy, decreases in the periodic table toward the left and downward in each vertical file, with the result that the most easily ionizable element in the entire table is francium, followed closely by cesium. The alkali metals, which make up the extreme left-hand file, have ionization energies ranging from 124.3 kilocalories per mole (kcal/mole) in lithium to 89.7 kcal/mole in cesium (omitting the rare radioactive element francium). The alkaline-earth metals, the next group to the right, have higher ionization energies ranging from 214.9 in beryllium to 120.1 kcal/mole in barium.
The electronegativity scale of the elements compares the ability of the atoms of the various elements to attract electrons to themselves. In the periodic table the electronegativities range from 0.7 for cesium, the least electronegative of the elements, to 4.0 for fluorine, the most electronegative. Metals are ordinarily considered to be those elements having values less than 2.0 on the electronegativity scale. As a group the alkali metals are the least electronegative of the elements, ranging from 0.7 to 1.0 on the scale, while the alkaline earths, the next group on the table, have electronegativities ranging from about 0.9 to 1.5.
The table summarizes the important physical and thermodynamic properties of the alkali metals. At atmospheric pressure these metals are all characterized by a body-centred cubic crystallographic arrangement (a standard pattern of atoms in their crystals), with eight nearest neighbours to each atom. The closest distance between atoms, a characteristic property of crystals, increases with increasing atomic weight of the alkali metal atoms. As a group, the alkali metals have a looser crystallographic arrangement than any of the other metallic crystals, and cesium—because of its greater atomic weight—has an interatomic distance that is greater than that of any other metal.
In the Solar System
Estimated abundances of the chemical elements in the Solar system. Hydrogen and helium are most common, from the Big Bang. The next three elements (lithium, beryllium, and boron) are rare because they are poorly synthesised in the Big Bang and also in stars. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance in elements as they have even or odd atomic numbers, and (2) a general decrease in abundance, as elements become heavier. Iron is especially common because it represents the minimum energy nuclide that can be made by fusion of helium in supernovae.
The Oddo–Harkins rule holds that elements with even atomic numbers are more common that those with odd atomic numbers, with the exception of hydrogen. This rule argues that elements with odd atomic numbers have one unpaired proton and are more likely to capture another, thus increasing their atomic number. In elements with even atomic numbers, protons are paired, with each member of the pair offsetting the spin of the other, enhancing stability. All the alkali metals have odd atomic numbers and they are not as common as the elements with even atomic numbers adjacent to them (the noble gases and the alkaline earth metals) in the Solar System. The heavier alkali metals are also less abundant than the lighter ones as the alkali metals from rubidium onward can only be synthesised in supernovae and not in stellar nucleosynthesis. Lithium is also much less abundant than sodium and potassium as it is poorly synthesised in both Big Bang nucleosynthesis and in stars: the Big Bang could only produce trace quantities of lithium, beryllium and boron due to the absence of a stable nucleus with 5 or 8 nucleons, and stellar nucleosynthesis could only pass this bottleneck by the triple-alpha process, fusing three helium nuclei to form carbon, and skipping over those three elements.
Spodumene, an important lithium mineral
The Earth formed from the same cloud of matter that formed the Sun, but the planets acquired different compositions during the formation and evolution of the solar system. In turn, the natural history of the Earth caused parts of this planet to have differing concentrations of the elements. The mass of the Earth is approximately 5.98×1024 kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to planetary differentiation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.
The alkali metals, due to their high reactivity, do not occur naturally in pure form in nature. They are lithophiles and therefore remain close to the Earth’s surface because they combine readily with oxygen and so associate strongly with silica, forming relatively low-density minerals that do not sink down into the Earth’s core. Potassium, rubidium and caesium are also incompatible elements due to their large ionic radii.
Sodium and potassium are very abundant in earth, both being among the ten most common elements in Earth’s crust; sodium makes up approximately 2.6% of the Earth’s crust measured by weight, making it the sixth most abundant element overall and the most abundant alkali metal. Potassium makes up approximately 1.5% of the Earth’s crust and is the seventh most abundant element. Sodium is found in many different minerals, of which the most common is ordinary salt (sodium chloride), which occurs in vast quantities dissolved in seawater. Other solid deposits include halite, amphibole, cryolite, nitratine, and zeolite. Many of these solid deposits occur as a result of ancient seas evaporating, which still occurs now in places such as Utah’s Great Salt Lake and the Dead Sea.: 69 Despite their near-equal abundance in Earth’s crust, sodium is far more common than potassium in the ocean, both because potassium’s larger size makes its salts less soluble, and because potassium is bound by silicates in soil and what potassium leaches is absorbed far more readily by plant life than sodium.: 69
Despite its chemical similarity, lithium typically does not occur together with sodium or potassium due to its smaller size.: 69 Due to its relatively low reactivity, it can be found in seawater in large amounts; it is estimated that seawater is approximately 0.14 to 0.25 parts per million (ppm) or 25 micromolar. Its diagonal relationship with magnesium often allows it to replace magnesium in ferromagnesium minerals, where its crustal concentration is about 18 ppm, comparable to that of gallium and niobium. Commercially, the most important lithium mineral is spodumene, which occurs in large deposits worldwide.: 69
Rubidium is approximately as abundant as zinc and more abundant than copper. It occurs naturally in the minerals leucite, pollucite, carnallite, zinnwaldite, and lepidolite, although none of these contain only rubidium and no other alkali metals.: 70 Caesium is more abundant than some commonly known elements, such as antimony, cadmium, tin, and tungsten, but is much less abundant than rubidium.
Francium-223, the only naturally occurring isotope of francium, is the product of the alpha decay of actinium-227 and can be found in trace amounts in uranium minerals. In a given sample of uranium, there is estimated to be only one francium atom for every 1018 uranium atoms. It has been calculated that there are at most 30 grams of francium in the earth’s crust at any time, due to its extremely short half-life of 22 minutes.[