Alkali metal

2008/9 Schools Wikipedia Selection. Related subjects: Chemical elements

The alkali metals are a series of elements comprising Group 1 ( IUPAC style) of the periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). (Hydrogen, although nominally also a member of Group 1, very rarely exhibits behavior comparable to the alkali metals). The alkali metals provide one of the best examples of group trends in properties in the periodic table, with well characterized homologous behaviour down the group.

The alkali metals are all highly reactive and are rarely found in elemental form in nature. As a result, in the laboratory they are stored under mineral oil. They also tarnish easily and have low melting points and densities. Potassium and rubidium possess a weak radioactive characteristic (harmless) due to the presence of long duration radioactive isotopes.

The alkali metals are silver-colored (cesium has a golden tinge), soft, low- density metals, which react readily with halogens to form ionic salts, and with water to form strongly alkaline (basic) hydroxides. These elements all have one electron in their outermost shell, so the energetically preferred state of achieving a filled electron shell is to lose one electron to form a singly charged positive ion, or cation.

Hydrogen, with a solitary electron, is usually placed at the top of Group 1 of the periodic table, but it is not considered an alkali metal; rather it exists naturally as a diatomic gas. Removal of its single electron requires considerably more energy than removal of the outer electron for the alkali metals. As in the halogens, only one additional electron is required to fill in the outermost shell of the hydrogen atom, so hydrogen can in some circumstances behave like a halogen, forming the negative hydride ion. Binary compounds of hydride with the alkali metals and some transition metals have been prepared. Under extremely high pressure, such as is found at the core of Jupiter, hydrogen does become metallic and behaves like an alkali metal; see metallic hydrogen.

Alkali metals have the lowest ionization potentials in their respective periods, as removing the single electron from the outermost shell gives them the stable inert gas configuration. Their second ionization potentials are very high, as removing an electron from a species having a noble gas configuration is very difficult.

Series of alkali metals, stored in mineral oil (note "natrium" is sodium.)

Alkali metals are famous for their vigorous reactions with water, and these reactions become increasingly violent as one moves down the group. The reaction with water is as follows:

Alkali metal + water → Alkali metal hydroxide + hydrogen gas

With potassium as an example:

2K (s) + 2H₂O (l) → 2KOH (aq) + H₂ (g)

Reaction in ammonia

Alkali metals dissolve in liquid ammonia to give blue solutions that are paramagnetic. As the solution approaches saturation, it becomes deep purple, then metallic.

K + NH₃ → K⁺ + e^-

Because the solution contains free electrons, it occupies more space than the sum of the volumes of the metal and ammonia. The presence of free electrons also makes these solutions very good reducing agents and good electrical conductors. Since they are easier to handle than the metals themselves they are sometimes used as substitutes.

The solution is not stable over long periods, and the dissolved alkali metal will react to form the corresponding amide. This reaction, when accelerated with a catalyst (usually iron(III) nitrate), is used for the production of sodium amide:

2 Na + 2 NH₃ → 2 NaNH₂ + H₂

The amide can be extracted, or it can be converted to sodium azide by bubbling nitrous oxide through the ammonia solution:

2 NaNH₂ + N₂O → NaN₃ + NaOH + NH₃

Trends

The alkali metals show a number of trends when moving down the group - for instance, decreasing electronegativity, increasing reactivity, and decreasing melting and boiling point. Density generally increases, with the notable exception of potassium being less dense than sodium, and the possible exception of francium being less dense than caesium.

Biological occurrences

• The metal lithium is not essential for any biological functions, but was found to exist in extremely tiny quantities in umbilical cord blood. This was found during a study by taking blood samples of newborns from both the umbilical cord and the mother at the time of birth, and subjecting the samples to a number of tests. They found that there was 7 times the amount of lithium in blood than previously believed. (Krachler and Rossipal 488) While it is considered a non-essential trace element (an element needed in extremely tiny amounts for proper growth), lithium also has medicinal uses. A double-blind, placebo-controlled, two-week long study conducted by a group of doctors on severely depressed patients proved that lithium is a key component in the treatment of severe depression. (Joffe, Levitt, and Sokolov, 791) It has also been proven to play a role in the treatment of other mental disorders. (Bildik et al. 277) One recorded instance was the case of a girl who developed a condition known as Neuroleptic malignant syndrome, who was given an antipsychotic drug (a tranquilizing drug that induces a state of relative calm). She quickly developed the symptoms of NMS, and was quickly hospitalized. There, doctors began treating her for the disorder, and in the final stage of her treatment, lithium was administered to her to help stabilize her mental functions. Weeks later, she was released and was in good physical health, but retained her previous diagnosis of Bipolar Disorder (a mental disorder that encompasses periods of mania and depression interspersed by periods of normal behaviour). (Bildik et al. 278)

• Sodium and potassium are very common alkali metals. These elements are essential for the existence of all known life. They are found in the cytoplasm (organic fluid, mainly water) of all living cells, and they are critical to everyday cell operation and regulation. (Chang and Tsong 587) Also, a common sodium salt, sodium chloride (table salt), has been identified many times as a contributor to hypertension in humans. (Sharp 727) A double-blind, two-month study was conducted on a group of elderly adults by modestly cutting their salt intake, to find if cutting salt intake would lower blood pressure. Some in the group were given half the salt that others were given for the duration of the study. The findings indicated that those who were given only half the amount of salt experienced a drop in blood pressure of up to 7.2 mm Hg (millimeters Mercury). (Cappacio and Markandu 850)

• Rubidium is not required for any known biological functions, but is known to be an unnecessary and toxic trace-element in humans. (Krachler and Rossipal 488) A recent study conducted on rubidium levels in freshwater ecosystems from Lake Erie and two Arctic lakes indicate that this element biomagnifies (the concentration grows as you move higher in the food chain) in marine food chains. This was found by collecting samples of fish, birds and plankton, and conducting tests on them. They found that the predatory animals (the small-mouth bass, for example) had higher rubidium concentrations than animals commonly considered to be prey. (Campbell et al. 1163)

• Caesium (also spelled cesium) is not typically found in any biological systems. However, caesium salt (caesium chloride) is a popular medicinal supplement taken to counteract the effects of damaged or diseased cells on the body. (Ackerman et al. 1011) Furthermore, this supplement has been shown to be toxic to human physiology, and has caused very serious side effects in people, and potentially can be deadly if not taken carefully. (Ackerman et al. 1011)

• Francium is an extremely rare element, and its chemical and physical properties have not been characterized to the same extent as the other alkali metals. It is known to have the same chemistry as caesium and is primarily an exotic curiosity in the world of physics. At any one moment in time, less than one ounce of francium is believed to exist on Earth, and its most stable isotope has a half life of a little less than 23 minutes.("Physics update" 9) This makes working with the element dangerous and difficult and radiochemistry techniques must be used to characterize its chemical properties. It has not been found to exist in any biological systems, and while there is no direct evidence of this, it is assumed to be true due to the rarity and instability of the element.