Alkali metal

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Group	1
Period
1	1 H
2	3 Li
3	11 Na
4	19 K
5	37 Rb
6	55 Cs
7	87 Fr

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). (Note that hydrogen, although nominally also a member of Group 1, very rarely exhibits behaviour 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 are very weakly radioactive (harmless) due to the presence of long duration radioactive isotopes.

The alkali metals are silver-colored (caesium 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.

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. But their second ionization potentials are very high, as removing an electron from a species having a noble gas configuration is very difficult.

1 Reactions
- 1.1 Reactions in water
- 1.2 Reaction in ammonia
2 Trends
3 Biological occurrences
4 See also
5 External links

[edit] Reactions

[edit] Reactions in water

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)

In this reaction, enough energy is produced to ignite the hydrogen, creating a lilac flame above the potassium. If rubidium, cesium or francium react with water though, the subsequent explosion tends to be very violent, a fact not helped by its rapidity in coming about once the metal and water start reacting.

[edit] Reaction in ammonia

Alkali metals dissolve in liquid ammonia to give blue solutions that are paramagnetic. Saturated solutions are a deep purple color.

K (s) + NH₃ (l) → K⁺ (aq) + e^- (aq)

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

[edit] 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.

Alkali metal	Standard Atomic Weight (u)	Melting Point (K)	Boiling Point (K)	Electronegativity (Pauling)
Lithium	6.941	453.69	1615	0.98
Sodium	22.990	370.87	1156	0.93
Potassium	39.098	336.53	1032	0.82
Rubidium	85.468	312.46	961	0.82
Caesium	132.905	301.59	944	0.79
Francium	(223)	? 300	? 950	0.7

[edit] Biological occurrences

Lithium is not normally found in biological systems, however lithium salts have been used in psychiatry to treat manic depression.

With a high natural abundance and acqueous solubility, sodium and potassium are ubiquitous and essential in all known living systems. They are generally present in cell plasma and electrolytes. In multi-celluar organisms (eukaryotes), sodium/potassium ion pumps are a common mechanism for controlling electrochemical potential.

Rubidium and caesium, with lower natural abundance, have not been found in biological systems, and are generally considered moderately toxic.

Francium is highly radioactive, and with a short half-life, only occurs on earth in minute traces. It would be highly toxic to living cells.

[edit] See also

[edit] External links

Science aid:Alkali metals A simple look at alkali metals
Doc Brown, Alkali metals
Video showing various alkali metals in water

**Explanation of above periodic table slice:**
Alkali metals	Atomic numbers in black are solids	Solid borders indicate primordial elements (older than the Earth)	Dashed borders indicate natural radioactive elements with no isotopes older than the Earth