<P> 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 the minutest 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, but they have all met with failure . 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 . </P> <P> 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 are the most accurate and precise representation of time . 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 . 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 . </P> <P> The physical and chemical properties of the alkali metals can be readily explained by their having an ns valence electron configuration, which results in weak metallic bonding . Hence, all the alkali metals are soft and have low densities, melting and boiling points, as well as heats of sublimation, vaporisation, and dissociation . They all crystallise in the body - centered cubic crystal structure, and have distinctive flame colours because their outer s electron is very easily excited . The ns configuration also results in the alkali metals having very large atomic and ionic radii, as well as very high thermal and electrical conductivity . Their chemistry is dominated by the loss of their lone valence electron in the outermost s - orbital to form the + 1 oxidation state, due to the ease of ionising this electron and the very high second ionisation energy . Most of the chemistry has been observed only for the first five members of the group . The chemistry of francium is not well established due to its extreme radioactivity; thus, the presentation of its properties here is limited . What little is known about francium shows that it is very close in behaviour to caesium, as expected . The physical properties of francium are even sketchier because the bulk element has never been observed; hence any data that may be found in the literature are certainly speculative extrapolations . </P> <Table> Properties of the alkali metals <Tr> <Th> Name </Th> <Th> Lithium </Th> <Th> Sodium </Th> <Th> Potassium </Th> <Th> Rubidium </Th> <Th> Caesium </Th> <Th> Francium </Th> </Tr> <Tr> <Td> Atomic number </Td> <Td> </Td> <Td> 11 </Td> <Td> 19 </Td> <Td> 37 </Td> <Td> 55 </Td> <Td> 87 </Td> </Tr> <Tr> <Td> Standard atomic weight (u) </Td> <Td> 6.94 (1) </Td> <Td> 22.98976928 (2) </Td> <Td> 39.0983 (1) </Td> <Td> 85.4678 (3) </Td> <Td> 132.9054519 (2) </Td> <Td> (223) </Td> </Tr> <Tr> <Td> Electron configuration </Td> <Td> (He) 2s </Td> <Td> (Ne) 3s </Td> <Td> (Ar) 4s </Td> <Td> (Kr) 5s </Td> <Td> (Xe) 6s </Td> <Td> (Rn) 7s </Td> </Tr> <Tr> <Td> Melting point (° C) </Td> <Td> 180.54 </Td> <Td> 97.72 </Td> <Td> 63.38 </Td> <Td> 39.31 </Td> <Td> 28.44 </Td> <Td>? </Td> </Tr> <Tr> <Td> Boiling point (° C) </Td> <Td> 1342 </Td> <Td> 883 </Td> <Td> 759 </Td> <Td> 688 </Td> <Td> 671 </Td> <Td>? </Td> </Tr> <Tr> <Td> Density (g cm) </Td> <Td> 0.534 </Td> <Td> 0.968 </Td> <Td> 0.89 </Td> <Td> 1.532 </Td> <Td> 1.93 </Td> <Td>? </Td> </Tr> <Tr> <Td> Heat of fusion (kJ mol) </Td> <Td> 3.00 </Td> <Td> 2.60 </Td> <Td> 2.321 </Td> <Td> 2.19 </Td> <Td> 2.09 </Td> <Td>? </Td> </Tr> <Tr> <Td> Heat of vaporisation (kJ mol) </Td> <Td> 136 </Td> <Td> 97.42 </Td> <Td> 79.1 </Td> <Td> 69 </Td> <Td> 66.1 </Td> <Td>? </Td> </Tr> <Tr> <Td> Heat of formation of monatomic gas (kJ mol) </Td> <Td> 162 </Td> <Td> 108 </Td> <Td> 89.6 </Td> <Td> 82.0 </Td> <Td> 78.2 </Td> <Td>? </Td> </Tr> <Tr> <Td> Electrical resistivity at 25 ° C (n Ω cm) </Td> <Td> 94.7 </Td> <Td> 48.8 </Td> <Td> 73.9 </Td> <Td> 131 </Td> <Td> 208 </Td> <Td>? </Td> </Tr> <Tr> <Td> Atomic radius (pm) </Td> <Td> 152 </Td> <Td> 186 </Td> <Td> 227 </Td> <Td> 248 </Td> <Td> 265 </Td> <Td>? </Td> </Tr> <Tr> <Td> Ionic radius of hexacoordinate M ion (pm) </Td> <Td> 76 </Td> <Td> 102 </Td> <Td> 138 </Td> <Td> 152 </Td> <Td> 167 </Td> <Td>? </Td> </Tr> <Tr> <Td> First ionisation energy (kJ mol) </Td> <Td> 520.2 </Td> <Td> 495.8 </Td> <Td> 418.8 </Td> <Td> 403.0 </Td> <Td> 375.7 </Td> <Td> 392.8 </Td> </Tr> <Tr> <Td> Electron affinity (kJ mol) </Td> <Td> 59.62 </Td> <Td> 52.87 </Td> <Td> 48.38 </Td> <Td> 46.89 </Td> <Td> 45.51 </Td> <Td>? </Td> </Tr> <Tr> <Td> Enthalpy of dissociation of M (kJ mol) </Td> <Td> 106.5 </Td> <Td> 73.6 </Td> <Td> 57.3 </Td> <Td> 45.6 </Td> <Td> 44.77 </Td> <Td>? </Td> </Tr> <Tr> <Td> Pauling electronegativity </Td> <Td> 0.98 </Td> <Td> 0.93 </Td> <Td> 0.82 </Td> <Td> 0.82 </Td> <Td> 0.79 </Td> <Td>? </Td> </Tr> <Tr> <Td> Standard electrode potential (E ° (M → M); V) </Td> <Td> − 3.04 </Td> <Td> − 2.71 </Td> <Td> − 2.93 </Td> <Td> − 2.98 </Td> <Td> − 3.03 </Td> <Td>? </Td> </Tr> <Tr> <Td> Flame test colour Principal emission / absorption wavelength (nm) </Td> <Td> Crimson 670.8 </Td> <Td> Yellow 589.2 </Td> <Td> Violet 766.5 </Td> <Td> Red - violet 780.0 </Td> <Td> Blue 455.5 </Td> <Td>? </Td> </Tr> </Table>

Why alkali metals have low melting and boiling points
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