alkali metal sevies
The alkali metals are a group (column) in the periodic table consisting of the chemical elements lithium (Li), sodium (Na), potassium (K),rubidium (Rb), caesium (Cs),and francium (Fr). This group lies in the s-block of the periodic table of elements as all alkali metals have their outermost electron in an s-orbital: this shared electron configuration results in them having very similar characteristic properties. Indeed, the alkali metals provide the best example of group trends in properties in the periodic table, with elements exhibiting well-characterised homologous behaviour.
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. 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. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements,[note 3] excluding hydrogen (H), which is nominally a group 1 element but not normally considered to be an alkali metal as it rarely exhibits behaviour comparable to that of the alkali 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 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, 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.
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 very efficient light. 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.
| Name | Lithium | Sodium | Potassium | Rubidium | Caesium | |
|---|---|---|---|---|---|---|
| Atomic number | 3 | 11 | 19 | 37 | 55 | |
| Standard atomic weight (u)[note 4][10][11] | 6.94(1)[note 5] | 22.98976928(2) | 39.0983(1) | 85.4678(3) | 132.9054519(2) | |
| Electron configuration | [He] 2s1 | [Ne] 3s1 | [Ar] 4s1 | [Kr] 5s1 | [Xe] 6s1 | |
| Melting point (°C) | 180.54 | 97.72 | 63.38 | 39.31 | 28.44 | |
| Boiling point (°C) | 1342 | 883 | 759 | 688 | 671 | |
| Density (g·cm−3) | 0.534 | 0.968 | 0.89 | 1.532 | 1.93 | |
| Heat of fusion (kJ·mol−1) | 3.00 | 2.60 | 2.321 | 2.19 | 2.09 | |
| Heat of vaporisation (kJ·mol−1) | 136 | 97.42 | 79.1 | 69 | 66.1 | |
| Heat of formation of monatomic gas (kJ·mol−1) | 162 | 108 | 89.6 | 82.0 | 78.2 | |
| Electrical resistivity at 25 °C (nΩ·cm) | 94.7 | 48.8 | 73.9 | 131 | 208 | |
| Atomic radius (pm) | 152 | 186 | 227 | 248 | 265 | |
| Ionic radius of hexacoordinate M+ ion (pm) | 76 | 102 | 138 | 152 | 167 | |
| First ionisation energy (kJ·mol−1) | 520.2 | 495.8 | 418.8 | 403.0 | 375.7 | |
| Electron affinity (kJ·mol−1) | 59.62 | 52.87 | 48.38 | 46.89 | 45.51 | |
| Enthalpy of dissociation of M2 (kJ·mol−1) | 106.5 | 73.6 | 57.3 | 45.6 | 44.77 | |
| Pauling electronegativity | 0.98 | 0.93 | 0.82 | 0.82 | 0.79 | |
| Standard electrode potential (E°(M+→M0); V) | −3.0405 | −2.714 | −2.925 | −2.925 | −2.923 | |
| Flame test colour Principal emission/absorption wavelength (nm) |
Crimson 670.8 |
Yellow 589.2 |
Violet 766.5 |
Red-violet 780.0 |
Blue 455.5 |
Francium
| Group | 1 | Melting point | 21°C, 70°F, 294 K |
| Period | 7 | Boiling point | 650°C, 1202°F, 923 K |
| Block | s | Density (g cm−3) | Unknown |
| Atomic number | 87 | Relative atomic mass | [223] |
| State at 20°C | Solid | Key isotopes | 223Fr |
| Electron configuration | [Rn] 7s1 | CAS number | 7440-73-5 |
The alkali metals are more similar to each other than the elements in any other group are to each other.Indeed, the similarity is so great that it is quite difficult to separate potassium, rubidium, and caesium, due to their similar ionic radii; lithium and sodium are more distinct. For instance, when moving down the table, all known alkali metals show increasing atomic radius, decreasing electronegativity,increasing reactivity,[and decreasing melting and boiling points as well as heats of fusion and vaporisation.75 In general, their densities increase when moving down the table, with the exception that potassium is less dense than sodium.One of the very few properties of the alkali metals that does not display a very smooth trend is their reduction potentials: lithium’s value is anomalous, being more negative than the others.:75 This is because the Li+ ion has a very high hydration energy in the gas phase: though the lithium ion disrupts the structure of water significantly, causing a higher change in entropy, this high hydration energy is enough to make the reduction potentials indicate it as being the most electropositive alkali metal, despite the difficulty of ionising it in the gas phase.