The alkali metals form Group 1 of the periodic table. The six alkali metals are Lithium (Li) atomic number 3, Sodium (Na) atomic number 11, Potassium (K) atomic number 19, Rubidium (Rb) atomic number 37, Cesium (Cs) atomic number 55 and Francium (Fr) atomic number 87. All the alkali metals have a single electron in their outer electron shell. This electron configuration means that they readily donate their outer electron to another element. In doing so, the alkali metal atom becomes a positive ion.
As they are all so reactive, alkali metals never occur in the metallic state in nature. While the chemical reactions of five of the elements are well known, those of francium are unknown. Francium is radioactive and its longest lived isotope has a half-life of just 22 minutes. Scientists estimate that at any time, there is just one ounce of francium present on Earth, so little is known of its chemical reactions or properties.
All of the alkali metals are very soft and cut easily with a knife. They are not very dense metals, three of them lithium, sodium and potassium are less dense than water.
All of the alkali metals react with water. As the atomic number increases this reaction becomes more violent. The reaction results in the forming of a positive alkali metal ion a negative hydroxyl ion (OH-) and hydrogen gas. Hydroxyl ions are responsible for the alkalinity of solutions, hence the name alkali metals.
Atomic radius of an atom increases with the atomic number. As the atomic radius increases the distance between the negatively charged electron in the outer shell and the positively charged atomic nucleus also increases. This means that the outer electron can escape from the atom more easily with the increase in atomic number. Therefore, the higher numbered members of the alkali metals, such as cesium, are more reactive than the lower numbered members such as lithium.
The violent reactions incurred by the addition of water to sodium and the other alkali metals with greater atomic numbers may cause the resulting hydrogen gas to ignite. To prevent them reacting with water vapor present in the air pure alkali metals are stored in mineral oil.
As their atomic numbers increase, the melting points and boiling points of the alkali metals decrease. Lithium has a melting point of 453.65 K (180.50C or 356.90F) and a boiling point of 1615 K (1342C or 2448F) while those of cesium are 301.59 K (28.44C or 83.19F) and 944 K (671C or 1240F). Both rubidium, with a melting point of 312.46 K (39.31C or 102.76F), and cesium are close to melting at room temperature.
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