Thread Content
Alkali metal elements include six elements: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr); they are the elements of Group 1 in the periodic table. Since the hydroxides of sodium and potassium are highly soluble in water, they ionize almost completely and possess strong basicity, which is why they are called alkali metals. Other elements in this group also possess similar properties, which is why this group is collectively referred to as alkali metals. Among them, rubidium and cesium are rare elements, while francium is a radioactive element. The prominent feature that distinguishes alkali metal elements from other elements is: (1) they have strong metallic properties and are the most typical metals. Metallic elements are highly chemically reactive and possess strong reducing power (i.e., the ability to lose electrons). (2) Their oxides and hydroxides are soluble in water and exhibit strong basicity. (3) Their salts are typical ionic compounds, and the vast majority are soluble in water. The atomic structure of alkali metal elements is characterized by 8 electrons in the penultimate shell, in the configuration s2p6 (lithium has only 2 electrons in its penultimate shell, ls2). This is similar to the electron configuration of the outermost shell in noble gases, which consists of just one electron, in the configuration ns1. In each period, the atomic radius of alkali metal elements is the largest. Alkali metals readily lose their outermost electrons in chemical reactions to become +I valence cations. They do not change value, nor do they readily form complexes. It is difficult for their cations to gain an electron and be reduced to elemental metal. Therefore, exhibiting the most typical chemical properties of metals, elements in the same group see an increase in atomic radius as the atomic number increases from top to bottom, and their metallic character strengthens accordingly. The common physical characteristic of alkali metals is their low density; they are the lightest among metals, which is why they are classified as light non-ferrous metals. Their melting points are also lower. Free electrons in alkali metal lattices are highly active and exhibit good electrical conductivity; when exposed to light, these electrons can be emitted from the surface. Utilizing this property, rubidium and cesium are used in the manufacture of phototubes. Alkali metals can also form alloys that are liquid at room temperature; the most important ones are potassium-sodium alloys (77.2% K and 22.8% Na, with a melting point of 260.7 K) and sodium amalgam (with a melting point of 236.2 K). The former is used as a coolant in nuclear reactors due to its high specific heat. The latter is often used as a reducing agent in organic synthesis due to its slow reducibility. When alkali metals and their volatile compounds are burned, they emit special light that gives the flame a characteristic color; for example, the flame of lithium is red, that of sodium is yellow, while those of potassium, rubidium, and cesium are purple. In analytical chemistry, the color of the flame is commonly used to identify sodium and potassium.