The elements in Group 1 of the Periodic Table—Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs), and Francium (Fr)—are known as the alkali metals. These metals are soft, silvery-white, and have extremely low densities. The alkali metals are the most reactive group of metals on the Periodic Table. Their extreme chemical instability means they are never found in their pure, elemental form in nature but only as compounds, a direct consequence of their atomic structure.
The Single Valence Electron Problem
The primary reason for the intense reactivity of Group 1 elements lies in their atomic structure, specifically the number of electrons in their outermost shell. Every alkali metal atom possesses exactly one electron in its valence shell. Atoms seek the stable configuration of a noble gas, which usually means having eight electrons in the outer shell (the octet rule). Gaining seven electrons is energetically prohibitive for alkali metals.
Instead, the path to stability is to lose that single electron. Shedding this electron reveals a filled inner shell, achieving the stable electron configuration of the preceding noble gas. This loss creates a positively charged ion (cation) with a charge of +1. The ease with which this electron is given up directly causes the group’s high chemical reactivity.
Low Ionization Energy and Atomic Size
The ease of losing the single valence electron is quantified by ionization energy, the minimum energy required to remove an electron. Group 1 elements have the lowest first ionization energies of any elements in their respective periods. This low energy requirement makes electron loss highly favorable.
As one moves down Group 1 from Lithium to Cesium, the atomic size increases because each subsequent element adds a new electron shell. This greater distance places the single outer electron farther from the positively charged nucleus. Additionally, the growing number of inner electron shells shields the outermost electron, blocking the nucleus’s attractive force. This combination of distance and shielding weakens the pull on the valence electron, causing ionization energy to drop and reactivity to increase dramatically down the group.
The Violent Chemical Reactions of Alkali Metals
The high reactivity explained by their atomic structure manifests in violent chemical reactions. A common demonstration is the reaction of alkali metals with water, which is a highly exothermic process that releases significant heat. The metal readily donates its single electron to water molecules, forming a metal hydroxide and liberating hydrogen gas.
This reaction becomes progressively more vigorous moving down the group. Lithium reacts moderately, while Sodium reacts aggressively enough that the heat produced ignites the released hydrogen gas. Cesium reacts instantly and explosively upon contact with water, demonstrating the trend of increasing reactivity. Due to this intense instability, alkali metals must be stored under mineral oil or kerosene to prevent reaction with air and moisture. They also react vigorously with non-metals, such as Group 17 halogens like chlorine, to form stable ionic salts.