The elements found in the first column of the periodic table, excluding hydrogen, are known as the alkali metals. This group of six metallic elements—lithium, sodium, potassium, rubidium, cesium, and francium—share chemical behaviors due to their electron configuration. They become progressively heavier as you move down the column.
Characteristics of Alkali Metals
Alkali metals are united by a single, defining characteristic: they each possess only one electron in their outermost shell. This electron configuration means they readily lose this single valence electron to form a cation with a positive one (+1) charge. This tendency to give up an electron makes them highly reactive, especially with water and air. These metals are relatively soft, can be easily cut with a knife, and possess low melting points compared to most other metals. As atomic size increases moving from top to bottom, the outermost electron is held less tightly by the nucleus, resulting in both the atomic size and the chemical reactivity increasing down the column.
Francium: The Heaviest Alkali Metal
The heaviest element in this family is Francium (Fr), which occupies the seventh period with an atomic number of 87. This element is extremely rare; estimates suggest that no more than 30 grams of it are present in the Earth’s crust at any single time. Francium is defined by its intense instability and radioactivity. Its longest-lived isotope, Francium-223, has a half-life of only about 22 minutes. Because of this rapid decay, Francium has never been isolated in a large enough quantity to be viewed by the naked eye. Scientists must study its behavior using radiochemical techniques, observing its chemical properties in trace amounts.
Cesium: The Practical Heavyweight
Because Francium is so fleeting, Cesium (Cs) is often considered the heaviest alkali metal that can be studied and utilized in a practical sense. With an atomic number of 55, Cesium is the last non-radioactive element in the group. It is a soft, silvery-gold metal that is so reactive it ignites explosively upon contact with water, making it the most reactive of the stable metals. Cesium has the lowest ionization energy of all the stable elements. This property is exploited in several applications, including its use in highly accurate atomic clocks. Cesium-133 is used to define the second, with one second equal to 9,192,631,770 cycles of radiation from the atom. Cesium compounds are also used in high-density drilling fluids for the petroleum industry and as “getters” to remove trace gases in vacuum tubes.