Caesium in the context of "Rubidium"

Mercury is a chemical element; it has symbol Hg and atomic number 80. It is commonly known as quicksilver. A heavy, silvery d-block element, mercury is the only metallic element that is known to be liquid at standard temperature and pressure; the only other element that is liquid under these conditions is bromine, one of the halogens, though metals such as caesium, gallium, and rubidium melt just above room temperature.

Mercury occurs in deposits throughout the world mostly as cinnabar (mercuric sulfide). The red pigment vermilion is obtained by grinding natural cinnabar or synthetic mercuric sulfide. Exposure to mercury and mercury-containing organic compounds is toxic to the nervous system, immune system and kidneys of humans and other animals; mercury poisoning can result from exposure to water-soluble forms of mercury (such as mercuric chloride or methylmercury) either directly or through mechanisms of biomagnification.

The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Together with hydrogen they constitute group 1, which lies in the s-block of the periodic table. 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. This family of elements is also known as the lithium family after its leading element.

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 (and in the case of lithium, nitrogen). 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. All the alkali metals react with water, with the heavier alkali metals reacting more vigorously than the lighter ones.

A 'unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom. The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, Δν_Cs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s."

Historically, many units of time were defined by the movements of astronomical objects.

Suboxides are a class of oxides wherein the electropositive element is in excess relative to the “normal” oxides. When the electropositive element is a metal, the compounds are sometimes referred to as “metal-rich”. Thus the normal oxide of caesium is Cs₂O, which is described as a Cs salt of O. A suboxide of caesium is Cs₁₁O₃, where the charge on Cs is clearly less than 1+, but the oxide is still described as O. Suboxides typically feature extensive bonding between the electropositive element, often leading to clusters.

Examples of suboxides other than alkali metal derivatives:

Alkali metal nitrates are chemical compounds consisting of an alkali metal (lithium, sodium, potassium, rubidium and caesium) and the nitrate ion. Only two are of major commercial value, the sodium and potassium salts. They are white, water-soluble salts with melting points ranging from 255 °C (LiNO
₃) to 414 °C (CsNO
₃) on a relatively narrow span of 159 °C

The melting point of the alkali metal nitrates tends to increase from 255 °C to 414 °C (with an anomaly for rubidium being not properly aligned in the series) as the atomic mass and the ionic radius (naked cation) of the alkaline metal increases, going down in the column. Similarly, but not presented here in the table, the solubility of these salts in water also decreases with the atomic mass of the metal.

The Borate Minerals are minerals which contain a borate anion group. The borate (BO₃) units may be polymerised similar to the SiO₄ unit of the silicate mineral class. This results in B₂O₅, B₃O₆, B₂O₄ anions as well as more complex structures which include hydroxide or halogen anions. The [B(O,OH)₄] anion exists as well.

Many borate minerals, such as borax, colemanite, and ulexite, are salts: soft, readily soluble, and found in evaporite contexts. However, some, such as boracite, are hard and resistant to weathering, more similar to the silicates.

Caesium (₅₅Cs) has 41 known isotopes, ranging in mass number from 112 to 152. Only one isotope, Cs, is stable. The longest-lived radioisotopes are Cs with a half-life of 1.33 million years,
Cs with a half-life of 30.04 years and Cs with a half-life of 2.0650 years. All other isotopes have half-lives less than 2 weeks, most under an hour.

Caesium is an abundant fission product (135 and 137 are directly produced) and various isotopes are of concern as such, see the sections below.

Rydberg matter is an exotic phase of matter formed by Rydberg atoms; it was predicted around 1980 by É. A. Manykin, M. I. Ozhovan and P. P. Poluéktov. It has been formed from various elements like caesium, potassium, hydrogen and nitrogen; studies have been conducted on theoretical possibilities like sodium, beryllium, magnesium and calcium. It has been suggested to be a material that diffuse interstellar bands may arise from. Circular Rydberg states, where the outermost electron is found in a planar circular orbit, are the most long-lived, with lifetimes of up to several hours, and are the most common.