Rubidium is a chemical element with symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group, with a standard atomic weight of 85.4678. Elemental rubidium is reactive, with properties similar to those of other alkali metals, including rapid oxidation in air. On Earth, natural rubidium comprises two isotopes: 72% is the stable isotope, 85Rb. German chemists Robert Bunsen and Gustav Kirchhoff discovered rubidium in 1861 by the newly developed technique, flame spectroscopy; the name comes from the Latin word rubidus, meaning the color of its emission spectrum. Rubidium's compounds have various chemical and electronic applications. Rubidium metal is vaporized and has a convenient spectral absorption range, making it a frequent target for laser manipulation of atoms. Rubidium is not a known nutrient for any living organisms. However, rubidium ions have the same charge as potassium ions, are taken up and treated by animal cells in similar ways. Rubidium is a soft, silvery-white metal.
It is the second most electropositive of the stable alkali metals and melts at a temperature of 39.3 °C. Like other alkali metals, rubidium metal reacts violently with water; as with potassium and caesium, this reaction is vigorous enough to ignite the hydrogen gas it produces. Rubidium has been reported to ignite spontaneously in air, it forms amalgams with mercury and alloys with gold, caesium and potassium, but not lithium. Rubidium has a low ionization energy of only 406 kJ/mol. Rubidium and potassium show a similar purple color in the flame test, distinguishing the two elements requires more sophisticated analysis, such as spectroscopy. Rubidium chloride is the most used rubidium compound: among several other chlorides, it is used to induce living cells to take up DNA. Other common rubidium compounds are the corrosive rubidium hydroxide, the starting material for most rubidium-based chemical processes. Rubidium silver iodide has the highest room temperature conductivity of any known ionic crystal, a property exploited in thin film batteries and other applications.
Rubidium forms a number of oxides when exposed to air, including rubidium monoxide, Rb6O, Rb9O2. Rubidium forms salts with halides, producing rubidium fluoride, rubidium chloride, rubidium bromide, rubidium iodide. Although rubidium is monoisotopic, rubidium in the Earth's crust is composed of two isotopes: the stable 85Rb and the radioactive 87Rb. Natural rubidium is radioactive, with specific activity of about 670 Bq/g, enough to expose a photographic film in 110 days. Twenty four additional rubidium isotopes have been synthesized with half-lives of less than 3 months. Rubidium-87 has a half-life of 48.8×109 years, more than three times the age of the universe of ×109 years, making it a primordial nuclide. It substitutes for potassium in minerals, is therefore widespread. Rb has been used extensively in dating rocks. During fractional crystallization, Sr tends to concentrate in plagioclase, leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residual magma may increase over time, the progressing differentiation results in rocks with elevated Rb/Sr ratios.
The highest ratios occur in pegmatites. If the initial amount of Sr is known or can be extrapolated the age can be determined by measurement of the Rb and Sr concentrations and of the 87Sr/86Sr ratio; the dates indicate the true age of the minerals only if the rocks have not been subsequently altered. Rubidium-82, one of the element's non-natural isotopes, is produced by electron-capture decay of strontium-82 with a half-life of 25.36 days. With a half-life of 76 seconds, rubidium-82 decays by positron emission to stable krypton-82. Rubidium is the twenty-third most abundant element in the Earth's crust as abundant as zinc and rather more common than copper, it occurs in the minerals leucite, pollucite and zinnwaldite, which contain as much as 1% rubidium oxide. Lepidolite contains between 0.3% and 3.5% rubidium, is the commercial source of the element. Some potassium minerals and potassium chlorides contain the element in commercially significant quantities. Seawater contains an average of 125 µg/L of rubidium compared to the much higher value for potassium of 408 mg/L and the much lower value of 0.3 µg/L for caesium.
Because of its large ionic radius, rubidium is one of the "incompatible elements." During magma crystallization, rubidium is concentrated together with its heavier analogue caesium in the liquid phase and crystallizes last. Therefore, the largest deposits of rubidium and caesium are zone pegmatite ore bodies formed by this enrichment process; because rubidium substitutes for potassium in the crystallization of magma, the enrichment is far less effective than that of caesium. Zone pegmatite ore bodies containing mineable quantities of caesium as pollucite or the lithium minerals lepidolite are a source for rubidium as a by-product. Two notable sources of rubidium are th
In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. Selection rules have been derived for electromagnetic transitions in molecules, in atoms, in atomic nuclei, so on; the selection rules may differ according to the technique used to observe the transition. The selection rule plays a role in chemical reactions, where some are formally spin forbidden reactions, that is, reactions where the spin state changes at least once from reactants to products. In the following atomic and molecular transitions are considered. In quantum mechanics the basis for a spectroscopic selection rule is the value of the transition moment integral ∫ ψ 1 ∗ μ ψ 2 d τ,where ψ 1 and ψ 2 are the wave functions of the two states involved in the transition and µ is the transition moment operator; this integral represents the propagator of the transition between states. In practice, the integral itself does not need to be calculated to determine a selection rule.
It is sufficient to determine the symmetry of transition moment function, ψ 1 ∗ μ ψ 2. If the symmetry of this function spans the symmetric representation of the point group to which the atom or molecule belongs its value is not zero and the transition is allowed. Otherwise, the transition is forbidden; the transition moment integral is zero if the transition moment function, ψ 1 ∗ μ ψ 2, is anti-symmetric or odd, i.e. y = -y holds. The symmetry of the transition moment function is the direct product of the parities of its three components; the symmetry characteristics of each component can be obtained from standard character tables. Rules for obtaining the symmetries of a direct product can be found in texts on character tables; the Laporte rule is a selection rule formally stated as follows: In a centrosymmetric environment, transitions between like atomic orbitals such as s-s, p-p, d-d, or f-f, transitions are forbidden. The Laporte rule applies to electric dipole transitions, so the operator has u symmetry.
P orbitals have u symmetry, so the symmetry of the transition moment function is given by the triple product u×u×u, which has u symmetry. The transitions are therefore forbidden. D orbitals have g symmetry, so the triple product g×u×g has u symmetry and the transition is forbidden; the wave function of a single electron is the product of a space-dependent wave function and a spin wave function. Spin can be said to have odd parity, it follows. In formal terms, only states with the same total spin quantum number are "spin-allowed". In crystal field theory, d-d transitions that are spin-forbidden are much weaker than spin-allowed transitions. Both can be observed, in spite of the Laporte rule, because the actual transitions are coupled to vibrations that are anti-symmetric and have the same symmetry as the dipole moment operator. In vibrational spectroscopy, transitions are observed between different vibrational states. In a fundamental vibration, the molecule is excited from its ground state to the first excited state.
The symmetry of the ground-state wave function is the same as that of the molecule. It is, therefore, a basis for the symmetric representation in the point group of the molecule, it follows that, for a vibrational transition to be allowed, the symmetry of the excited state wave function must be the same as the symmetry of the transition moment operator. In infrared spectroscopy, the transition moment operator z; the excited state wave function must transform as at least one of these vectors. In Raman spectroscopy, the operator transforms as one of the second-order terms in the right-most column of the character table, below; the molecule methane, CH4, may be used as an example to illustrate the application of these principles. The molecule has Td symmetry; the vibrations of methane span the representations A1 + E + 2T2. Examination of the character table shows that all four vibrations are Raman-active, but only the T2 vibrations can be seen in the infrared spectrum. In the harmonic approximation, it can be shown that overtones are forbidden in both infrared and Raman spectra.
However, when anharmonicity is taken into account, the transitions are weakly allowed. The selection rule for rotational transitions, derived from the symmetries of the rotational wave functions in a rigid rotor, is ΔJ = ±1, where J is a rotational quantum number. There are many types of coupled transition such as are observed in vibration-rotation spectra; the excited-state wave function is the product of two wave functions such as vibrational and rotational. The general principle is that the symmetry of the excited state is obtained as the direct product of the symmetries of the component wave functions. In rovibronic transitions, the excited states involve three wave functions; the infrared spectrum of hydrogen chloride gas shows rotational fine structure superimposed on the vibrational spectrum. This is typical of the infrared spectra of heteronuclear diatomic molecules, it shows the s
Fused quartz or fused silica is glass consisting of silica in amorphous form. It differs from traditional glasses in containing no other ingredients, which are added to glass to lower the melt temperature. Fused silica, has high working and melting temperatures. Although the terms fused quartz and fused silica are used interchangeably, the optical and thermal properties of fused silica are superior to those of fused quartz and other types of glass due to its purity. For these reasons, it finds use in situations such as semiconductor fabrication and laboratory equipment, it transmits ultraviolet better than other glasses, so is used to make lenses and optics for the ultraviolet spectrum. The low coefficient of thermal expansion of fused quartz makes it a useful material for precision mirror substrates. Fused quartz is produced by fusing high-purity silica sand. There are four basic types of commercial silica glass: Type I is produced by induction melting natural quartz in a vacuum or an inert atmosphere.
Type II is produced by fusing quartz crystal powder in a high-temperature flame. Type III is produced by burning SiCl4 in a hydrogen-oxygen flame. Type IV is produced by burning SiCl4 in a water vapor-free plasma flame. Quartz contains only silicon and oxygen, although commercial quartz glass contains impurities; the most dominant impurities are titanium. Melting is effected at 1650°C using either an electrically heated furnace or a gas/oxygen-fuelled furnace. Fused silica can be made from any silicon-rich chemical precursor using a continuous process which involves flame oxidation of volatile silicon compounds to silicon dioxide, thermal fusion of the resulting dust; this results in a transparent glass with an ultra-high purity and improved optical transmission in the deep ultraviolet. One common method involves adding silicon tetrachloride to a hydrogen–oxygen flame, but this precursor results in environmentally unfriendly byproducts including chlorine and hydrochloric acid. Fused quartz is transparent.
The material can, become translucent if small air bubbles are allowed to be trapped within. The water content is determined by the manufacturing process. Flame-fused material always has a higher water content due to the combination of the hydrocarbons and oxygen fuelling the furnace, forming hydroxyl groups within the material. An IR grade material has an content of <10 parts per million. Most of the applications of fused silica exploit its wide transparency range, which extends from the UV to the near IR. Fused silica is the key starting material for optical fiber, used for telecommunications; because of its strength and high melting point, fused silica is used as an envelope for halogen lamps and high-intensity discharge lamps, which must operate at a high envelope temperature to achieve their combination of high brightness and long life. Vacuum tubes with silica envelopes allowed for radiation cooling by incandescent anodes; because of its strength, fused silica was used in deep diving vessels such as the bathysphere and benthoscope.
Fused silica is used to form the windows of manned spacecraft, including the Space Shuttle and International Space Station. The combination of strength, thermal stability, UV transparency makes it an excellent substrate for projection masks for photolithography, its UV transparency finds uses in the semiconductor industry. EPROMs are recognizable by the transparent fused quartz window which sits on top of the package, through which the silicon chip is visible, which permits exposure to UV light during erasing. Due to the thermal stability and composition, it is used in semiconductor fabrication furnaces. Fused quartz has nearly ideal properties for fabricating first surface mirrors such as those used in telescopes; the material behaves in a predictable way and allows the optical fabricator to put a smooth polish onto the surface and produce the desired figure with fewer testing iterations. In some instances, a high-purity UV grade of fused quartz has been used to make several of the individual uncoated lens elements of special-purpose lenses including the Zeiss 105mm f/4.3 UV Sonnar, a lens made for the Hasselblad camera, the Nikon UV-Nikkor 105mm f/4.5 lens.
These lenses are used for UV photography, as the quartz glass has a lower extinction rate than lenses made with more common flint or crown glass formulas. Fused quartz can be metallised and etched for use as a substrate for high-precision microwave circuits, the thermal stability making it a good choice for narrowband filters and similar demanding applications; the lower dielectric constant than alumina allows thinner substrates. Fused quartz is the material used for modern glass instruments such as the glass harp and the verrophone, is used for new builds of the historical glass harmonica. Here, the superior strength and structure of fused quartz gives it a greater dynamic range and a clearer sound than the used lead crystal. Fused silica as an industrial raw material is used to make various refractory shapes such as crucibles, trays and rollers for many high-temperature thermal processes including steelmaking, investment casting, glass manufacture. Refractory shapes made from fused silica hav
A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. Molecules are distinguished from ions by their lack of electrical charge. However, in quantum physics, organic chemistry, biochemistry, the term molecule is used less also being applied to polyatomic ions. In the kinetic theory of gases, the term molecule is used for any gaseous particle regardless of its composition. According to this definition, noble gas atoms are considered molecules as they are monatomic molecules. A molecule may be homonuclear, that is, it consists of atoms of one chemical element, as with oxygen. Atoms and complexes connected by non-covalent interactions, such as hydrogen bonds or ionic bonds, are not considered single molecules. Molecules as components of matter are common in organic substances, they make up most of the oceans and atmosphere. However, the majority of familiar solid substances on Earth, including most of the minerals that make up the crust and core of the Earth, contain many chemical bonds, but are not made of identifiable molecules.
No typical molecule can be defined for ionic crystals and covalent crystals, although these are composed of repeating unit cells that extend either in a plane or three-dimensionally. The theme of repeated unit-cellular-structure holds for most condensed phases with metallic bonding, which means that solid metals are not made of molecules. In glasses, atoms may be held together by chemical bonds with no presence of any definable molecule, nor any of the regularity of repeating units that characterizes crystals; the science of molecules is called molecular chemistry or molecular physics, depending on whether the focus is on chemistry or physics. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds, while molecular physics deals with the laws governing their structure and properties. In practice, this distinction is vague. In molecular sciences, a molecule consists of a stable system composed of two or more atoms.
Polyatomic ions may sometimes be usefully thought of as electrically charged molecules. The term unstable molecule is used for reactive species, i.e. short-lived assemblies of electrons and nuclei, such as radicals, molecular ions, Rydberg molecules, transition states, van der Waals complexes, or systems of colliding atoms as in Bose–Einstein condensate. According to Merriam-Webster and the Online Etymology Dictionary, the word "molecule" derives from the Latin "moles" or small unit of mass. Molecule – "extremely minute particle", from French molécule, from New Latin molecula, diminutive of Latin moles "mass, barrier". A vague meaning at first; the definition of the molecule has evolved. Earlier definitions were less precise, defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical properties; this definition breaks down since many substances in ordinary experience, such as rocks and metals, are composed of large crystalline networks of chemically bonded atoms or ions, but are not made of discrete molecules.
Molecules are held together by ionic bonding. Several types of non-metal elements exist only as molecules in the environment. For example, hydrogen only exists as hydrogen molecule. A molecule of a compound is made out of two or more elements. A covalent bond is a chemical bond; these electron pairs are termed shared pairs or bonding pairs, the stable balance of attractive and repulsive forces between atoms, when they share electrons, is termed covalent bonding. Ionic bonding is a type of chemical bond that involves the electrostatic attraction between oppositely charged ions, is the primary interaction occurring in ionic compounds; the ions are atoms that have lost one or more electrons and atoms that have gained one or more electrons. This transfer of electrons is termed electrovalence in contrast to covalence. In the simplest case, the cation is a metal atom and the anion is a nonmetal atom, but these ions can be of a more complicated nature, e.g. molecular ions like NH4+ or SO42−. An ionic bond is the transfer of electrons from a metal to a non-metal for both atoms to obtain a full valence shell.
Most molecules are far too small to be seen with the naked eye. DNA, a macromolecule, can reach macroscopic sizes, as can molecules of many polymers. Molecules used as building blocks for organic synthesis have a dimension of a few angstroms to several dozen Å, or around one billionth of a meter. Single molecules cannot be observed by light, but small molecules and the outlines of individual atoms may be traced in some circumstances by use of an atomic force microscope; some of the largest molecules are supermolecules. The smallest molecule is the diatomic hydrogen, with a bond length of 0.74 Å. Effective molecular radius is the size; the table of permselectivity for different substances contains examples. The chemical formula for a molecule uses one line of chemical element symbols and sometimes al
In chemistry and atomic physics, an electron shell, or a principal energy level, may be thought of as an orbit followed by electrons around an atom's nucleus. The closest shell to the nucleus is called the "1 shell", followed by the "2 shell" the "3 shell", so on farther and farther from the nucleus; the shells correspond with the principal quantum numbers or are labeled alphabetically with letters used in the X-ray notation. Each shell can contain only a fixed number of electrons: The first shell can hold up to two electrons, the second shell can hold up to eight electrons, the third shell can hold up to 18 and so on; the general formula is. Since electrons are electrically attracted to the nucleus, an atom's electrons will occupy outer shells only if the more inner shells have been filled by other electrons. However, this is not a strict requirement: atoms may have two or three incomplete outer shells. For an explanation of why electrons exist in these shells see electron configuration; the electrons in the outermost occupied shell determine the chemical properties of the atom.
Each shell consists of one or more subshells, each subshell consists of one or more atomic orbitals. The shell terminology comes from Arnold Sommerfeld's modification of the Bohr model. Sommerfeld retained Bohr's planetary model, but added mildly elliptical orbits to explain the fine spectroscopic structure of some elements; the multiple electrons with the same principal quantum number had close orbits that formed a "shell" of positive thickness instead of the infinitely thin circular orbit of Bohr's model. The existence of electron shells was first observed experimentally in Charles Barkla's and Henry Moseley's X-ray absorption studies. Barkla labeled them with the letters K, L, M, N, O, P, Q; the origin of this terminology was alphabetic. A "J" series was suspected, though experiments indicated that the K absorption lines are produced by the innermost electrons; these letters were found to correspond to the n values 1, 2, 3, etc. They are used in the spectroscopic Siegbahn notation; the physical chemist Gilbert Lewis was responsible for much of the early development of the theory of the participation of valence shell electrons in chemical bonding.
Linus Pauling generalized and extended the theory while applying insights from quantum mechanics. The electron shells are labeled K, L, M, N, O, P, Q. Electrons in outer shells have higher average energy and travel farther from the nucleus than those in inner shells; this makes them more important in determining how the atom reacts chemically and behaves as a conductor, because the pull of the atom's nucleus upon them is weaker and more broken. In this way, a given element's reactivity is dependent upon its electronic configuration; each shell is composed of one or more subshells. For example, the first shell has one subshell, called 1s; the various possible subshells are shown in the following table: The first column is the "subshell label", a lowercase-letter label for the type of subshell. For example, the "4s subshell" is a subshell of the fourth shell, with the type described in the first row; the second column is the azimuthal quantum number of the subshell. The precise definition involves quantum mechanics, but it is a number that characterizes the subshell.
The third column is the maximum number of electrons. For example, the top row says. In each case the figure is 4 greater than the one above it; the fourth column says. For example, looking at the top two rows, every shell has an s subshell, while only the second shell and higher have a p subshell; the final column gives the historical origin of the labels s, p, d, f. They come from early studies of atomic spectral lines; the other labels, namely g, h and i, are an alphabetic continuation following the last originated label of f. Although it is stated that all the electrons in a shell have the same energy, this is an approximation. However, the electrons in one subshell do have the same level of energy, with subshells having more energy per electron than earlier ones; this effect is great enough. Each subshell is constrained to hold 4ℓ + 2 electrons at most, namely: Each s subshell holds at most 2 electrons Each p subshell holds at most 6 electrons Each d subshell holds at most 10 electrons Each f subshell holds at most 14 electrons Each g subshell holds at most 18 electronsTherefore, the K shell, which contains only an s subshell, can hold up to 2 electrons.
Although that formula gives the maximum in principle, in fact that maximum is only achieved for the first four shells (K, L, M
Polarization is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. A simple example of a polarized transverse wave is vibrations traveling along a taut string. Depending on how the string is plucked, the vibrations can be in a vertical direction, horizontal direction, or at any angle perpendicular to the string. In contrast, in longitudinal waves, such as sound waves in a liquid or gas, the displacement of the particles in the oscillation is always in the direction of propagation, so these waves do not exhibit polarization. Transverse waves that exhibit polarization include electromagnetic waves such as light and radio waves, gravitational waves, transverse sound waves in solids. In some types of transverse waves, the wave displacement is limited to a single direction, so these do not exhibit polarization. An electromagnetic wave such as light consists of a coupled oscillating electric field and magnetic field which are always perpendicular.
In linear polarization, the fields oscillate in a single direction. In circular or elliptical polarization, the fields rotate at a constant rate in a plane as the wave travels; the rotation can have two possible directions. Light or other electromagnetic radiation from many sources, such as the sun and incandescent lamps, consists of short wave trains with an equal mixture of polarizations. Polarized light can be produced by passing unpolarized light through a polarizer, which allows waves of only one polarization to pass through; the most common optical materials are isotropic and do not affect the polarization of light passing through them. Some of these are used to make polarizing filters. Light is partially polarized when it reflects from a surface. According to quantum mechanics, electromagnetic waves can be viewed as streams of particles called photons; when viewed in this way, the polarization of an electromagnetic wave is determined by a quantum mechanical property of photons called their spin.
A photon has one of two possible spins: it can either spin in a right hand sense or a left hand sense about its direction of travel. Circularly polarized electromagnetic waves are composed of photons with only one type of spin, either right- or left-hand. Linearly polarized waves consist of photons that are in a superposition of right and left circularly polarized states, with equal amplitude and phases synchronized to give oscillation in a plane. Polarization is an important parameter in areas of science dealing with transverse waves, such as optics, seismology and microwaves. Impacted are technologies such as lasers and optical fiber telecommunications, radar. Most sources of light are classified as incoherent and unpolarized because they consist of a random mixture of waves having different spatial characteristics, frequencies and polarization states. However, for understanding electromagnetic waves and polarization in particular, it is easiest to just consider coherent plane waves. Characterizing an optical system in relation to a plane wave with those given parameters can be used to predict its response to a more general case, since a wave with any specified spatial structure can be decomposed into a combination of plane waves.
And incoherent states can be modeled stochastically as a weighted combination of such uncorrelated waves with some distribution of frequencies and polarizations. Electromagnetic waves, traveling in free space or another homogeneous isotropic non-attenuating medium, are properly described as transverse waves, meaning that a plane wave's electric field vector E and magnetic field H are in directions perpendicular to the direction of wave propagation. By convention, the "polarization" direction of an electromagnetic wave is given by its electric field vector. Considering a monochromatic plane wave of optical frequency f, let us take the direction of propagation as the z axis. Being a transverse wave the E and H fields must contain components only in the x and y directions whereas Ez = Hz = 0. Using complex notation, the instantaneous physical electric and magnetic fields are given by the real parts of the complex quantities occurring in the following equations; as a function of time t and spatial position z these complex fields can be written as: E → =
Isotopes are variants of a particular chemical element which differ in neutron number, in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom; the term isotope is formed from the Greek roots isos and topos, meaning "the same place". It was coined by a Scottish doctor and writer Margaret Todd in 1913 in a suggestion to chemist Frederick Soddy; the number of protons within the atom's nucleus is called atomic number and is equal to the number of electrons in the neutral atom. Each atomic number identifies a specific element, but not the isotope; the number of nucleons in the nucleus is the atom's mass number, each isotope of a given element has a different mass number. For example, carbon-12, carbon-13, carbon-14 are three isotopes of the element carbon with mass numbers 12, 13, 14, respectively; the atomic number of carbon is 6, which means that every carbon atom has 6 protons, so that the neutron numbers of these isotopes are 6, 7, 8 respectively.
A nuclide is a species of an atom with a specific number of protons and neutrons in the nucleus, for example carbon-13 with 6 protons and 7 neutrons. The nuclide concept emphasizes nuclear properties over chemical properties, whereas the isotope concept emphasizes chemical over nuclear; the neutron number has large effects on nuclear properties, but its effect on chemical properties is negligible for most elements. In the case of the lightest elements where the ratio of neutron number to atomic number varies the most between isotopes it has only a small effect, although it does matter in some circumstances; the term isotopes is intended to imply comparison, for example: the nuclides 126C, 136C, 146C are isotopes, but 4018Ar, 4019K, 4020Ca are isobars. However, because isotope is the older term, it is better known than nuclide, is still sometimes used in contexts where nuclide might be more appropriate, such as nuclear technology and nuclear medicine. An isotope and/or nuclide is specified by the name of the particular element followed by a hyphen and the mass number.
When a chemical symbol is used, e.g. "C" for carbon, standard notation is to indicate the mass number with a superscript at the upper left of the chemical symbol and to indicate the atomic number with a subscript at the lower left. Because the atomic number is given by the element symbol, it is common to state only the mass number in the superscript and leave out the atomic number subscript; the letter m is sometimes appended after the mass number to indicate a nuclear isomer, a metastable or energetically-excited nuclear state, for example 180m73Ta. The common pronunciation of the AZE notation is different from how it is written: 42He is pronounced as helium-four instead of four-two-helium, 23592U as uranium two-thirty-five or uranium-two-three-five instead of 235-92-uranium; some isotopes/nuclides are radioactive, are therefore referred to as radioisotopes or radionuclides, whereas others have never been observed to decay radioactively and are referred to as stable isotopes or stable nuclides.
For example, 14C is a radioactive form of carbon, whereas 12C and 13C are stable isotopes. There are about 339 occurring nuclides on Earth, of which 286 are primordial nuclides, meaning that they have existed since the Solar System's formation. Primordial nuclides include 32 nuclides with long half-lives and 253 that are formally considered as "stable nuclides", because they have not been observed to decay. In most cases, for obvious reasons, if an element has stable isotopes, those isotopes predominate in the elemental abundance found on Earth and in the Solar System. However, in the cases of three elements the most abundant isotope found in nature is one long-lived radioisotope of the element, despite these elements having one or more stable isotopes. Theory predicts that many "stable" isotopes/nuclides are radioactive, with long half-lives; some stable nuclides are in theory energetically susceptible to other known forms of decay, such as alpha decay or double beta decay, but no decay products have yet been observed, so these isotopes are said to be "observationally stable".
The predicted half-lives for these nuclides greatly exceed the estimated age of the universe, in fact there are 27 known radionuclides with half-lives longer than the age of the universe. Adding in the radioactive nuclides that have been created artificially, there are 3,339 known nuclides; these include 905 nuclides that are either stable or have half-lives