Neon was discovered in 1898 by British chemists William Ramsay (1852-1916) and Morris Travers (1872-1961) at University College London. Just a few weeks after their discovery of krypton and xenon, the two scientists cooled a sample of liquid air and collected the gas that escaped during progressive evaporation. By placing this gas in a discharge tube, they observed a bright red-orange glow. Ramsay's 13-year-old son, who was present during the experiment, exclaimed: "It's a wonderful light!"
Ramsay named this new element neon (from the Greek neos = new) because of its recent discovery. Neon was the last of the stable noble gases to be discovered (after helium, argon, krypton, and xenon). In 1910, the French physicist Georges Claude (1870-1960) developed the first neon sign, thus initiating a revolution in advertising and urban lighting. This invention gave rise to the famous "neon signs" that would soon illuminate cities around the world.
Neon (symbol Ne, atomic number 10) is a noble gas of group 18 in the periodic table, consisting of ten protons, usually ten neutrons (for the most common isotope), and ten electrons. The three stable isotopes are neon-20 \(\,^{20}\mathrm{Ne}\) (≈ 90.48%), neon-21 \(\,^{21}\mathrm{Ne}\) (≈ 0.27%), and neon-22 \(\,^{22}\mathrm{Ne}\) (≈ 9.25%).
At room temperature, neon is a monatomic gas (Ne), colorless, odorless, and completely chemically inert. Its complete electronic configuration (outer shell saturated with eight electrons) gives it exceptional stability. Neon is the second lightest noble gas after helium and has the smallest temperature range between its melting and boiling points of all elements (only 2.6 K). Ne gas has a density of about 0.900 g/L at standard temperature and pressure.
The temperature at which the liquid and solid states can coexist (melting point): 24.56 K (−248.59 °C). The temperature at which it transitions from liquid to gas (boiling point): 27.104 K (−246.046 °C). Liquid neon is used as a cryogenic refrigerant in some specialized applications, although less common than liquid nitrogen or helium.
| Isotope / Notation | Protons (Z) | Neutrons (N) | Atomic mass (u) | Natural abundance | Half-life / Stability | Decay / Remarks |
|---|---|---|---|---|---|---|
| Neon-18 — \(\,^{18}\mathrm{Ne}\,\) | 10 | 8 | 18.005708 u | Unnatural | 1.672 s | Radioactive β\(^+\) decay to \(\,^{18}\mathrm{F}\); artificially produced in accelerators. |
| Neon-19 — \(\,^{19}\mathrm{Ne}\,\) | 10 | 9 | 19.001880 u | Unnatural | 17.22 s | Radioactive β\(^+\); used in nuclear research. |
| Neon-20 — \(\,^{20}\mathrm{Ne}\,\) | 10 | 10 | 19.992440 u | ≈ 90.48 % | Stable | Ultra-majority isotope; produced by fusion of carbon and oxygen in massive stars. |
| Neon-21 — \(\,^{21}\mathrm{Ne}\,\) | 10 | 11 | 20.993847 u | ≈ 0.27 % | Stable | Rare isotope; used as a tracer in geochemistry and cosmochemistry. |
| Neon-22 — \(\,^{22}\mathrm{Ne}\,\) | 10 | 12 | 21.991385 u | ≈ 9.25 % | Stable | Produced in massive stars; its ratio with Ne-20 reveals nucleosynthesis history. |
| Neon-23 — \(\,^{23}\mathrm{Ne}\,\) | 10 | 13 | 22.994467 u | Unnatural | 37.24 s | Radioactive β\(^-\) decay to \(\,^{23}\mathrm{Na}\); relatively long half-life for a light radioactive isotope. |
| Neon-24 — \(\,^{24}\mathrm{Ne}\,\) | 10 | 14 | 23.993610 u | Unnatural | 3.38 minutes | Radioactive β\(^-\); produced in nuclear reactors and accelerators. |
| Other isotopes — \(\,^{16}\mathrm{Ne},\,^{17}\mathrm{Ne},\,^{25}\mathrm{Ne}-\,^{34}\mathrm{Ne}\) | 10 | 6-7, 15-24 | — (resonances) | Unnatural | \(10^{-21}\) — 0.602 s | Very unstable states observed in nuclear physics; some have neutron halo structures. |
N.B. :
Electron shells: How electrons organize around the nucleus.
Neon has 10 electrons distributed across two electron shells. Its full electronic configuration is: 1s² 2s² 2p⁶, or simplified as: [He] 2s² 2p⁶. This configuration can also be written as: K(2) L(8).
K Shell (n=1): contains 2 electrons in the 1s subshell. This inner shell is complete and highly stable.
L Shell (n=2): contains 8 electrons distributed as 2s² 2p⁶. The 2s and 2p orbitals are completely filled, providing maximum stability. This 8-electron configuration in the outer shell is called an octet and represents an optimal energy state.
Neon has 8 electrons in its outer shell (2s² 2p⁶), forming a saturated electronic configuration. This configuration explains its exceptional chemical properties:
Neon neither loses nor gains electrons under normal conditions, which explains the total absence of stable oxidation states.
The complete valence shell gives neon absolute chemical inertness, hence its classification among the noble gases (or rare gases).
No stable chemical compound of neon has ever been synthesized, even under extreme laboratory conditions. Neon is the most inert noble gas after helium.
The electronic configuration of neon, with its outer shell completely filled with 8 electrons, makes it the reference noble gas for the second period of the periodic table. This structure gives it characteristic properties: absolute chemical stability (neon is totally inert and forms no compounds), extremely high ionization energy (it is practically impossible to remove an electron), and total absence of reactivity under all known conditions. Neon represents the most stable energy state for 10 electrons, which is why many neighboring elements (sodium, fluorine, oxygen, magnesium) tend to adopt this [Ne] configuration by gaining or losing electrons. The neon configuration defines the octet rule in chemistry: atoms tend to acquire 8 electrons in their outer shell to achieve maximum stability. Neon is primarily used for its physical properties rather than chemical ones: it produces a characteristic red-orange light when electrically excited, hence its iconic use in neon signs and fluorescent tubes. It also serves as a refrigerant gas in certain cryogenic applications and as an inert atmosphere in specialized industrial processes.
Neon, like all noble gases, has a complete outer electron shell with eight electrons (octet configuration), giving it exceptional chemical stability. This configuration makes neon one of the most chemically inert elements: it forms virtually no stable chemical bonds under normal or even extreme conditions.
Unlike heavier noble gases (krypton, xenon, radon) that can form some chemical compounds under very specific conditions, no true stable chemical compound of neon has ever been synthesized or observed in nature. Even the most sophisticated attempts with the most powerful oxidizing agents (such as fluorine) or under high pressure have failed to force neon to form true chemical bonds.
Neon can form inclusion compounds (clathrates) where neon atoms are physically trapped in a molecular cage formed by other molecules (such as ice), but without the formation of true chemical bonds. Ephemeral molecular ions containing neon (such as NeH⁺, NeAr⁺) have been detected in mass spectrometry, but these species are extremely unstable and only exist under high-energy conditions.
This total chemical inertness makes neon an ideal gas for creating protective atmospheres where no chemical reaction is desired. Neon is also non-toxic, non-flammable, and poses no chemical or environmental hazard, although it can cause asphyxiation by displacing oxygen in confined spaces.
Neon is the fifth most abundant element in the universe (after hydrogen, helium, oxygen, and carbon), but its detection and study in space present particular challenges. Neon represents about 0.13% of the baryonic mass of the universe.
Neon is mainly produced by stellar nucleosynthesis in massive stars. Neon-20, the ultra-majority isotope, is formed by two main processes: the fusion of two carbon-12 nuclei (C + C → Ne-20 + He-4) and the capture of an alpha particle by oxygen-16 (O-16 + He-4 → Ne-20). These reactions occur at temperatures of about 600 million kelvin in the cores of massive stars at the end of their lives.
In very massive stars (greater than 8 solar masses), neon can itself serve as nuclear fuel during neon burning at temperatures exceeding 1.2 billion kelvin. This phase produces oxygen and magnesium and lasts only a few years, or even a few days for the most massive stars. Neon is then dispersed into the interstellar medium during the supernova explosion, enriching the matter that will form future generations of stars and planets.
The "missing neon problem" has long intrigued astrophysicists. In the interstellar medium and stellar atmospheres, the observed abundance of neon is often lower than theoretical predictions. Unlike other elements, neon does not easily form detectable molecular compounds, and its atomic spectral lines are difficult to observe because they are in the far ultraviolet, absorbed by Earth's atmosphere. Additionally, a significant portion of neon may be trapped in interstellar dust grains, making it invisible to conventional spectroscopic observations.
Space missions equipped with UV spectrometers (such as the Hubble Space Telescope, FUSE, and X-ray observatories) have allowed better characterization of neon abundance in various cosmic environments: HII regions, planetary nebulae, supernova remnants, and the diffuse interstellar medium.
The isotopic ratio ²⁰Ne/²²Ne varies according to astrophysical sources and provides valuable information about nucleosynthesis processes. Massive stars produce neon-22 by neutron capture on neon-20 and magnesium-25, thus modifying the isotopic ratio. The study of these ratios in meteorites, presolar grains, and the solar wind reveals the complex history of material mixing from different stellar generations before the formation of the solar system.
In the solar wind, neon has a ²⁰Ne/²²Ne ratio of about 13.8, different from that found in Earth's atmosphere or in primitive meteorites. These variations testify to the isotopic fractionation processes that occurred during the formation of the Sun and the solar system.
Neon also plays a role in cosmochemistry. The three components of neon (Ne-A, Ne-B, Ne-C) identified in primitive meteorites have different origins: solar, cosmogenic (produced by cosmic rays), and nucleosynthetic. The analysis of these components allows tracing the history of the primitive material of the solar system and the irradiation processes it has undergone.
N.B.:
The neon sign, invented by Georges Claude in 1910, profoundly transformed the urban landscape of the 20th century. Although commonly called "neon signs," most modern illuminated signs actually use various gases and fluorescent coatings to produce different colors: pure neon produces red-orange; argon with mercury gives blue; helium produces yellow-pink; krypton gives white-violet. Tubes can also be coated with phosphors that convert UV light into various visible colors. These signs have become cultural icons, symbolizing modern metropolises from Times Square to Las Vegas, from Tokyo to Hong Kong. Neon art has also evolved into a recognized form of contemporary art, with artists creating spectacular light installations in museums and galleries around the world. Despite competition from modern LEDs (more energy-efficient), true neon tubes retain a unique luminous quality and cultural nostalgia that ensure their longevity, at least as a form of art and creative expression.
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