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Atomic Mass Unit
The dalton or unified atomic mass unit (SI symbols: Da or u) is a unit of mass widely used in physics and chemistry. . It is approximately the mass of one nucleon (either a proton or neutron). A mass of 1 Da is also referred to as the atomic mass constant and denoted by mu. Several definitions of this unit have been used, implying slightly different values. The current IUPAC endorsed definition is the unified atomic mass unit, denoted by the symbol u. As of 2019, the International System of Units (SI) lists the dalton, symbol Da, as a unit acceptable for use with the SI unit system and secondarily notes that the dalton (Da) and the unified atomic mass unit (u) are alternative names (and symbols) for the same unit. The symbol Da is more widely used in most fields. It is defined precisely as 1/12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. Despite being an official abbreviation for a related obsolete unit and not widely used in the scientific literature, the abbreviation "amu" now often refers to the modern unit (Da or u) in many preparatory texts. As of June 2019, the value recommended by the Committee on Data for Science and Technology (CODATA) is 1.66053906660(50)×10−27 kg, or approximately 1.66 yoctograms. This unit is commonly used in physics and chemistry to express the mass of atomic-scale objects, such as atoms, molecules, and elementary particles. For example, an atom of helium has a mass of about 4 Da, and a molecule of acetylsalicylic acid (aspirin), C9H8O4, has a mass of about 180.16 Da. In general, the standard atomic weight of an element is the average weight of its atom as it occurs in nature, expressed in daltons. The molecular masses of proteins, nucleic acids, and other large polymers are often expressed with the units kilodalton (kDa), equal to 1000 daltons, megadalton (MDa), one million daltons, etc. Titin, one of the largest known proteins, has an atomic mass of between 3 and 3.7 megadaltons. The DNA of chromosome 1 in the human genome has about 249 million base pairs, each with an average mass of about 650 Da, or 156 GDa total. The mole is a unit of amount of substance, widely used in chemistry and physics, which was originally defined so that the mass of one mole of a substance, measured in grams, would be numerically equal to the average mass of one of its constituent particles, measured in daltons. That is, the molar mass of a chemical compound was meant to be numerically equal to its average molecular mass. For example, the average mass of one molecule of water is about 18.0153 daltons, and one mole of water is about 18.0153 grams. A protein whose molecule has an average mass of 64 kDa would have a molar mass of 64 kg/mol. However, while this equality can be assumed for almost all practical purposes, it is now only approximate, because of the way the mole was redefined on 20 May 2019. The mass in daltons of an atom is numerically very close to the number of nucleons A in its atomic nucleus. It follows that the molar mass of a compound (grams per mole) is also numerically close to the average number of nucleons per molecule. However, the mass of an atomic-scale object is affected by the binding energy of the nucleons in its atomic nuclei, as well as the mass and binding energy of the electrons. Therefore, this equality holds only for the carbon-12 atom in the stated conditions, and will vary for other substances. For example, the mass of one unbound atom of the common hydrogen isotope (hydrogen-1, protium) is 1.007825032241(94) Da, the mass of one free neutron is 1.008664915823(491) Da, and the mass of one hydrogen-2 (deuterium) atom is 2.014101778114(122) Da. In general, the difference (mass defect) is less than 0.1%; except for hydrogen (about 0.8%), helium-3 (0.5%), lithium (0.25%) and beryllium (0.15%). The atomic mass unit should not be confused with unit of mass in the atomic units systems, which is instead the electron rest mass (me).
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  • 31 Oct 2022
Topic Review Video
Physicochemical Processes Leading to Plasma-Driven Solution Electrolysis
A new type of electrolysis, initially known as the contact glow-discharge electrolysis (CGDE) and, more recently, as the plasma-driven solution electrolysis (PDSE), has attracted attention as an alternative method of hydrogen production. PDSE is a nontypical electrochemical process in which electric plasma is formed in the glow discharges excited by the direct or pulsed current in a gas–vapor envelope in the vicinity of the discharge electrode immersed in the electrolytic solution. The yield of chemicals in PDSE (i.e., the ratio of the moles of the product formed to the moles of electrons consumed in a chemical reaction) is several times higher than the Faradaic production of chemicals (predicted by Faraday’s law). In PDSE, new chemical compounds can also be synthesized, which does not happen using Faradaic electrolysis.
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Topic Review
Solid State Ionics
Solid-state ionics is the study of ionic-electronic mixed conductor and fully ionic conductors (solid electrolytes) and their uses. Some materials that fall into this category include inorganic crystalline and polycrystalline solids, ceramics, glasses, polymers, and composites. Solid-state ionic devices, such as solid oxide fuel cells, can be much more reliable and long-lasting, especially under harsh conditions, than comparable devices with fluid electrolytes. The field of solid-state ionics was first developed in Europe, starting with the work of Michael Faraday on solid electrolytes Ag2S and PbF2 in 1834. Fundamental contributions were later made by Walther Nernst, who derived the Nernst equation and detected ionic conduction in heterovalently doped zirconia, which he applied in his Nernst lamp. Another major step forward was the characterization of silver iodide in 1914. Around 1930, the concept of point defects was established by Yakov Frenkel, Walter Schottky and Carl Wagner, including the development of point-defect thermodynamics by Schottky and Wagner; this helped explain ionic and electronic transport in ionic crystals, ion-conducting glasses, polymer electrolytes and nanocomposites. In the late 20th and early 21st centuries, solid-state ionics focused on the synthesis and characterization of novel solid electrolytes and their applications in solid state battery systems, fuel cells and sensors. The term solid state ionics was coined in 1967 by Takehiko Takahashi, but did not become widely used until the 1980s, with the emergence of the journal Solid State Ionics. The first international conference on this topic was held in 1972 in Belgirate, Italy, under the name "Fast Ion Transport in Solids, Solid State Batteries and Devices".
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Topic Review
Fabrication of Semiconductor Optical Fibers
Because of their thermal and mechanical mismatch with typical glass materials, the fabrication of optical fibers from semiconductors is a considerable challenge for the field of materials science. As a result, the platform for optical fibers has been restricted to materials that are consistent with the typical fiber drawing techniques for an extended period. In 2006, a chemical deposition approach was used to create the initial example of a crystalline semiconductor optical fiber. As a result of this endeavor, different methods have been utilized in the fabrication of semiconductor optical fibers. Each of these methods has its own set of benefits and drawbacks. Since there have been no standardized techniques for fabricating these fibers, their production is relatively costly.
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  • 31 Oct 2022
Topic Review
Scanning Kelvin Probe
In microscopy, a scanning Kelvin probe (SKP) is a non-contact, non-destructive scanning probe microscopy (SPM) technique used to measure the work function of the sample under study. By raster scanning in the x,y plane the work function of the sample can be locally mapped for correlation with sample features. It is predominantly used to measure corrosion and coatings. It is closely related to the Kelvin probe force microscope (KPFM) technique.
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  • 30 Oct 2022
Topic Review
Traditional Chinese Timekeeping
The traditional Chinese time systems refers to the time standards for divisions of the day used in China until the introduction of the Shixian calendar in 1628 at the beginning of the Qing dynasty.
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Topic Review
Spectrochemistry
Spectrochemistry is the application of spectroscopy in several fields of chemistry. It includes analysis of spectra in chemical terms, and use of spectra to derive the structure of chemical compounds, and also to qualitatively and quantitively analyze their presence in the sample. It is a method of chemical analysis that relies on the measurement of wavelengths and intensity of electromagnetic radiation.
  • 478
  • 30 Oct 2022
Topic Review
Reaction
As described by the third of Newton's laws of motion of classical mechanics, all forces occur in pairs such that if one object exerts a force on another object, then the second object exerts an equal and opposite reaction force on the first. The third law is also more generally stated as: "To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts." The attribution of which of the two forces is the action and which is the reaction is arbitrary. Either of the two can be considered the action, while the other is its associated reaction.
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Topic Review
MicrOmega-IR
MicrOmega-IR is an infrared hyperspectral microscope that is part of the science payload on board the European Rosalind Franklin rover, tasked to search for biosignatures on Mars. The rover is planned to land on Mars in the mid- or late 2020s. MicrOmega-IR will analyse in situ the powder material derived from crushed samples collected by the rover's core drill.
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Topic Review
Cubic Mile of Oil
The cubic mile of oil (CMO) is a unit of energy, aiming to give the general public an understanding of large quantities of energy. It is approximately equal to 1.6×1020 joule. It was created by Hew Crane of SRI International to aid in public understanding of global-scale energy consumption and resources. Large scale sources of energy include wind, solar photovoltaic, solar thermal, nuclear, hydroelectric, oil, coal, natural gas, geothermal, and biomass (primarily the burning of wood). Traditionally, many different units are commonly used to measure these sources (e.g., joules, BTUs, kilowatt hours, therms) but only some of them are familiar to a global general public, and some argue that fewer are needed and a standard should be chosen. Still, these common energy units are mainly sized for everyday activities, for example a joule is the energy required to lift a small apple one metre vertically. For regional, national, and global scales, larger energy units, such as exajoule, terawatt-hour, billion barrels of oil equivalent (BBOE) and quad are used. Derived by multiplying the small common units by large powers of ten these larger units pose additional conceptual difficulties for many citizens. Crane intended the cubic mile of oil to provide a visualizable scale for comparing the contributions of these diverse energy components as a percentage of total worldwide, energy use. In 2005, the global economy was consuming approximately 30 billion barrels (4.8 billion cubic metres; 1.3 trillion US gallons) of oil each year. Numbers of this magnitude are difficult to conceive by most people. The volume occupied by 1 trillion US gallons (3.8 billion cubic metres) is about 1 cubic mile (4.2 billion cubic metres). Crane felt that a cubic mile would be an easier concept for the general public than a trillion gallons.
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