Arsenic is the twentieth most abundant element in Earth's crust, averaging a concentration of approximately 2 ppm. Arsenopyrite (FeAsS) is its most common mineral.
There are presently four artificial, or synthetic, sweeteners that have been approved by the U.S. Food and Drug Administration (FDA): saccharin, aspartame, acesulfame-K, and sucralose.
Ascorbic acid or vitamin C is an antiscorbutic agent. Scurvy is a disease that potentially ranks as the second most important nutritional deficiency, after protein-calorie malnutrition.
Astatine is a radioactive halogen (the heaviest of the halogen elements) and is a solid at room temperature. Dale R.
In the night sky, the expanses of space between the stars of the Milky Way appear to be empty. In fact this space is occupied by a very thin gas that is mostly hydrogen and that has mere traces (less than 0.1% by number of atoms) of other elements such as oxygen, carbon, and nitrogen.
With each breath, the lungs inhale air filled with nitrogen and oxygen, the most abundant natural gases in the atmosphere. Also inhaled, however, are small quantities of gases and particles that are pollutants.
The atomic nucleus is a tiny massive entity at the center of an atom. Occupying a volume whose radius is 1/100,000 the size of the atom, the nucleus contains most (99.9%) of the mass of the atom.
The atom is now known to consist of three primary particles: protons, neutrons, and electrons, which make up the atoms of all matter. A series of experimental facts established the validity of the model.
An atom is the smallest possible unit of an element. Since all forms of matter consist of a combination of one or more elements, atoms are the building blocks that constitute all the matter in the universe.
The fifty-sixth element in the Periodic Table, barium has been known in various mineral forms since the 1600s. However, it was not until the 1770s that Carl W.
deoxyribose and phosphates groups occupying (or being oriented toward) the outer surfaces, interacting with water. The 20-angstrom (7.9 × 10−8-inch) diameter of the helix was consistent with the presence of two adjacent strands and supported the hypothesis that a purine base resided on one strand and a pyrimidine base on the equivalent (homologous) site of the complementary strand.
Bases are considered the chemical opposite of acids because of their ability to neutralize acids. In 1887 the Swedish physicist and chemist Svante Arrhenius defined a base as the chemical substance that produces hydroxide ions (OH−) and cations.
Berkelium, element 97, is a synthetic radioelement, first synthesized by Glenn Seaborg's group in 1949. A target of a few milligrams of an isotope of americium (241Am) was bombarded with α-particles within a cyclotron at the University of California at Berkeley.
Beryllium was identified as a unique element and as a constituent of the mineral beryl and the gem emerald by the French chemist Louis Vauquelin in 1797. Metallic beryllium was isolated in 1828 by the scientists (working independently of one another) Antoine Bussy and Friedrich Wöhler.
Bioluminescence is the emission of visible light by biological systems, which arises from enzyme-catalyzed chemical reactions. Bioluminescence can be distinguished from chemiluminescence in that it occurs in living organisms and requires an enzyme catalyst.
Bismuth is a brittle, crystalline metal that is white with a pinkish tint. It is the heaviest and only nontoxic member of the heavy metals.
When chlorine gas is bubbled through a cylinder of tomato juice, the chlorine/tomato juice mixture turns almost completely white within five minutes. This spectacular change is a result of the chemical action of chlorine, acting as an oxidizing bleaching agent, on the pigments in tomato juice.
By the early 1940s, scientists knew that chromosomes existed and that they were composed of smaller units called genes. Chemical analysis had revealed that the eucaryotic chromosome consists of about 50 percent protein and 50 percent deoxyribonucleic acid (DNA).
In 1811, just three years after John Dalton published his atomic theory, a brilliant theoretician named Amedeo Avogadro proposed his molecular theory. Avogadro's molecular theory related gas densities to molecular weights, explained reacting proportions by volume in terms of molecular ratios and compositions, and suggested methods for determining both molecular weights and compositions.
Born in St. Martens-Latem, Belgium, Leo Hendrick Baekeland was the son of a cobbler (Karel Baekeland) and a housemaid (Rosalia Merchie).
The name of Johann Jakob Balmer is immortalized in the Balmer series of spectral lines emitted from the hydrogen atom. Atoms that are excited to higher energies return to lower energies by emitting electromagnetic radiation at specific frequencies.
In 1972 John Bardeen did something that no other physicist, not even Albert Einstein, had ever done. He won his second Nobel Prize in physics.
Antoine-Henri Becquerel was born the son of the physicist Alexandre-Edmond Becquerel, and the grandson of the physicist Antoine-César Becquerel, and it is not surprising that he followed in their footsteps. It is also not surprising that his research interests centered around solar radiation and phosphorescence, as these are phenomena that his father had investigated.
Paul Berg is considered to be one of the few pioneers in molecular biology, which is essentially the application of chemistry to biological systems. His work with recombinant DNA provided scientists with a very valuable laboratory technique.
Claude-Louis Berthollet was influential in four areas: theoretical chemistry, experimental chemistry, practical chemistry, and chemical writing. He was also a chemistry teacher and, with his contemporary Pierre-Simon de Laplace, a patron of young French scientists.
Before the early 1800s the symbols used to denote chemical elements and compounds were obscure. Alchemists wanted to keep their work secret and so devised symbols for the chemicals they used that would not reveal anything about them.
Joseph Black was trained as a medical doctor. One of his early scientific undertakings was investigating means of treating "the stone" (kidney stones and gallstones).
Niels Bohr was one of the founders of modern atomic and nuclear physics. He was born into a family of intellectual and academic distinction.