Brinell Hardness

The Brinell scale characterises the indentation hardness of materials through the scale of penetration of an indenter, loaded on a material test-piece. It is one of several definitions of hardness in materials science.

Brinelling refers to surface fatigue caused by repeated impact or overloading. It is a common cause of roller bearing failures, and loss of preload in bolted joints when a hardened washer is not used. Engineers will use the Brinell hardness of materials in their calculations to avoid this mode of failure. Fretting corrosion can cause a similar-looking kind of damage and is called false brinelling since the mechanism is different.

Proposed by Swedish engineer Johan August Brinell in 1900, it was the first widely used and standardised hardness test in engineering and metallurgy. The large size of indentation and possible damage to test-piece limits its usefulness.

Atomic Mass

An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of 12C in its nuclear and electronic ground state.

A sample of any element consists of one or more isotopes of that element. Each isotope is a different weight. The relative amounts of each isotope for any element represents the isotope distribution for that element. The atomic weight is the average of the isotope weights weighted for the isotopedistribution and expressed on the 12C scale as mentioned above.

The standard atomic weights apply to the elements as they exist naturally on Earth, and the uncertainties take into account the isotopic variation found in most laboratory samples.

Atomic Charge

Since the number of protons (positive charges) always equals the number of electrons (negative charges) in an atom, positive charges equal negative charges and atoms in the elemental state have no charge. Only when an atom takes an electron from another atom does the particle become charged. This charged form of the atom is known as an ion.

Positively charged ions are called cations, and negatively charged ions are called anions. For instance, when chlorine accepts an electron from sodium, the sodium ion that is formed will have one more proton than electrons. It will therefore have a positive charge and be called a cation. The chlorine (or chloride) ion will have one more electron than protons. It will take on a negative charge and be called an anion. The compound formed by this transfer of electrons is sodium chloride or table salt, which is nothing like the highly reactive sodium or extremely poisonous chlorine from which it was formed.

Aqua regia

Aqua regia  (Latin: royal water)

Aqua regia is a higly corrosive, fuming yellow or red solution. It is a mixture of (usually) 1 part nitric acid (HNO3) to three parts hydrochloric acid (HCl). It is one of the few reagents that can dissolve gold and platinum.

Aqua regia is itself very corrosive and has been implicated in several explosions as well due to mishandling and it should not be used unless gentler cleaning techniques are inadequate.

Due to the reaction between its components resulting in its decomposition, aqua regia quickly loses its effectiveness. As such, its components should only be mixed immediately before use.

Alpha Decay

Alpha decay is one process that unstable atoms can use to become more stable. During alpha decay, an atom's nucleus sheds two protons and twoneutrons in a packet that scientists call an alpha particle.

Since an atom loses two protons during alpha decay, it changes from one element to another. For example, after undergoing alpha decay, an atom ofuranium (with 92 protons) becomes an atom of thorium (with 90 protons).

Alpha decay is a form of radioactive decay in which an atomic nucleus ejects an alpha particle through the electromagnetic force and transforms into a nucleus with mass number 4 less and atomic number 2 less. For example:

this is usually written as;

Note that an alpha particle IS a helium nucleus, and that both mass number and atomic number are conserved. Alpha decay can essentially be thought of as nuclear fission where the parent nucleus splits into two daughter nuclei. Alpha decay is fundamentally a quantum tunneling process. Unlike beta decay, alpha decay is governed by the strong nuclear force.

Alpha particles with their typical kinetic energy of 5 MeV have a speed of 15,000 km/s.

Because of alpha decay, virtually all of the helium produced in the United States and elsewhere comes from trapped underground deposits associated with minerals containing uranium or thorium, and brought to the surface as a by-product of natural gas production.

Alpha particles emitted by radioactive nuclei are among the most hazardous forms of radiation, if these nuclei are incorporated within a human body. As any heavy charged particle, alpha particles lose their energy within a very short distance in dense media, causing significant damage to surrounding biomolecules. On the other hand, external alpha irradiation is not harmful because alpha particles are completely absorbed by a very thin (micrometers) dead layer of skin as well as by few centimeters of air. However, if a substance radiating alpha particles is injected, ingested or inhaled by an organism it may become a risk, potentially inflicting very serious damage to the organisms' genetic makeup.

Alloy

An alloy is a combination, either in solution or compound, of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. An alloy with two components is called a binary alloy; one with three is a ternary alloy; one with four is a quaternary alloy. The resulting metallic substance generally has properties significantly different from those of its components.

Alloys are usually designed to have properties that are more desirable than those of their components. For instance, steel is stronger than iron, one of its main elements. It 'inherits' some of the characteristics of the elements it was made from, usually physical properties like density, reactivity and electrical and thermal conductivity. However, its engineering properties (Tensile strength, Young's modulus, shear strength) can be vastly different from its constituent materials. Among other factors, this is due to the differing sizes of the atoms in the alloy - larger atoms exert a compressive force on neighbouring atoms, and smaller atoms will exert a tensile force on their neighbours. Unlike a pure metal, where the atoms are more free to move, this helps the alloy resist deformation.

Unlike pure metals, most alloys do not have a single melting point. Instead, they have a melting range in which the material is a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and that at which melting is complete is called the liquidus. However, for most pairs of elements, there is a particular ratio which has a single melting point, and this is called a eutectic mixture.

In practice, some alloys are used so predominantly with respect to their base metals that the name of the primary constituent is also used as the name of the alloy. For example, 14 carat (58%) gold is an alloy of gold with other elements. Similarly, the silver used in jewellery and the aluminiumused as a structural building material are also alloys.

Allotrope

An allotrope is a variant of a substance consisting of only one type of atom. It is a new molecular configuration, with new physical properties. Allotropes of a given substance will often have substantial differences between each other. Generally one allotrope will be far more abundant than another.

Oxygen has three known allotropes, O2 which is far more abundant than O3, ozone and O4, tetraoxygen.

Phosphorous comes in at least 3 allotropic forms; red, black (or purple, or violet), white (or yellow). Red and white phosphorous are the most common, both of which consist of tetrahedrally arranged groups of four phosphorous. The tetrahedral arrangements in red phosphorous are linked into chains, whereas those in white phosphorous are separate. Black phosphorous is arranged in 2-dimensional hexagonal sheets, much like graphite. White prosphorous reacts immediately to the air, oxiding and producing phosphorus pentoxide.

Carbon is the substance with the greatest number of allotropes, with 8 discovered so far. It possesses allotropes most radically different from one another, ranging from soft to hard, opaque to transparent, abrasive to smooth, inexpensive to costly. These allotropes include the amorphous carbonallotrope, carbon nanofoam, carbon nanotube, the diamond allotrope, fullerene allotrope, graphite, lonsdaleite, and ceraphite allotrope. Coal and soot are both forms of amorphous carbon, one of the most common carbon allotropes. Diamond is an allotrope in which atoms are linked in a 3-D crystalline network of covalent carbon bonds. Diamond, of course, is both very expensive, rare, and strong. Carbon fullerenes are among the strongest and lightest materials known. Carbon nanofoam has an extremely low density, only a few times heavier than air.

Alkali Metal

Alkali Metal     A metal in the first column of the periodic table (i.e., lithium, sodium, potassium, rubidium, caesium and francium). With the exception of francium, these metals are all soft and silvery.

They may be readily fused and volatilized with their melting and boiling points becoming lower with increasing atomic mass. They are the strongest electropositive metals. These elements react vigorously, even violently with water.

The alkali metals are silver-colored (caesium has a golden tinge), soft, low-density metals, which react readily with halogens to form ionic salts, and with water to form strongly alkaline (basic) hydroxides. These elements all have one electron in their outermost shell, so the energetically preferred state of achieving a filled electron shell is to lose one electron to form a singly charged positive ion, or cation.

Hydrogen, with a solitary electron, is usually placed at the top of Group 1 of the periodic table, but it is not considered an alkali metal; rather it exists naturally as a diatomic gas. Removal of its single electron requires considerably more energy than removal of the outer electron for the alkali metals. As in the halogens, only one additional electron is required to fill in the outermost shell of the hydrogen atom, so hydrogen can in some circumstances behave like a halogen, forming the negative hydride ion. Binary compounds of hydride with the alkali metals and some transition metals have been prepared. Under extremely high pressure, such as is found at the core of Jupiter, hydrogen does become metallic and behaves like an alkali metal Alkali Metal (alkaline-earth metals)  Elements in the second column (from the left) of the periodic table all fall into this series. These elements are in general white, differing by shades of colour or casts; they are malleable, extrudable and machinable. These elements may be made into rods, wire or plate. Also, these elements are less reactive than the alkali metals and have higher melting points and boiling points. The alkaline earth metals are silvery coloured, soft, low-density metals, which react readily with halogens to form ionic salts, and with water, though not as rapidly as the alkali metals, to form strongly alkaline (basic) hydroxides. For example, where sodium and potassium react with water at room temperature, magnesium reacts only with steam and calcium with hot water. All the alkaline earth metals have two electrons in their outermost shell, so the energetically preferred state of achieving a filled electron shell is to lose two electrons to form doubly charged positive ions.

Air

Earth's atmosphere is a layer of gases surrounding the planet Earth and retained by the Earth's gravity. It contains roughly 78% nitrogen and 21%oxygen, trace amounts of other gases, and water vapour. This mixture of gases is commonly known as air. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night.

The atmosphere has no abrupt cut-off. It slowly becomes thinner and fades away into space. There is no definite boundary between the atmosphere and outer space. Three-quarters of the atmosphere's mass is within 11 km of the planetary surface. In the United States, persons who travel above an altitude of 50.0 miles (80.5 km) are designated as astronauts. An altitude of 120 km (75 mi or 400,000 ft) marks the boundary where atmospheric effects become noticeable during re-entry. The Karman line, at 100 km (62 mi), is also frequently used as the boundary between atmosphere and space.

Actinides

IUPAC currently recommends the name actinoid rather than actinide. The suffix "-ide" generally indicates negative ions whereas the suffix "-oid" indicates similarity to one of the members of the containing family of elements.

The fourteen elements in the bottom row of the inner-transition elements of the periodic table that follow the element Actinium (Ac #89). Some reference sources include actinium in this series others do not. For these elements the 5f orbital is the filling orbital. This series is a sub-series of thetransition metals. All actinides are radioactive.

Only thorium and uranium occur naturally in the earth's crust in anything more than trace quantities. Neptunium and plutonium have been known to show up naturally in trace amounts in uranium ores as a result of decay or bombardment. The remaining actinides were discovered in nuclear fallout, or were synthesized in particle colliders. The latter half of the series possess exceedingly short half-lives. Actinoids The fourteen elements in the bottom row of the inner-transition elements of the periodic table that follow the element Actinium (Ac #89). Some reference sources include actinium in this series others do not. For these elements the 5f orbital is the filling orbital. This series is a sub-series of thetransition metals. All actinoids are radioactive. Only thorium and uranium occur naturally in the earth's crust in anything more than trace quantities. Neptunium and plutonium have been known to show up naturally in trace amounts in uranium ores as a result of decay or bombardment. The remaining actinoids were discovered in nuclear fallout, or were synthesized in particle colliders. The latter half of the series possess exceedingly short half-lives.

Base

Bases can be thought of as the chemical opposite of acids. A reaction between an acid and base is called neutralization. Bases and acids are seen as opposites because the effect of an acid is to increase the hydronium ion (H3O+) concentration in water, whereas bases reduce this concentration. Bases react with acids to produce water and salts (or their solutions).

A weak base is a chemical base that does not ionize fully in an aqueous solution. As bases are proton acceptors, a weak base may also be defined as a chemical base in which protonation is incomplete. This results in a relatively low pH level compared to strong bases. Bases range from a pH of greater than 7 (7 is neutral, like pure water) to 14 (though some bases are greater than 14).

A strong base is a base which hydrolyzes completely, raising the pH of the solution towards 14. Strong bases, like strong acids, attack living tissue and cause serious burns.

Acid

An acid is traditionally considered any chemical compound that when dissolved in water, gives a solution with a pH of less than 7.

Generally, acids have the following chemical and physical properties: Taste: Acids generally are sour when dissolved in water. Touch: Acids produce a stinging feeling, particularly strong acids. Reactivity: Acids react aggressively with or corrode most metals. Electrical conductivity: Acids, while not normally ionic, are electrolytes.

Strong Acids; Hydrochloric Acid (HCl), Nitric Acid (HNO3) and Sulfuric Acid (H2SO4).

Common acids; Acetic acid (E260, C2H4O2) found in vinegar, Ascorbic acid (vitamin C, E300, C6H8O6) found in fruits, Carbonic acid (E290, H2CO3) found in carbonated soft drinks, Citric acid (E330, C6H8O7) found in citrus fruits, Formic acid (E236, CH2O2HCOOH) found in bee and ant stings,Lactic acid (E270, C3H6O3) found in dairy products such as yoghurt and sour milk, also is product of cellular fermentation, the reason muscles burn,Oxalic acid (C2H2O4) found in spinach and rhubarb, Pectic acid (C17H24O16) found in fruits and some vegetables, Sorbic acid (E200, C6H8O2) found in foods and drinks, Stearic acid (E570, C18H36O2), a type of fatty acid that comes from many animal and vegetable fats and oils, Tannic acid(C76H52O46) found in tea and Tartaric acid (E334, C4H6O6) found in grapes.

Abundance

 

Abundance in context of the Periodic Table Isotope pages refers to Natural Abundance; the prevalence of different isotopes of an element as found in Earth naturally. The weighted (by natural abundance) average mass of these isotopes is the atomic weight listed for the element in the periodic table. The abundance of an element varies from planet to planet.

Percent natural abundances refer to the relative proportions, expressed as percentages by number, in which the isotopes of an element are found in natural sources.

"Synthetic" refers to a synthetic isotope, that is, a radionuclide that is not found in nature: no natural process or mechanism exists which produces it, or it is so unstable that it decays away in a very short period of time. Examples include Technetium-95 and Promethium-146. Many of these are found in, and harvested from, spent nuclear fuel assemblies. Some must be manufactured in particle accelerators.

Absorption Spectrum

A material's absorption spectrum shows the fraction of incident electromagnetic radiation absorbed by the material over a range of frequencies. An absorption spectrum is, in a sense, the opposite of an emission spectrum. Every chemical element has absorption lines at several particular wavelengths corresponding to the differences between the energy levels of its atomic orbitals. For example, an object that absorbs blue, green and yellow light will appear red when viewed under white light. Absorption spectra can therefore be used to identify elements present in a gas or liquid.

This method is used in deducing the presence of elements in stars and other gaseous objects which cannot be measured directly.