Chemistry & Environmental Dictionary

Absolute Error - Atomic Mass Unit

Absolute Error

The difference between the approximate and exact value in any calculation.


In measurement indicates that a set of measurements are close to the true answer, though they are not necessarily precise.


Compounds that contain hydrogen (H #1) and when dissolved in water (H2O), they increase the concentration of hydrogen ions, H+ (hydronium ions H3O+). Acids are proton donors. Substances with a pH less than 7 are considered to be acidic.


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 the transition metals.

Alkali Earth 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 color 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.

Alkali Metal

A metal in the first column of the periodic table (i.e., lithium, sodium, potassium, rubidium, cesium 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.

Alpha Particle

Consists of the nucleus of an atom of helium (two protons and two neutrons) and is sometimes emitted from the nucleus of some atoms as part of those atoms' radioactive decay. Alpha particles have an energy range of 4-8 MeV. This energy is dissipated in only a few centimeters of air or less than 0.005mm of aluminum (Al).

Ångstrom (Å)

One ten-billionth of a meter (10-10m or 0.0000000001 m) or 1/10th of a nanometer.

Angular Momentum Quantum Number

See azimuthal quantum number


A negatively charged ion.

Antimatter (antiparticle)

Species of subatomic particles that have the same mass and spin as normal particles, however, they have opposite electrical charges from their normal matter counterparts. In the case of antineutrons they are opposite of neutrons in magnetic moment. Positrons, which are the counterpart to electrons, have a positive charge and antiprotons have a negative charge. Photons are their own antimatter counterpart. When a particle of matter collides with a particle of antimatter, both particles are destroyed and their masses are converted to photons of equivalent energy.


A lung disease (pneumoconiosis) resulting from inhaling fibers of asbestos and marked by interstitial fibrosis of the lung. (More information)


The smallest possible unit of matter that still maintains an element's identity during chemical reactions. Atoms contain one or more protons and neutrons (except hydrogen (H), which normally contains no neutrons) in a nucleus around which one or more electrons revolve.

Atomic Mass (Atomic Mass Average)

The average mass of all nuclides of an element determined by the proportions in which each nuclide of the element are present within the earth and its atmosphere.

Atomic Mass Unit (AMU)

A mass unit that is exactly 1/12th the mass of a carbon 12 (C12) atom (approximately 1.67E-24g).

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