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abundance

Overview page. Subjects: Chemistry — Physics.

1 The ratio of the total mass of a specified element in the earth's crust to the total mass of the earth's crust, often expressed as a percentage. For example, the abundance of...

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accumulator

Overview page. Subjects: Chemistry — Physics.

A type of voltaic cell or battery that can be recharged by passing a current through it from an external d.c. supply. The charging current, which is passed in the opposite direction to that...

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actinium

Overview page. Subjects: Chemistry — Physics.

Symbol Ac. A radioactive metallic element belonging to group 3 of the periodic table; a.n. 89; mass number of most stable isotope 227 (half-life 21.7 years); m.p. 1050 ± 50°C; b.p. 3200°C...

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actinoid contraction

Overview page. Subjects: Chemistry — Physics.

A smooth decrease in atomic or ionic radius with increasing proton number found in the actinoids.

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activation analysis

Overview page. Subjects: Chemistry — Physics.

An analytical technique that can be used to detect most elements when present in a sample in milligram quantities (or less). In neutron activation analysis the sample is exposed to a flux...

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activity

Overview page. Subjects: Chemistry — Physics.

1. Symbol a. A thermodynamic function used in place of concentration in equilibrium constants for reactions involving nonideal gases and solutions. For example, in a reaction

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adiabatic approximation

Overview page. Subjects: Chemistry — Physics.

An approximation used in quantum mechanics when the time dependence of parameters, such as the internuclear distance between atoms in a molecule, is slowly varying. This approximation means...

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adiabatic demagnetization

Overview page. Subjects: Chemistry — Physics.

A technique for cooling a paramagnetic salt, such as potassium chrome alum, to a temperature near absolute zero. The salt is placed between the poles of an electromagnet and the heat...

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americium

Overview page. Subjects: Chemistry — Physics.

Symbol Am. A radioactive metallic transuranic element belonging to the actinoids; a.n. 95; mass number of most stable isotope 243 (half-life 7.95 × 103 years); r.d. 13.67 (20°C); m.p. 994 ±...

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ammonia clock

Overview page. Subjects: Chemistry — Physics.

A form of atomic clock in which the frequency of a quartz oscillator is controlled by the vibrations of excited ammonia molecules. The ammonia molecule (NH3) consists of a pyramid with a...

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