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Actinide elements

(a series of radioactive metallic elements in Group 3 of the periodic table . Members of the series are often called actinides, although actinium (at.

Open Actinium

( ăktĭn'ēӘm ) [Gr.,=like a ray], radioactive chemical element; symbol Ac; at. no. 89; mass number of most stable isotope 227; m.p. about 1,050 degrees

Activation energy

in chemistry, minimum energy needed to cause a chemical reaction . A chemical reaction between two substances occurs only when an atom, ion, or

Open Adams, Walter S. (Walter Sydney), 1876-1956
Adams, Walter S. (Walter Sydney), 1876-1956

US astronomer. He developed the use of spectroscopy in the study of stars and planets, and found that luminosity and the relative intensities of

Open Adaptation (Biology)
Adaptation (Biology)

In biology, a trait of an organism that increases its chances of success in its environment. Animals and plants adapt to changes in their environment

Open Addition reactions
Addition reactions

Chemical reaction in which the atoms of an element or compound react with a double bond or triple bond in an organic compound by opening up one of the


Bonding agents are called adhesives or glues. They may be made from natural substances or artificial materials. They bind together metals, wood,

Open Adolescence

Historically, adolescent development has been described as tumultuous, which is sometimes attributed to increased emotions and an undeveloped

Open Adrenal glands
Adrenal glands

Triangular endocrine gland situated on top of the kidney . (The name derives from the Latin word for kidney – ad-renal and supra-renal meaning next to

Adriatic Sea

Large arm of the Mediterranean Sea, lying northwest to southeast between the Italian and the Balkan peninsulas. The western shore is Italian; the