单词	sigma
释义	sigma /ˈsɪɡmə / noun 1The eighteenth letter of the Greek alphabet (Σ, σ), transliterated as ‘s’. The form ς is used instead of σ at the end of a word. The uncial form, resembling the letter c, is also sometimes used. He coined this latter term and employed the Greek letter lower case sigma to denote its population parameter.... Alpha, rho, tau, sigma, omega, Ash went over the letters he saw in his mind while Lari just stared, apparently in her own little world. Finally, even if epsilon is the correct reading, in the letter space to its right, where Meritt read part of an iota, are traces of what appears to be a sigma. 1.1 (Sigma) [followed by Latin genitive] Astronomy The eighteenth star in a constellation: Sigma Octantis... Finally, the final green beam of light was shot straight up at the constellation Sigma. 1.2 Chemistry & Physics Relating to or denoting an electron or orbital with zero angular momentum about an internuclear axis: the σ orbital is localized symmetrically... These conical intersections are fundamentally different from the ‘H-vinyl’ structures found in simple alkenes because they involve the interaction of four sigma electrons, as opposed to two pi electrons and two sigma electrons. The sigma bond occupies the internuclear space, and the two pi bonds are at 90° angles to each other surrounding the carbon-carbon internuclear space. The triple bond is composed of one sigma bond from overlap of sp hybrid orbitals, and two pi bonds, from overlap of two sets of p orbitals. symbol 1 (Σ) Mathematical sum. 2 (σ) Standard deviation. Origin Greek. Rhymes enigma, stigma
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