sigma bond sigma bond English English English ویکی‌پدیای انگلیسی English σ sigma bond English sigma bonds English sigma σ English English English http://en.wikipedia.org/wiki/Sigma_bond English English σ bonds English English σ Sigma bond sigma bond English σ English English English English English sigma bond sigma bond English English Ïƒ valence bond English sigma bond English English sigma bond σ English English English sigma bond English sigma sigma bond