In chemistry, the arrangement in space of the atom s in a molecule .
It is especially important in organic chemistry (see organic compound ), in which each carbon atom in a molecule can form from two to four covalent bonds (see bonding ) with as many as four other atoms. A carbon atom with four single bonds, for example, lies at the centre of a tetrahedron, with one bond extending to each corner; if three or all four atoms bonded to the carbon are different, the structure that results has two different mirror-image (see optical activity ; isomer ) forms, similar to left-and right-handed gloves. Configuration issues also apply to some inorganic compounds. Until late in the 20th century, chemists struggled to determine the true three-dimensional form (absolute configuration) of a molecule experimentally, but modern optical and chemical methods have greatly simplified the task. The term electronic configuration refers to the number of electrons in the various shells of an atom, which defines its chemical reactivity and the type of bonding in which it participates.