CHEMISTRY GLOSSARY

molecular shape    →   oblik molekule

Molecular shape is the three dimensional arrangement of atoms in space around a central atom. The molecular formula of a substance does not give an indication of its shape. For example, CO₂ is a linear molecule, but SO₂ is angular.

The three-dimensional shapes of many small molecules can be predicted by applying the valence shell electron pair repulsion theory (VSEPR). When atoms combine to form molecules, pairs of valence electrons arrange themselves as far from each other as possible. Another way to characterize molecular shape is in terms of hybrid orbitals.

Onsager relations    →   Onsagerove relacije

Onsager relations are an important set of equations in the thermodynamics of irreversible processes. They express the symmetry between the transport coefficients describing reciprocal processes in systems with a linear dependence of flux (J_i) on driving forces (X_j).

In Onsager’s theory the coupling coefficients are equal, L_ij = L_ji. This is known as reciprocal relations. The theory was developed by the Norwegian chemist Lars Onsager (1903-1976) in 1931.

photoelectric effect    →   fotoelektrični efekt

Photoelectric effect is the complete absorption of a photon by a solid with the emission of an electron. The energy of a photon (hν) is

valence bond    →   valentna veza

In the valence bond theory, a valence bond is a chemical bond formed by overlap of half-filled atomic orbitals on two different atoms.

VSEPR    →   VSEPR

Valence shell electron pair repulsion theory (VSEPR) is a model that explains the shapes of molecules by assuming that electron pairs arrange themselves around atoms in a way that minimizes electron-electron repulsions.

sigma bond    →   sigma veza

Most single bonds are sigma bonds (σ-bond). In the valence bond theory, a sigma bond is a valence bond that is symmetrical around the imaginary line between the bonded atoms.

spin    →   spin

Spin is the intrinsic angular momentum of an elementary particle, or system of particles such as nucleus, that is also responsible for the magnetic moment; or, a particle or nucleus possessing such a spin. The spins of nuclei have characteristic fixed values. Pairs of neutrons and protons align to cancel out their spins, so that nuclei with an odd number of neutrons and/or protons will have a net non-zero rotational component characterized by a non-zero quantum nuclear spin number.

Stern-Gerlach experiment: a beam of silver atoms is split into two beams when it traverses a nonuniform magnetic field. Atoms with spin quantum number m_s=+1/2 follow one trajectory, and those with m_s=+1/2 follow another.

triple bond    →   trostruka veza

Triple bond. (≡) is a covalent bond that involves 3 bonding pairs. In the valence bond theory, one of the bonds in a triple bond is a sigma bond and the other two are pi bonds. For example, the central bond in acetylene is a triple bond: H-C≡C-H.