CHEMISTRY GLOSSARY

molecular weight    →   molekularna masa

See Relative molecular mass

molecule    →   molekula

Molecule is the smallest part of a chemical compound that can take part in a chemical reaction. The atoms of a molecule are held together by chemical bonds.

percentage composition of atom in the molecule    →   postotni sastav atoma u molekuli

Percentage composition of atom in the molecule is a structure of compound presented in the shape of a percentage of its mass, which comes from every element.

polyvalent molecule    →   polivalentna molekula

Polyvalent molecule is a molecule which having multiple binding sites. The antibodies of our immune system are one example.

structural isomer    →   strukturni izomer

Structural isomers are compounds that have the same formula, but different constituents. Dimethyl ether (CH₃OCH₃) and ethanol (CH₃CH₂OH), for example, are constitutional isomers with general formula C₃H₆O.

nonpolar molecule    →   nepolarna molekula

Nonpolar molecule is a molecule which has no separation of charge, so no positive or negative poles are formed. For example, the Cl₂ molecule has no polar bonds (molecule with one type of atom), CH₄ is a non-polar molecule (due to its symmetry). Nonpolar molecules do not dissolve in water as they cannot form hydrogen bonds (thus are hydrophobic) but do dissolve in lipids or fats (lipophilic).

polar molecule    →   polarna molekula

Polar molecules are molecules at which centres of gravity of positive and negative charge are not in the same point.

relative molecular mass    →   relativna molekularna masa

Relative molecular mass (M_r) is the ratio of the average mass per molecule or specified entity of a substance to 1/12 of the mass of nuclide ¹²C. Also called molecular weight. It is equal to the sum of the relative atomic masses of all the atoms that comprise a molecule. For example

M_r(H₂SO₄) = 2·A_r(H) + A_r(S) + 4·A_r(O)

= 2·1.0079 + 32.066 + 4·15.999

= 2.0158 + 32.066 + 63.996

= 98.078

alanine    →   alanin

Alanine is hydrophobic amino acids with aliphatic side chain. It is the second simplest amino acid, but used the most in proteins. The nonpolar hydrophobic amino acids tend to cluster together within proteins, stabilizing protein structure by means of hydrophobic interactions. Alanine is a nonessential amino acid, meaning it can be manufactured by the human body, and does not need to be obtained directly through the diet.

• Abbreviations: Ala, A
• IUPAC name: 2-aminopropanoic acid
• Molecular formula: C₃H₇NO₂
• Molecular weight: 89.09 g/mol

allotropy    →   alotropija

Allotropy (Gr. allos, other, and tropos, manner) is the phenomenon of an element existing in two or more physical forms in the same physical state. The difference between the forms involves either crystaline structure (white, red and black phosphorus), the number of atoms in the molecule of a gas (diatomic oxygen and triatomic ozone), or the molecular structure of a liquid (liquid helium an helium II).

In some cases, the allotropes are stable over a temperature range, with a definite transition point at which one changes into the other. For instance, tin has two allotropes: white (metallic) tin stable above 13.2 °C and grey (nonmetallic) tin stable below 13.2 °C. This form allotropy is called enantiotropy. Form of allotropy, in which there is no transition temperature at which the two are in equilibrium, is called monotropy.

Allotropy does not apply to the substance existing in different physical states as, for example, when ice melts and changes from solid ice to liquid water.

Allotropy is generally restricted to describing polymorphic behaviour in elements, while polymorphism may refer to any material having multiple crystal structures.