CHEMISTRY GLOSSARY

Geiger counter    →   Geigerov brojač

Geiger counter (Geiger-Muller counter) is a device used to detect and measure ionising radiation. It consists of a tube containing a low-pressure gas (usually argon or neon with methane) and a cylindrical hollow cathode through the centre of which runs a fine-wire anode. A potential difference of about 1 000 V is maintained between the electrodes. An ionising particle or photon passing through a window into the tube will cause an ion to be produced and the high potential will accelerate it towards its appropriate electrode, causing an avalanche of further ionisations by collision. The consequent current pulses can be counted in electronic circuits or simply amplified to work a small loudspeaker in the instrument. It was first devised in 1908 by the German physicist Hans Geiger (1882-1945). Geiger and W. Muller produced an improved design in 1928.

Graham’s law    →   Grahamov zakon

Graham’s law is the rates at whish gases diffuse are inversely proportional to the square roots of their densities. This principle is made use of in the diffusion method of separating isotopes. The law was formulated in 1829 by British chemist Thomas Graham (1805-1869).

lead-acid battery    →   olovni akumulator

Lead-acid battery is a electrical storage device that uses a reversible chemical reaction to store energy. It was invented in 1859 by French physicist Gaston Planté. Lead-acid batteries are composed of a lead(IV) oxide cathode, a sponge metallic lead anode and a sulphuric acid solution electrolyte.

In charging, the electrical energy supplied to the battery is changed to chemical energy and stored. The chemical reaction during recharge is normally written:

In discharging, the chemical energy stored in the battery is changed to electrical energy. During discharge, lead sulfate (PbSO4) is formed on both the positive and negative plates. The chemical reaction during discharge is normally written:

Lead acid batteries are low cost, robust, tolerant to abuse, tried and tested. For higher power applications with intermittent loads however, they are generally too big and heavy and they suffer from a shorter cycle life.

luminescence    →   luminiscencija

Luminescence (from Latin lumen, light) is the emission of electromagnetic radiation (UV, visible or IR) from atoms or molecules as a result of the transition of an electronically excited state to a lower energy state, usually the ground state. Luminescence can be divided into categories by duration (fluorescence or phosphorescence) or by the mechanism that creates the light (radioluminescence, electroluminescence, photoluminescence, thermoluminescence, triboluminescence, chemiluminescence, bioluminescence). The prefix identifies the energy source responsible for generating or releasing the light.

Phosphorescence is emission of light from a substance exposed to radiation and persisting as an afterglow after the source of excitation has been removed. Fluorescence, on the other hand, is an almost instantaneous effect, ending within about 10^-8 second after excitation.

mercury    →   živa

Mercury has been known since ancient times. The origin of the name comes from the Latin word hydrargyrum meaning liquid silver. It is heavy, silver-white metal, liquid at ordinary temperatures. Stable in air and water. Unreactive with alkalis and most acids. Gives off poisonous vapour. Chronic cumulative effects. Mercury only rarely occurs free in nature. The chief ore is cinnabar or mercury sulfide (HgS). Used in thermometers, barometers and batteries. Also used in electrical switches and mercury-vapour lighting products.

metal    →   metal

Metals are materials in which the highest occupied energy band (conduction band) is only partially filled with electrons.

Their physical properties generally include:

- They are good conductors of heat and electricity. The electrical conductivity of metals generally decreases with temperature.

- They are malleable and ductile in their solid state.

- They show metallic lustre.

- They are opaque.

- They have high density.

- They are solids (except mercury)

- They have a crystal structure in which each atom is surrounded by eight to twelve near neighbours

Their chemical properties generally are:

- They have one to four valence electrons.

- They have low ionisation potentials; they readily lose electrons.

- They are good reducing agents.

- They have hydroxides which are bases or amphoteric.

- They are electropositive.

Metallic characteristics of the elements decrease and non-metallic characteristics increase with the increase of valence electrons. Also metallic characteristics increase with the number of electron shells. Therefore, there is no sharp dividing line between the metals and non-metals.

Of the 114 elements now known, only 17 show primarily non-metallic characteristics, 7 others are metalloids, and 89 may be classed as metals.

non-metal    →   nemetal

Non-metals are defined as elements that are not metals.

Their physical properties generally include:

• They are poor conductors.
• They are brittle, not ductile in their solid state.
• They show no metallic lustre.
• They may be transparent or translucent.
• They have low density.
• They form molecules which consists of atoms covalently bonded; the noble gases are monoatomic.

Their chemical properties are generally:

• They usually have four to eight valence electrons.
• They have high electron affinities (except the noble gases)
• They are good oxidising agents (except the noble gases)
• They have hydroxides which are acidic (except the noble gases)
• They are electronegative.

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.

precision balance    →   tehnička vaga

Precision balances typically display results from three to one decimal places (0.001 g up to 0.1 g). The readability precision balances are reduced when compared to analytical balances but, precision balances accommodate higher capacities (up to several kilograms). In its traditional form, it consists of a pivoted horizontal lever of equal length arms, called the beam, with a weighing pan, also called scale, suspended from each arm.

In electronic top pan, or toploader balances, mass is determined not by mechanical deflection but by electronically controlled compensation of an electric force. The signal generated enables the mass to be read from a digital display. The mass of the empty container can be stored in the balance’s computer memory and automatically deducted from the mass of the container plus its contents.

promethium    →   prometij

Promethium was discovered by J. A. Marinsky, Lawrence Glendenin and Charles D. Coryell (USA) in 1945. Named after Prometheus in Greek mythology, who stole fire from the gods. It is rare earth metal of synthetic origin on the earth, naturally made in stars. Poison. Radiotoxic. Radioactive. Promethium does not occur naturally. Found among fission products of uranium, thorium and plutonium. It has been used as a source of radioactivity for thickness-measuring gages.