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Physical Properties of Calcium

Calcium crystallises in hexagonal or rhombohedric plates, which, when freshly cut, have a bright silver-white appearance. X-ray analysis shows that the calcium atoms are arranged as a face-centred cube, each being surrounded by twelve equi-distant atoms. The diameter of the atoms - that is, the distance from centre to centre of contiguous atoms - is 3.40×10-8 cm.

Calcium is malleable and can be drawn into wire of mm. diameter. The hardness is 2.2 – 2.5 on Mohs' scale - that is, between lead and Iceland spar, or about equal to that of aluminium. The tensile strength is 612 kgm. per sq. cm.

The following values have been given for the specific electrical resistance: 3.43×10-6 ohms per c.c. at 0° C., temperature coefficient 0.00457; 4.6×10-6 ohms per c.c. at ordinary temperature, increasing linearly up to about 13.6×10-3 ohms at 600° C., with a temperature coefficient of 0.00364; 6.77×10-6 ohms per c.c. at ordinary temperature.

Swisher also determined the thermo-electric power to be positive with respect to lead, the potential varying from 8.9 microvolts per degree at 50° C., to 14.0 microvolts at 400° C.

Calcium has two resonance potentials: 1.90 volts, the wave-length determining it being 6572.78 Å; and 2.85 volts, wave-length 4226.73 Å. The ionisation potential is 6.1 volts, and the corresponding wave-length 2027.56 Å. McLennan and Young found 6.12 volts for the ionisation potential and λ = 2028.2 Å.

The ionic mobility of ½Ca is 51.

Using a dropping mercury cathode, Heyrovsky showed the deposition potential of the calcium ion to be -2.023 volts.

The photo-electric emission of electrons by calcium is normal over wave-lengths 2300 Å to 7000 Å, and has a maximum value at λ = 3650 Å. This maximum becomes more pronounced as the angle of incidence diminishes.

When heated, calcium becomes positively charged.

The density of calcium appears to be in the neighbourhood of 1.54. Higher values given by earlier investigators are evidently due to impurities, which will, for the most part, be heavier than calcium. For the redistilled metal Arndt obtained the value 1.52, and Biltz and Hohorst,1.542 at 25° C. Brace found 1.46 for the density of the cold rolled material at 22° C.

The melting-point of calcium is 810° C., but it will sublime below the melting-point and is very volatile above it. It is more volatile than either barium or strontium. The vapour pressure of calcium at the melting-point is 2 mm., and, by calculation from the vapour-pressure curve, the boiling-point is 1240° C.

More extensive specific heat investigations have been carried out by Eastman and Rodebush, employing Nernst's method for low temperatures. The specific heat at -205.4° C. is 0.098, corresponding to an atomic heat of 3.93, and this value gradually increases with temperature until, at 20.5° C., the value 0.169, atomic heat 6.75, is reached.

The most intense lines in the spectrum of calcium, measured in Angstrom units, are the following: -

Arc: Ultra-violet: 2197.791, 2208.606, 3006.864, 3158.877, 3361.918, 3644.400; Visible: 3706.022, 3933.68, 3968.482, 4226.731, 4283.008, 4302.527, 4425.428, 4434.947, 4454.765, 4585.868, 5265.559, 5349.470, 5588.746, 5857.476, 6162.177, 6439.086, 6462.576, 6493.789, 6572.783; Infra-red: 8499.3, 8542.6, 8662.6, 10345.0.

Spark: Ultra-violet: 2197.94, 2208.83, 2398.61, 3158.864, 3179.345; Visible: 3933.67, 3968.476, 4226.730, 4283.004, 4289.361, 4298.985, 4302.523, 4435.960, 4454.780.
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