Atomic properties
The unalloyed magnesium has the following atomic characteristics:
1. Symbol: Mg.
2. Atomic number: 12.
3. Electron configuration in free atoms: (2) (8) 2.
4. Orbital electron states in free atoms: 1s2, 2s2, 2p6, 3s2.
5. Atomic isotopes 78.99% Mg24 (mass = 23.98504), 10.00% Mg25 (mass = 24.98584) and 11.07% Mg26 (mass = 25.98259).
6. Atomic weight: 24.3050(6).
7. Atomic volume: 14.0 cm3/moll.
Crystal Structure
The crystal structure of pure magnesium under atmospheric pressure is hexagonal close-packed. The lattice parameters of pure magnesium at 25 oC (77 oF) were in agreement within ±0.01% of the assessed values of a (side length) = 0.32092 nm and c (height) = 0.52105 nm. The actual c/a ratio at room temperature is 1.6236 making magnesium nearly perfectly closed packed.
 Table 1
Mass Characteristics
The density of magnesium at 20 oC (68 oF) is 1.738 g/cm3 (0.0628 lb/in.3). At the melting temperature (650 oC, or 1202 oF), the density in solid state is approximately 1.65 g/cm3 (0.0596 lb/in.3); in liquid state, it is approximately 1.58 g/cm3 (0.0571 lb/in.3).
Volume change on freezing. Volumetric shrinkage of 4.2% occurs on freezing (1.5% linear shrinkage).
Volume change on cooling. Volumetric shrinkage of 5% occurs on cooling solid form from 650 oC to 20 oC (1202 oF to 68 oF) (1.7% linear shrinkage).
Thermal properties
The melting point of pure magnesium under atmospheric pressure is 650 oC ± 1 oC (1202 oF ± 1.8 oF).
The boiling point of pure magnesium under atmospheric pressure is 1090 oC (1994 oF).
Thermal expansion. The coefficients of linear thermal expansion for polycrystalline magnesium at low temperatures can be expressed as:
αt = (25.0 + 0.0188t oC) μm/m x oC
or
αt = (13.7 + 0.0058t oF) μ in/in. x oF
Thermodynamic properties
Specific heat capacities at constant pressure (Cp) in units of J/mol x K are:
1. Solid (298-923 K): 26.19 - 1.01 x 10-3 T - 1.60 x 105/T2 + 8.41 x 10-6 T2
2. Liquid (923-1600 K): 212.74 - 205.66 x 10-3T - 350.15 x 105/T2 + 61.56 x 106
3. Monatomic gas (298-2000 K): 20.79
4. Monatomic gas (2000-3000 K): 22.12 - 1.24 x 10-3T
5. Diatomic gas (298-900 K): 20.45 + 0.48 x 10-3T + 3.23 x 105/T2 - 0.19 x 106T
6. Diatomic gas (900-3000 K): 20.78 + 2.71 x 105/T2
Electrical properties
Electrical conductivity is 38.6% IACS.
Electrical resistivity (ρ) for single crystals of magnesium at 20 oC (68 oF) is 45.3 nΩ · m along the a-axis and 37.8 nΩ · m along the c-axis. The temperature coefficient at 20 oC (68 oF) is 0.143 nΩ · m/K along the c-axis.
The contact potential is +44mV versus saturated calomel electrode at 25 oC (77 oF) and -0.222 mV versus copper at 27 oC (81 oF).
The electrochemical equivalent is 126 mg/C.
The standard electrode potential is -2.40 V versus hydrogen.
The ionization potential is 7.65 eV for Mg+ and 15.05 eV for Mg2+.
The electrolytic solution potential is 1.63 mV versus saturated calomel electrode at 25 oC (77 oF) in aerated NaCl solution.
The Work Function. Two values have been published: 3.61 eV and 3.66 eV.
Magnetic properties
Magnetic susceptibility is 0.00627 to 0.00632 mks (mass)
Magnetic permeability is 1.000012.
The Hall constant is -1.06 x 10 -16 Ω · m/(A/m)
Optical properties
Reflectivity is 0.72 at λ = 0.500 μm, 0.74 at λ = 1.00 μm, 0.80 at λ = 3.0 μm, 0.93 at λ = 9.0 μm.
Solar absorptivity is 0.31.
Emissivity is 0.07 at 22 oC (72 oF).
The absorption constant is 4.42 at λ = 0.589 μm.
The refractive index is 0.37 at λ = 0.589 μm.
Nuclear properties
Neutron absorption cross-section values in barns per atom (1 barn = 10-24 cm2) for thermal neutron radiation are:
0.063 ± 0.004 for natural magnesium
0.03 ± for Mg24
0.27 ± for Mg25
0.03 ± for Mg26
Magnesium has the following radioactive isotopes: Mg21, Mg22, Mg23, Mg27, and Mg28.
The x-ray absorption coefficient μm is 32.9 m2/kg.
Mechanical properties of magnesium are listed in Table 2. Its dynamic modulus of elasticity is 45 GPa and its static modulus of elasticity is 43 GPa.
 Table 2
Poisson's ratio (v) is 0.35.
The velocity of sound for drawn and annealed magnesium is 5.77 km/s for a longitudinal wave in bulk, 3.05 km/s for a transverse (shear) wave, and 4.94 km/s for a longitudinal (extensional) wave.
The friction coefficient for magnesium versus magnesium at 20 oC (68 oF) is 0.36.
Approximate dynamic viscosity of liquid magnesium is 1.23 mPa x s at 700 oC (1292 oF).
Surface Tension. In the temperature range 660 oC to 852 oC (1220 oF to 1566 oF), four sources reported values ranging from 0.545 N/m to 0.563 N/m. Two sources reported 0.502 N/m and 0.504 N/m for the temperature range 894 oC to 1120 oC (1641 oF to 2228 oF).
Pure magnesium is not used for commercial structural applications, although the metal has a high damping capacity and is easily machined and formed into shapes by casting or hot forming processes.
Chemical properties
Magnesium burns in air with an intense white flame. The ignition temperature is 645 oC in dry air but decreases with increasing moisture content. The rate of oxidation in moist air at different temperatures is illustrated in Figure 1.
Burning magnesium reacts violently with water. Fire is extinguished with magnesium chloride, alkali chlorides, dry sand, or dry iron sponge. Magnesium reacts with gaseous chlorine to form magnesium chloride and with nitrogen at ca. 500 oC to form Mg3 N2.
 Figure 1
Pure magnesium has a high resistance to corrosion because its galvanic activity is low. Contamination with heavy metals (usually copper, iron, and nickel), chlorides, and oxide or nitride inclusions combined with exposure to chloride-containing solutions on untreated surfaces, strongly promote corrosion due to enhanced galvanic activity.
The metal is readily dissolved by most organic and inorganic acids. A protective layer of water-insoluble magnesium hydroxide is formed when magnesium is exposed to moist air or clean water at room temperature.
Magnesium is resistant to alkali hydroxide solutions, hydrofluoric acid, fluorine, and fluorine compounds (including ammonium hydrogen difluoride) due to the formation of protective hydroxide and fluoride films.
The ability of magnesium to form stable protective oxide, chromate, phosphate, sulfate, and fluoride films is employed commercially in protective coating systems used as primers for paints or as final coatings. |
