The Physical Properties Of Beryllium
Beryllium is a light and rare metal with a small atomic number and low density (only 1.847g/cm³), which is about 2/3 of aluminum and 1/2 of titanium. The melting point b is higher (1283 ℃). Beryllium is α-Be at room temperature and has a close-packed hexagonal structure; at 1254 °C, a phase transition occurs and it is a β-Be structure. Beryllium has the largest heat capacity of all metals. With a specific heat capacity of 1.8828 J/gK at room temperature, beryllium absorbs more heat than other metals, and this property is maintained until its melting point. The thermal conductivity of beryllium at room temperature is 0.15kW/(m.K). The thermal expansion coefficient of beryllium is comparable to that of stainless steel and Ni-Co alloy; the thermal diffusivity is also very good.
Beryllium has a reflectance of 50% for visible light, 55% for ultraviolet, and 98% for infrared (10.6m). The X-ray penetration rate is very high (almost transparent), about 17 times that of aluminum, and it is an indispensable material for X-ray windows.
Beryllium has a very high modulus of elasticity (309,000 MPa), about 4 times that of aluminum, 2.5 times that of titanium, and 1.5 times that of steel. Especially in the temperature range from room temperature to 615 ℃, the specific stiffness is about 6 times that of steel, aluminum and titanium. In addition, the thermal neutron absorption rate of beryllium is the smallest among all metals, and the scattering cross section is very large.
In addition, beryllium is a steel gray metallic light metal. Beryllium is harder than other metals in the same group, unlike calcium, strontium, and barium, which can be cut with a knife.
The Chemical Properties Of Beryllium
Beryllium is a very active metal with a great affinity for oxygen, and can react with oxygen at room temperature to form a thin oxide film with protective properties on its surface. When the temperature is lower than 600℃, beryllium can be oxidized for a long time in dry air, and the oxidation rate will be gradually accelerated when the temperature is higher than 600℃. When the temperature reaches 800°C, when it stays for a short time, the degree of oxidation is not too serious.
It can be seen that beryllium, like lithium, forms a protective oxide layer in the air, so it is very stable even when it is red hot in the air. Insoluble in cold water, slightly soluble in hot water, soluble in dilute hydrochloric acid, dilute sulfuric acid and potassium hydroxide solution to release hydrogen. Metal beryllium has significant corrosion resistance to oxygen-free sodium metal even at higher temperatures. Beryllium has a positive 2 valence state and can form polymers as well as a class of covalent compounds with significant thermal stability.
The Anomalous Properties Of Beryllium
The valence electron shell structure of Be atom is 2s(2), its atomic radius is 89pm, the ionic radius of Be(2+) is 31pm, and the electronegativity of Be is 1.57. Beryllium has a particularly small atomic radius and ionic radius (not only smaller than other elements of the same family, but also smaller than alkali metal elements) and relatively high electronegativity (not only higher than alkali metal elements, but also higher than other elements in the same family), so beryllium is The tendency to form covalent bonds is significant, unlike other elements of the same family that mainly form ionic compounds. Therefore, beryllium often exhibits anomalous properties that are different from other elements of the same family.
- Beryllium is easy to form a dense protective film on the surface and does not interact with water, while other metals of the same family, magnesium, calcium, strontium and barium, are easy to react with water.
- Beryllium hydroxide is amphoteric, while the hydroxides of other elements in the same family are medium or strong alkaline.
- Beryllium salts are strongly hydrolyzed to form tetrahedral ions [Be(H2O)2]2+, and the Be-O bond is very strong, which weakens the O-H bond, so the hydrated beryllium ion has a tendency to lose protons:
Therefore beryllium salts are acidic in pure water. The salts of other elements of the same family (except magnesium) have no hydrolysis effect.