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Beryllium and iodine make an ionic compound which is a solid. It is prepared by reacting beryllium metal with iodine in the presence of hydrogen at high temperatures. Its oxidation number is 2.
It is a greyish-white solid. It combines with water to form a solution of beryllium hydroxide. When this is added to acid it forms a precipitate of beryllium chloride. This is then redissolved in sodium hydroxide solution to give the colourless solution of beryllium tetrahydroxoberyllate. The name derives from the fact that it contains four beryllium hydroxide ions. The beryllium tetrahydroxoberyllate ions are then oxidised by adding iodine to produce the iodide ion. This process is reversible and the beryllium tetrahydroxoberyllate hydroxide ions can again be precipitated from solution by adding acid. This is because the iodide ion has both basic and acidic properties – it is amphoteric.
In ionic compounds the atoms have a positive charge and are attracted to each other across the ionic bond. This gives them a more dense structure than covalent molecules and lower melting and boiling points. However, the electron clouds of some ionic substances can be diffuse and “fluffy”. These distortions can lead to an ionic bond that has properties more like a covalent one.
For example, calcium chloride can be a mixture of ionic and covalent compounds because the electron cloud is diffuse. This can cause the ions to be displaced, resulting in an uneven distribution of electrons and the formation of a complex ion. This may also cause a slightly lower melting point, but it will still be an ionic compound.