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Thallium, because it is heavier, has two different
so-called

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oxidation states. It can react with one atom
of chlorine per

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thallium, or say three, and the different
chemistry between

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thallium-one and thallium-three. Thallium
salts are very

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poisonous, and there was quite a notorious
case some years

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ago when a technician at some company, who
turned out to

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be mad, was discovered to have been poisoning
many of his

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colleagues with thallium and so, but it is
not a widely used

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element, apart from a few materials that are
used. There is a

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rather nice red material that is made of thallium,
which has

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the rather unromantic name of KRS-5, this
is mixed bromide

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and iodine with thallium which is used as
windows for many

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infrared experiments. But because thallium
is so poisonous,

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one has to be quite careful about handling.