Barium is a metal with good electric conductance, like copper or gold, but it is much softer than those elements. Its Mohs hardness is 1.25, which means you can scatch it with your fingernail. It is also way more reactive than gold: if you put pure silvery-golden coloured barium in air, it immediately tarnishes to almost black as the barium reacts with oxygen. This reactivity has been used in the past in the manufacture of vacuum tubes for pre-transistor electronics; barium is a 'getter' that can suck up gases to improve the vacuum in the tube.
The best known use of barium is in X-ray imaging. Patients are given a 'barium meal' of barium sulphate so that medics can see details of the gut. It has to be barium sulphate, because soluble barium compounds are quite poisonous, as barium ions will block potassium sites in the nervous system leading to tremors, heart irregularities and even paralysis.
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If you want to find out more about some of the terms and concepts we have a Frequently Asked Questions Page.