You may have heard a dialogue something like this during a chemistry lesson at school.

Teacher: "What group of the Periodic System is hydrogen in?"

Pupil: "In the first. This is so because like the other elements of the first group, the alkali metals (lithium, sodium, potassium, rubidium, cesium, and francium), the hydrogen atom has only one electron in its only electron shell. Like them, hydrogen displays a positive valence of one in chemical compounds. Finally, hydrogen is capable of displacing some metals from their salts."

   What is there in common between hydrogen and the alkali metals? Only their positive valence of 1. Only the similar arrangement of their outer electron shells. As to the rest, they bear no resemblance at all. Hydrogen is a gas and a nonmetal. Hydrogen forms a diatomic molecule. The rest of the elements of the first group are classical metals, and the most active ones as far as chemical reactions go. Brandishing its only electron, hydrogen tries to don the toga of an alkali metal. But actually it is a wolf in sheep's clothing.

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