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Stories About Chemistry


3. A Two-Faced Element

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.”

Is this true? Yes, but only half true. Chemistry is a precise science: and chemists do not like half-truths. Hydrogen is a convincing example.

Hydrogen tower building

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.

The Big House is so arranged that kindred elements live one above the other off each stair well, comprising the groups and subgroups of the Periodic System. This is the law for the inhabitants of the Big House. By falling into Group I, hydrogen inevitably breaks this law. But where is poor hydrogen to go? There are all in all nine groups, nine stair wells in the Big House. Helium, hydrogen’s first-floor neighbour, found its flat only in what is now called the zero group. The places in the rest of the groups are vacant. See how many possibilities there are for replanning the first floor to find hydrogen a real place under the sun!”

Couldn’t it be lodged in the second group, with the alkaline-earth metals headed by beryllium? No, they feel absolutely no kinship towards hydrogen. The third, fourth, fifth and sixth groups also refuse to have anything to do with it. What about the seventh group? Wait! The halogens occupying this group, fluorine, chlorine, bromine, etc., are ready to extend a friendly hand to hydrogen.

Imagine a meeting between two children. “How old are you?” “So much.” “So am I.” “I’ve got a bicycle!” “Me too!” “What’s your Dad?” “A truck driver!” “Whee - so is mine!” “Let’s be friends?” “Let’s!”

“Are you a non-metal?” fluorine asks hydrogen. “Yes!” “Are you a gas?” “That’s right.” “So are we,” says fluorine, nodding at chlorine. “My molecule consists of two atoms!” contributes hydrogen.

“Well, what do you know about that!” says fluorine in surprise. “Just like ours. And can you show negative valence, accept additional electrons? We are awfully fond of doing that!”

“Of course I can. I form hydrogen compounds known as hydrides with the very alkali metals that dislike me so, and my valence in them is minus one.”

“All right then, pitch right in with us and let’s be friends!”

And hydrogen takes up its abode in the seventh group. But not for long. After getting to know its new relation a little better one of the halogens remarks a little disappointedly: “See here, brother. You don’t seem to have many electrons in your outer shell, do you? Only one as a matter of fact - like them blokes in group one. Hadn’t you better get back to the alkali metals?”

See what difficult straits hydrogen is in: there are plenty of rooms but none it can occupy permanently, with full rights. But why? What is the reason for this surprising two-facedness of hydrogen? What makes hydrogen behave so eccentrically?

The specific properties of any chemical element become evident when it combines with other elements. It then yields or accepts electrons which either leave its outer shell or enter it. When an element loses all the electrons of its outer shell, the rest of its shells usually remain unchanged. Such is the case with all the elements except hydrogen. When hydrogen parts with its only electron, all that remains is its atomic nucleus. What is left is a proton, this being, as a matter of fact, all the hydrogen atom nucleus consists of. (Actually it does not always consists of only a proton, but we shall come to this important point later.) Hence the chemistry of hydrogen is the only chemistry of its kind, as it were, the chemistry of an elementary particle, the proton. Thus, reactions involving hydrogen proceed under the influence of protons. And that is why hydrogen behaves so inconsistently.

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