Hydrogen

Hydrogen (chemical symbol H), named after the Greek word for "water-forming", is the most abundant element in the Universe. In fact more than nine-tenths of all atoms are hydrogen atoms.
It is the stuff of stars, of cold "empty" space and of the Earth. Hydrogen is found everywhere and in most compounds. It is the most universal element.

Hydrogen is also the simplest element in existence. Its atom contains just one proton in its core, and only one electron is associated with it. As a result, hydrogen is a tiny atom. But its simplicity explains why it is so universal: it is the building block from which other elements are made.

Although it is so universal, hydrogen is rarely found as a gas on Earth. It makes up only three-quarters of 1% of the mass of the planet. This is because hydrogen molecules (combinations of two hydrogen atoms, described by the symbol H₂) weigh so little that they can escape from the Earth's atmosphere. The only free hydrogen that survives is in pockets deep underground, part of the decay process that also forms oil and natural gas.

Hydrogen is a gas that easily catches fire and can be explosive. Yet it is made in huge quantities each year. About half of it is used to make ammonia (a compound of nitrogen and hydrogen), which is the basis for many fertilisers. About one-third of the hydrogen produced is used in refining metals. Another main use of hydrogen is to make a liquid called methanol, one of the starting materials in making artificial fibres.

Helium

Helium (symbol He) is another element with a simple structure. It is the second most abundant element in the Universe, and yet its presence was not suspected until

relatively recently. It is the only element ever to have been identified in space before it was found on Earth. Scientists noticed a mysterious element they could not identify in the light shining from stars. Eventually this was identified as helium.

Helium, like hydrogen, is lighter than air. But it is an inert gas, which means it rarely reacts and cannot burn. Being inert, it is not found in any compounds on Earth, which is why it took so long to identify. But being an inert gas also means it can be used in places where hydrogen would be dangerous. Thus, helium, rather than hydrogen, fills the floating balloons seen at funfairs.

Helium is one of a group of inert, or noble, gases, whose physical and chemical properties are closely related. The other noble gases, which together make about 1% of the Earth's atmosphere, are neon, argon, krypton, xenon and radon. All other elements react to form stable substances. The noble gases are unreactive because they each have just enough electrons to be perfectly stable on their own, without needing to react with other elements.

The noble gases

The noble, or inert, gases are helium, neon, argon, krypton, xenon and radon. They make up a complete group (0) in the Periodic Table and the properties of
each member of the group are closely related to the others. The noble gases make up nine-tenths of 1% of the volume of the Earth's atmosphere.

The noble gases are called inert gases because they do not usually react with any other element. Most elements react with one another because the combined atoms are more stable than the individual atoms. In fact, the most stable state generally occurs when the number of electrons in the outermost shell of an atom is eight. Reactions occur so that atoms can achieve eight electrons, either by losing electrons or gaining them from their partner. The noble gases are so stable, and react so little because, unusually for elements, the number of electrons in the outer part of the atom completely fill the shell. In all but helium the number of electrons in the outer shell is eight; for helium it is two ­ the maximum possible in its only shell.

Not all of the noble gases are equally stable. For example, xenon can be made to combine with a few other elements, but helium combines with none.

Hydrogen in water

Water is the most common chemical compound on the surface of the Earth, covering over two-thirds of the Earth's surface. People are also about two-thirds water.

Water is a compound of hydrogen and oxygen. Although we are so familiar with the way water behaves that we take it as normal, water, in fact, behaves differently from most other compounds. This is due to the strong attraction that each water molecule has for others. Water shrinks as it is cooled (down to 4°C) and then expands as it is cooled further still (to 0°C). It is also the only common substance that swells as it freezes.

All these unusual properties are due to the special way that the hydrogen atoms link water molecules together, the property called hydrogen bonding.

The atoms that make up water molecules are bound together very much more firmly than are the molecules to each other. Enormous amounts of energy are needed to break them apart. Heating, for example, only changes the physical state of water, converting solid ice to liquid water and then to steam. Only electrical energy will break bonds within the molecules.

The process of decomposing water (breaking it apart) is called electrolysis.

How electrolysis works

Pure water is a poor conductor of electricity. But water with impurities in it does conduct. Dilute solutions of acids, for example, are excellent conductors of electricity. Dilute sulphuric acid is used in vehicle batteries.

If two electrodes are dipped in a dilute acid solution, the electrical energy will break apart the hydrogen particles (ions) from the hydroxyl ions (pairs of hydrogen and oxygen particles). Because opposite charges attract, positively charged hydrogen ions drift through the dilute acid to the negative electrode. There they form hydrogen gas. Negatively charged hydroxyl ions drift to the positive electrode where oxygen gas forms.

The production of hydrogen gas from water comes at a price. Large amounts of electricity have to be used. However, the energy is not wasted, but simply stored in the hydrogen gas as chemical energy. This energy makes hydrogen gas very inflammable and even explosive if mixed with air or oxygen.

The demonstration on the pages of the Hydrogen book in the Elements set uses electrolysis to collect hydrogen and oxygen gases separately. The equipment is called Hoffman's voltameter.

Preparing hydrogen gas

Hydrogen gas can be produced in the laboratory by reacting a dilute acid with a metal. The demonstration below uses dilute hydrochloric acid and zinc. Dilute copper sulphate has been added to the hydrochloric acid to speed up the reaction; this is what makes the acid green. The copper does not take part in the reaction; it is a catalyst.

What the apparatus does

Hydrogen is produced by the reaction in the flask. The gas is then led through tubing into a water bath containing a gas jar supported on a beehive shelf. This is the standard way of collecting gases that are not soluble in water. As the gas bubbles through it, the water is displaced but the gas is contained.

At the end of the experiment a cover slip can be placed on the open end of the gas jar while it is still under water, thus sealing the gas in the jar.

EQUATION: Laboratory production of hydrogen

Hydrochloric acid + zinc = zinc chloride + hydrogen

2HCl(aq) + Zn(s) = ZnCl₂(aq) + H₂(g)