General geology. Elements in the Earth’s crust. Start-up. Part 1. Elements and compounds. 1. Read and translate the following words and word-combinations. Practice your pronunciation.

There are over 100 different elements, which are made up of atoms. Elements can be divided into metals and non-metals. Chemical symbols and formulae are used to represent elements and compounds.

Atoms. Everything is made from atoms. Atoms are tiny particles that are far too small to see, even with a microscope.

Elements. The atoms in a particular element are the same as each other, and they are different from the atoms of all other elements. For example, lead and gold are elements. A piece of pure gold contains only gold atoms. A piece of pure lead contains only lead atoms.

The atoms of some elements do not join together, but instead they stay as separate atoms. Helium is like this. The atoms of other elements, such as hydrogen and oxygen, join together to make molecules.

Compounds. A compound is a substance that contains atoms of two or more different elements, and these atoms are chemically joined together. For example, water is a compound of hydrogen and oxygen. Each of its molecules contains two hydrogen atoms and one oxygen atom. There are very many different compounds.

Chemical symbols. Each element is given its own chemical symbol, like H for hydrogen or O for oxygen. Chemical symbols are usually one or two letters long. Every chemical symbol starts with a capital letter, with the second letter written in lower case.

Sometimes it is easy to tell which element a symbol stands for. For example, C stands for carbon. But sometimes the symbol comes from a name for the element that is not an English word. For example, W stands for tungsten (from the word wolfram) and Na stands for sodium (from the word natrium). The same chemical symbols are used all over the world, no matter which language is spoken, which makes them very useful.

Chemical formulae. We use chemical symbols to stand for the elements. For example, C stands for carbon, O stands for oxygen, S stands for sulfur and Na stands for sodium. For a molecule, we use the chemical symbols of the atoms it contains to write down its formula. For example, the formula for carbon monoxide is CO. It tells you that each molecule of carbon monoxide consists of one carbon atom joined to one oxygen atom.

Some formulae are more complicated. For example, the formula for sodium sulfate is Na₂SO₄. It tells you that sodium sulfate contains two sodium atoms (Na₂), one sulfur atom (S) and four oxygen atoms (O₄).

The formula for a substance is always the same. All compounds have a definite composition. For example, a water molecule always contains two hydrogen atoms and one oxygen atom. It cannot be a water molecule if it has different numbers of these atoms. The formula for water is always H₂O.

Molecules of elements. Oxygen is an element. All its atoms are oxygen atoms. However, they do not exist separately. Instead, they pair up to form oxygen molecules (O₂). Hydrogen also has molecules with two atoms (H₂). We show this in their chemical formulae.

Molecules of compounds. Compounds are made from two or more atoms chemically joined together. For example, water is a compound of hydrogen and oxygen. Each water molecule contains two hydrogen atoms and one oxygen atom. We show this in its chemical formula, H₂O.

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