Atoms, Molecules and Chemical Symbols

1How many times can you cut a copper wire?

Take a 1 cm piece of copper wire from an old electric cable and cut it in half. Cut one half in half again, and again. With a sharp blade you can manage perhaps six or seven cuts before the piece is too small to hold. But suppose you had a perfect knife and perfect eyes. Would the copper go on being copper for ever, or would you reach a smallest piece?

About 2,400 years ago the Greek thinkers Leucippus and Democritus argued that you would reach a smallest piece, which they called atomos, meaning "uncuttable". They had no way to test the idea. Today we know the answer: after about 25 halvings, since $2^{25}\approx 3\times {10}^{7}2^\{25\}\; \backslash approx\; 3\; \backslash times\; 10^\{7\}$, your piece of copper would be a single atom wide, about $0.3\mathrm{n}\mathrm{m}0.3\backslash ,\backslash mathrm\{nm\}$ ($3\times {10}^{-10}\mathrm{m}3\; \backslash times\; 10^\{-10\}\backslash ,\backslash mathrm\{m\}$) across. Cut that atom and what is left is no longer copper.

2From an idea to a science

In 1808 the English teacher John Dalton turned the old Greek idea into a scientific theory. From careful weighing of substances before and after reactions, he proposed that:

• all matter is made of tiny atoms;
• the atoms of one element are all alike, and different from the atoms of every other element;
• atoms join in simple whole-number ratios to make compounds;
• in a chemical reaction atoms are rearranged, never created or destroyed.

Dalton drew each kind of atom as a circle with its own mark (the photo below). Later discoveries changed some of his ideas: atoms turned out to contain even smaller particles (Lesson 2), and some atoms of one element have different masses (isotopes). His last point is still the basis of every chemical equation you will write in Unit 7.

3Atoms join to make molecules

A few elements, the noble gases such as helium, neon and argon, exist as single atoms. Most other atoms are joined to other atoms. A group of atoms held together by chemical bonds is a molecule. If all the atoms in a molecule are the same, the substance is an element; if there are two or more kinds of atom, it is a compound (you met elements and compounds in Unit 4).

Figure 1.1: Atoms and molecules. Each coloured ball stands for one atom. Argon exists as single atoms; oxygen, water and ammonia are made of molecules.

Table 1.1: Molecules of elements and compounds

4Chemical symbols: one language for every chemist

In 1813 the Swedish chemist Jöns Jacob Berzelius replaced Dalton's circles with letters, and his system is the one used in every country today. A Rwandan chemist, a Chinese chemist and a Brazilian chemist all write iron as $\mathrm{F}\mathrm{e}\backslash mathrm\{Fe\}$. The rules are simple:

• A symbol has one or two letters (a few very heavy, recently made elements have had three).
• The first letter is always a capital; the second is always small.
• Most symbols come from the English name: H for hydrogen, C for carbon, Ca for calcium, Cl for chlorine.
• Some come from old Latin, German or Greek names (Table 1.3). Where two elements start with the same letter, the second letter tells them apart: C carbon, Ca calcium, Cl chlorine, Co cobalt, Cu copper.

Table 1.2: Names and symbols of the first 20 elements (learn these by heart)

Rwanda is one of the world's main producers of tin, tantalum and tungsten, the "3Ts". Two of the three have symbols that do not match their English names.

Table 1.3: Symbols that come from other names