Protons and neutrons have a relative mass of 1 and electrons have anegligible mass. Neutrons are neutral, but protons and electrons are electrically charged. Protons have a relative charge of +1 and electrons have a relative charge of -1.
Early ideas about atoms
The word atom comes from atomos, an ancient Greek word meaning indivisible. The Greek philosopher Demokritos (460-370 BCE) maintained that all matter could be divided and sub-divided into smaller and smaller units, and eventually there would be a tiny particle that could not be divided any further - an atom. This was remarkable because there was no way ancient Greeks could support this theory by observation or experiment.
John Dalton
Understanding of atoms didn’t progress much beyond Demokritos’ theory until the English chemist John Dalton (1766 - 1844) started to look at it in the 1800s. Dalton did experiments, worked out some atomic weights, and invented symbols for atoms and molecules. His most important conclusions are summarised below.
- all matter is made of atoms, and atoms are indestructible and cannot be broken down into pieces
- all the atoms of a particular element are identical to each other and different from the atoms of other elements
- atoms are rearranged in a chemical reaction
- compounds are formed when two or more different kinds of atoms join together
Dalton's theories about atoms took a long time to be accepted by scientists. Some of his ideas about gases were incorrect, and it was difficult for many years to do the experiments needed to support his theories, because atoms are too small to see.
Atoms and elements
Although the word 'atom' comes from the Greek for indivisible, we now know that atoms are not the smallest particles of matter. Instead, they have a small central nucleus surrounded by even smaller particles called electrons.
All substances are made from atoms. And, as Dalton suggested, any given element is made of atoms of just one particular sort. The atoms of any element are different from the atoms of any other element. So iron contains a different sort of atoms from those of sulfur, and the atoms in carbon are different from those of oxygen.
Chemical symbols
The atoms of each element are represented by chemical symbols. These usually consist of one or two different letters, but sometimes three letters are used for newly-discovered elements. The first letter in a chemical symbol is always anUPPERCASE letter, and the other letters are always lowercase. So, the symbol for magnesium is Mg and not mg, MG or mG.
Every element has its own chemical symbol. For example, iron is Fe, sulfur is S, sodium is Na and oxygen is O.
The periodic table
There are more than 100 different elements. The periodic table is a chart showing all the elements arranged in a particular way. The vertical columns in the periodic table are called groups. Each group contains elements that have similar properties.
The periodic table has eight main groups. For example, group 1 contains very reactive metals such as sodium - Na - while group 7 contains very reactive non-metals such as chlorine - Cl.
Note that you will never find a compound in the periodic table, because these consist of two or more different elements joined together by chemical bonds.
Reactions and compounds
New substances are formed by chemical reactions. When elements react together to form compounds their atoms join to other atoms using chemical bonds. For example, iron and sulfur - often spelt 'sulphur' - react together to form a compound called iron sulfide - often spelt 'sulphide' - and sodium and oxygen react together to form sodium oxide.
- electrons are transferred from one atom to another, so that one atom gives electrons and the other takes electrons, or
- electrons are shared between two atoms.
You don’t need to know any more details about these bonds for GCSE Science.
Chemical formulae
The chemical formula of a compound shows how many of each type of atom join together to make the units that make the compound up. For example, in iron sulfide every iron atom is joined to one sulfur atom, so we show its formula asFeS. In sodium oxide, there are two sodium atoms for every oxygen atom, so we show its formula as Na2O. Notice that the 2 is written as a subscript, so Na2O would be wrong.
The diagram below shows that one carbon atom and two oxygen atoms combine to make up the units of carbon dioxide - its chemical formula should therefore be written as CO2.
Sometimes you see more complex formulae such as Na2SO4 and Fe(OH)3:
- a unit of Na2SO4 contains two sodium atoms, one sulfur atom and four oxygen atoms joined together
- a unit of Fe(OH)3 contains one iron atom, three oxygen atoms and three hydrogen atoms - the brackets show that the 3 applies to O and H
Equations
When elements are joined to cause a chemical reaction, no atoms are made or lost during the process - but at the end of it they are joined differently from the way they were at the start. This means that the mass of the substances at the start - the reactants - is the same as the mass of the substances at the end -the products.
Copper and oxygen reaction - getting a balanced equation
We use balanced equations to show what happens to the different atoms in reactions. For example, copper and oxygen react together to make copper oxide.
Take a look at the word equation for the reaction, here:
copper + oxygen → copper oxide
You can see that copper and oxygen are the reactants, and copper oxide is the product.
If we just replace the words shown above by the correct chemical formulae, we will get an unbalanced equation, as shown here:
Cu + O2 → CuO
Notice that we have unequal numbers of each type of atom on the left-hand side compared with the right-hand side. To make things equal, we need to adjust the number of units of some of the substances until we get equal numbers of each type of atom on both sides of the arrow.
Here is the balanced symbol equation:
2Cu + O2 → 2CuO
You can see that now we have two copper atoms and two oxygen atoms on each side. This matches what happens in the reaction.
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