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THE PERIODIC TABLEINSIDE AN ELEMENTMAKING MOLECULESMOLECULES TODAY
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  What is the periodic table?

The periodic table is a classification system for the elements. Each element has unique properties and you can predict an element's physical and chemical properties based on its position in the periodic table.

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What do the symbols and numbers mean?

Lead - Pb - Atomic mass of 207 - Atomic number of 82When you look at the periodic table, the symbols and numbers look like the image on the left. The symbols are abbreviations of the origins of the word for each element. For example, the origin of lead's symbol, Pb, is the Latin word 'plumbum' meaning 'liquid silver'.

The number above the symbol is the atomic mass (or atomic weight). This is the total number of protons and neutrons in an atom.

The number below the symbol is the atomic number and this reflects the number of protons in the nucleus of each element's atom. Every element has a unique atomic number. Lead has 82 protons therefore its atomic number is 82. The elements are then arranged in order of increasing atomic numbers - so hydrogen (H), which only has one proton, is located first in the table, while sodium (Na), which has 11 protons, and is found further down the table.

We can use atomic number and atomic mass to find out the number of protons, neutrons and electrons there are in an atom. By subtracting the atomic number from the atomic mass we can discover the number of neutrons there are.

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How does it work?

There are 18 main columns of elements in the periodic table. These are called groups or families. Elements within the same group or family have similar - but not identical - properties. For example lithium, sodium and potassium are members of family 1. They are all highly reactive, and thus can easily combine with other elements to form compounds. Each horizontal row is called a period. Unlike elements in a family, elements in a period are not alike in properties. There are 7 periods of elements. The 2 rows that have been separated out of the main table are rare-earth elements, or lanthanons (lanthanides) in the first period and actinons (actinides) in the next period.

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Who invented it?

The journey towards the creation of the periodic table can be traced as far back as the Ancient Greeks of 400BC who used the words "element" and "atom" to describe the differences between things.

The periodic table we know today was created by 4 separate European scientists during the 1860s. In 1863, 44 year-old French geologist A. E. Béguyer de Chancourtois created a list of elements measured by their atomic weight. Meanwhile, in England, chemist John A. R. Newlands noticed that chemical groups repeated every 8 elements. He named this the octave rule, comparing it to musical notes.

Dimitri I. MendeleevBetween 1868 and 1869, working independently, Russian chemist Dmitri I. Mendeleev and German chemist Lothar Meyer arranged the elements into 7 columns relating to their chemical properties. Their tables were similar, but it was Mendeleev’s table that was adopted. It assembled observations and discoveries of earlier chemists, showing the symbols of the chemical elements in the order of their atomic masses, but his table was a huge scientific breakthrough because he left gaps and accurately predicted undiscovered elements. This greatly accelerated the discovery of new elements and our understanding of their properties, and it enabled scientists to accurately predict the elements – many elements have been discovered or artificially made which have fitted into the gaps in the table. Mendeleev predicted the properties of scandium, gallium and germanium: within 20 years scientists had discovered the existence of these elements and his predictions were found to be accurate. Modern versions of Mendeleev’s table have 109 elements, but new elements are being discovered all of the time.

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Discuss it ...
Do you think we can predict what elements will be discovered next using the periodic table?

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Teachers' Notes ...
For Teachers' Notes on this subject, click here.

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