The Periodic Table

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The Periodic Table

What is it?

THE PERIODIC TABLE

The Periodic Table is a method of organising all known elements by their atomic number, which is the number of protons in an atom of that element. Due to the nature of the elements, this means that they are grouped with elements of similar properties.The modern Periodic Table was developed by Dmitri Mendeleev, but prior work from John Newlands and Johann Dobereiner helped him form it. The three men's discoveries are described below.

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In 1817, Dobereiner published his law of triads. A 'triad' was a group of three elements which had similar properties - lithium, sodium and potassium, for example - but Dobereiner had also discovered that the middle element of these triads had a relative atomic mass (a precursor to atomic numbers that was not quite as accurate) similar to the average of the others. The system wasn't perfect, as it didn't involve all the elements, but these triads showed scientists the importance of atoms in ordering the elements.

Johann Dobereiner

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John Newlands invented the law of octaves in 1864. The law of octaves is so called because, when the elements are ordered by atomic mass, Newlands discovered that every eigth one had similar properties. The problem was that, towards the end of the table, the pattern broke down, meaning that , for example, iron was grouped with oxygen and sulfur, which are both non-metals.

John Newlands

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Published only five years after Newlands' law of octaves, Mendeleev's Periodic Table was similar in many ways, except one: he did not stick completely to ordering the elements by relative atomic mass, switching some round to better organise them into 'families'. His theory was laughed at to begin with, not least because his method of placing an element in a column with similar others left gaps in some of the rows. However, instead of seeing this as a problem, Mendeleev believed that the spaces were for elements which hadn't been discovered yet. He went on to predict the properties of these elements based on their atomic mass, and when gallium was discovered 15 years later, it had the same atomic mass and very similar properties to eka-aluminium, the made-up element Mendeleev had predicted.One other problem remained though: Mendeleev had switched round the position of some of the elements, so they were not strictly ordered by atomic mass. This problem was solved in 1913, when Henry Mosely suggested using his newly-discovered method of calculating atomic number, it fitted the periodic table exactly.

Dmitri Mendeleev

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Columns on the periodic table are known as groups, and contain elements with similar properties. For example, in group 18 (or group 0) all the elements are noble gases.

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Rows on the periodic table are called periods, and when you read them left to right, they are in ascending order of atomic number.


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