OCR AS Chemistry F331: Periodicity

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The Periodic Table
In the early 1800s, there were only 2 ways to categories elements- by their physical and
chemical properties and by their relative atomic mass.
An English chemist, John Newlands first arranged the elements in 1863. He noticed that if he
arranged the elements in order of mass, similar elements appeared at regular intervals ­
every eight element was similar.
He called this the law of octaves, and he listed some known elements in rows of seven so
that the similar elements lined up in columns.
Newlands presented the ideas to scientists who weren't convinced. The problem was that
the pattern broke down on the third row, with many transition metals messing it up
In 1869, Russian Chemist Dmitri Mendeleev produced an improved table.
He organised the 61 elements known at that time into a table made up of horizontal
rows and vertical columns.
Elements with similar properties were placed underneath each other in the columns.
Mendeleev left gaps in his Periodic Table. Not only did he predict that new elements
would be discovered to fill the gaps, he also predicted the properties of these yet
undiscovered elements.
In Mendeleev's Periodic Table the elements were arranged in order of increasing relative
atomic mass.
The modern periodic table is arranged into periods ­ rows and groups ­ columns.
All the elements within a period have the same number of electron shells. The number of
the period corresponds to the number of electron shells.
All the elements within a group have the same number of electrons in their outer
shell. This means they have similar physical and chemical properties.
Properties often change gradually as you go down each group.

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In the Periodic Table there are trends in properties as you go down a group, but also many
properties vary in a fairly regular way as you move across a period from left to right.
The pattern is then repeated as you go across the next period.
The occurrence of periodic pattern is called periodicity.
Periodic patterns include: density, melting points and boiling points, atomic size, periodicity
in chemical formula, periodicity in ionisation enthalpies and
periodicity in chemical properties.…read more

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Atomic Size
The size of an atom is determined by the space its electrons occupy.
Atoms become smaller as you move across a period, despite the increasing number of
This is because the successive electrons adding on crossing a period enter the same electron
In addition, the number of protons in the nucleus steadily increases across a period and the
increasing positive nuclear charge attracts the electrons more and more strongly.…read more


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