The periodic table

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1History of the periodic table
In the 1800's there was only two ways to categorise elements ­ by physical and chemical
properties and their relative atomic mass( mass number). They didn't know of atomic
structure, protons or electrons etc.
John Dalton
He put forward the atomic theory. He said:
­ All element are made up of very small solid particles called atoms
­ However he only knew 30 elements and listed in order of mass. He identified these
using symbols.
­ The atomic theory provided the basis for scientists like Dmitri Mendeleev to create the
periodic table
Newland's law of Octaves
John Newlands proposed that every eighth element had similar chemical and physical
properties e.g. lithium and sodium and potassium and magnesium and calcium. This was
called the law of octaves as it was similar to the musical scale. However the pattern did not
work for every element as the pattern broke on the third row. He also arranged them in order
of mass too. His work was criticized as:
­ He did not leave any gaps for undiscovered elements
­ He mixed up metals and nonmetals
­ His groups contained some elements that didn't have similar properties
Dmitri Mendeleev
Like Newlands and Dalton he arranged the elements in order of atomic mass. He wanted to
arrange them so that vertical columns had similar properties (but some elements did not fit
the pattern ) and so he left gaps and predicted the properties of undiscovered elements likely
to occupy these gaps. His ideas were not accepted at first as he had little proof to back him
up. It wasn't until one of the elements that matched his predictions was found, germanium, that
his theory was accepted. Many scientists then began to also use it as a tool to help complete
the gaps. He was also not believed as he left gaps and some people thought there were no
more elements. He reversed the order for some elements to put them in order of
chemically similar properties and didn't go up in atomic weight.
How did he know there were undiscovered elements? As some elements did not fit the
pattern/ have the correct properties so he left gaps.
Scientists initially thought of the periodic table as fun as there wasn't much evidence to
suggest that elements did fit together in a particular way. (Mendleev's was enough evidence
in favour of the periodic table.)
The modern periodic table arranges elements in terms of their electronic structure. Elements
in the same group have the same number of electrons in their outer shells and therefore share
the same chemical properties. Elements in the same period occupy the same number of
electron shells. It also goes in increasing atomic number.
The discovery of protons allowed the periodic table to be arranged in groups with
same number of electrons in outer shell, periods of same occupied shells and in
atomic mass order.

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Alkali metals are shiny and soft but go a dull colour when react with oxygen
­ They all have one electron in their outer orbit which makes them very reactive as they
really want to lose this electron to gain a full outer shell, this also gives them similar
­ If they lose this one electron they for + ions
­ Alkali metals are very reactive and so have to be stored in oil to prevent them reacting
with the air.…read more

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The Halogens ­ Group 7
As you go down group 7 the halogens are:
­ Less reactive. In a reaction with a metal halogens gain an electron to gain a full outer
orbit. As you go down the group the size of the atom increases are more electron
shells are added and the outer electrons are further and further away from the nucleus
and so the weaker the attraction between the outer electrons and the nucleus.…read more

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Transition metals ­ between groups 2 and 3 block of metals
­ They are metals with high melting an boiling points
­ The are good conductors of heat/electricity
­ The are dense, strong and shiny
­ They are much less reactive than group one metals as they do not react as violently
with water and oxygen
­ They are also stronger and more dense than group 1 metals as they have a much
higher melting points.…read more


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