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History of the Periodic Table
The two ways that elements used to be organised are :
By their physical and chemical properties
By their relative atomic mass
This is because back then they didn't know about protons, electrons, or atomic
structures.
The known elements were organised in order of relative atomic mass, and when
this was done they realised that there was a periodic pattern in the properties of
the elements, so they named it the periodic table.…read more

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History of the Periodic Table
Newlands came up with the Law of Octaves in 1864.
He found that when the elements were arranged in order of relative atomic
mass, every eighth element had similar properties. He organised the elements
into rows of seven because of this.
His pattern (of every eighth element having similar properties), broke down as
the table went on. He hadn't left any gaps for undiscovered elements , which
caused problems in his table.
His work has been critiqued because:
His groups contained elements with properties that weren't similar
He mixed the metals with non-metals
He didn't leave any gaps for undiscovered elements…read more

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History of the Periodic Table
In 1869 (5 years after Newlands), Dmitri Mendeleev came up with his own table
of elements.
He too put the elements in order of their relative atomic mass.
He noticed that the properties of the elements were related to their atomic
mass (in a periodic way), and arranged his table so that elements with similar
properties fell into the same vertical columns.
He found that he had to leave gaps in his table, so that elements with similar
properties stayed in the same vertical columns.
Mendeleev thought that the gaps left in his table would be filled with elements
that hadn't been discovered at the time.
He was able to predict the properties and work out the atomic masses of the
missing elements, and when the elements were discovered people realised that
his predictions were right.…read more

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The Modern Periodic Table
Some people weren't fond of the periodic table. When it was first
released there wasn't a lot of evidence to show that the elements
really fit into the table the way that they did. When newly discovered
elements fit into Mendeleev's table it provided some proof that the
table was correct.
When even more evidence surfaced scientists found out that the table
was useful for predicting the properties of elements.
In the 19th century, protons, neutrons, and electrons were discovered.
Scientists now accept that the periodic table is useful for summarising
that structure of atoms.…read more

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The Modern Periodic Table
Now that protons, neutrons, and electrons have been discovered the periodic table
is arranged according to atomic number.
Elements in the same group have the same number of elections in their outer shell (except
for transition metals)
Group numbers are equal to the number of electrons in the highest energy level
The nucleus is positively charged, and attracts the negative charge of the electrons, holding
them into place. The further away from the nucleus an electron is, the less attraction.
When there are more electron shells, the attraction of the nucleus to the outer shell is less.
The other shells get in the way and reduce the attraction. This is called shielding.
Electrons in the highest energy level are more likely to get lost because they have less
attraction to the nucleus. This is why group one metals get more reactive as you go down the
group.
The outer shell is also less likely to gain an electron when they're far away from the nucleus.
This is because there's less attraction from the nucleus that pulls electrons to the atom. This
is why group 7 elements get less reactive as you go down the group.…read more

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