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AQA Chemistry 3 ­
The Development of the Periodic Table…read more

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C3 1.1 The Early Periodic Table
· During the 19th century scientists were trying to find ways to classify elements.
· In 1863 Newlands proposed his theory, the law of octaves.
· The principle was that similar properties were repeated ever 8th element.
Relative atomic mass
· He put the 56 know elements into seven groups according to atomic weights.
· But sometimes within the group properties didn't match very well, so other scientists didn't
accept his ideas.
· In 1869 Mendeleev produced a better table by leaving gaps for undiscovered elements.
· Mendeleev could predict the properties these undiscovered elements would have because of
where the gaps were in the table.
· When the elements were discovered they matched the properties Mendeleev had predicted.
· As a result other scientists began to accept his ideas and Mendeleev's table began the basis
for the modern periodic table.…read more

Slide 3

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C3 1.2 The Modern Periodic Table
· Scientists found out about protons and electrons in the early 20th century.
· After this, they developed models of the arrangement of electrons in atoms.
· Elements were then arranged in order of their atomic number and lined up in groups.
· Elements in groups have similar properties because they have the same number of electrons
in their outer shell.
Image from BBC bitesize…read more

Slide 4

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C3 1.2 The Modern Periodic Table
· Reactivity depends upon the number of electrons in an atom.
· Going down a group atoms get larger/ contain more electrons.
· As the atoms get bigger there is more electrons between the outer electron and the nucleus.
· All these electrons shield the outer most electron from the positive charge of the nucleus.
· Thus weakening the attraction between them, making the electron easier to loose.
· When metals react they lose an electron.
· It is easier for the elements lower down in a group to lose an electron because the attraction
between it and the nucleus is weaker.
· Therefore these elements are more reactive
· Non-metals, however, gain an electron when they react.
· Non-metals higher up in their group find it easier to attract an electron as the charge from the
nucleus has not be shielded as much as those lower down.
· Non-metals are get more reactive going up their group…read more

Slide 5

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C3 1.3 The Alkali Metals.
· Alkali metals are soft solids at room temperature (some can be cut with a knife).
· They have low melting point and boiling points
· The melting and boiling point increase going down the group.
· They have low densities, lithium, sodium and potassium float on water.
· They react readily with air and water. With water they produce hydrogen gas and a metal
hydroxide that is a strong alkali.
· They all have one electron in their outer shell.
· They lose this electron in reactions to form ionic compounds in which their ions have a single
positive charge
· (Same number of protons, positive charge, and electrons, negative charge, this cancel each
other out. If an electron is lost the is a +1 charge as there is one more proton than electron.)
· They react with halogens to form salts that are white or colourless crystals
e.g sodium + chlorine = sodium chloride…read more

Slide 6

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C3 1.4 The Halogens
· The halogens are non-metal elements.
· They exist as small molecules made up of pairs of atoms.
· They have low melting and boiling points.
· Their melting and boiling points increase going down the table.
· At room temperature: fluorine is a pale yellow gas
chlorine is a green gas
bromine is a red-brown liquid
iodine is a grey solid (easily vaporises to violet gas)
· All of the halogens have seven electrons in their outer shell.
· They form ionic compounds with metals in which their ions have a charge of -1.
· They also bond covalently with other non-metals forming compounds.
· Reactivity of halogens increases going up the group…read more

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