chemistry c3 the periodic table

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The early periodic table

Newlands:

–Arranged elements according to atomic weights

–‘Law of Octaves’ similar properties repeated every 8th element

–Some elements placed in inappropriate groups, due to strictly following order of weight

Sometimes more than one element in each ‘position

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The early periodic table

Mendeleev:

Also arranged elements according to atomic weights

Elements repeat properties periodically (regularly), also placed in columns, or groups

Gaps left for undiscovered elements

Three new elements discovered in his lifetime that fitted these gaps

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The modern periodic table

Electrons, protons and neutrons were discovered early in the 20th century

Elements now arranged by atomic (proton)numbers

All elements now in appropriate groups

Elements in the same group have the same number of electrons in their highest occupied energy level (outer shell)

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Trends within the periodic table: Group 1 - the al

Physical properties:

Low density (the first three elements in the group are less dense than water)

The further down the group an element is:

the more reactive the element the lower its melting point and boiling point

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Trends within the periodic table: Group 1 - the al

Chemical properties:

All react with non-metals to form ionic compounds in which the metal ion carries a charge of +1

The compounds are white solids that dissolve in water to form colourless solutions

All react with water, releasing hydrogen

All form hydroxides that dissolve in water to give alkaline solutions

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Trends within the periodic table: The transition e

Compared with the elements in Group 1, transition elements:

have higher melting points (except for mercury) and higher densities

are stronger and harder

are much less reactive and so do not react as vigorously with water or oxygen

Many transition elements have ions with different charges, form coloured compounds and are useful as catalysts

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Trends within the periodic table: Group 7- the hal

  • The elements in Group 7 of the periodic table react with metals to form ionic compounds in which the halide ion carries a charge of –1
  • In Group 7, the further down the group an element is:

the less reactive the element

the higher its melting point and boiling point

  • A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt.
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Trends within the periodic table: Explaining the t

The trends in reactivity within groups in the periodic table can be explained because the higher the energy level of the outer electrons:

the more easily electrons are lost

the less easily electrons are gained

Group 1 elements react by losing an electron

Therefore Li is less reactive than K, because it its outer electron is nearer to the attractive nucleus

Group 7 elements react by gaining an electron

So F2 is more reactive than I2, because its outer electrons are nearer the positive nucleus

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