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C3 The Periodic Table

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The Periodic Table
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Keywords

Atomic number

Number of protons in the nucleus of an atom. Also the number of electrons an atom has.


Atomic mass

The combined number of protons and neutrons in an atom's nucleus.

Early Table

Newlands (Law of Octaves)

  • arranged his table in order of relative atomic mass
  • found a pattern every 8th element
  • to make the pattern fit, some elements where placed in groups that didn't match their chemical properties
  • he also assumed all the elements had been found
  • this table was not accepted by scientists

Mendeleev

  • arranged his table in order of relative atomic mass, but where elements were in groups that didn't match their properties he moved them
  • left gaps for undiscovered elements, rather than trying to fill spaces
  • predicted the properties of those elements... and got them right!
  • this convinced scientists to use his periodic table

Modern Table

The modern periodic table is based on the work of Mendeleev, and we see elements arranged in order of atomic number.


The Periodic Table is arranged in such a way that elements with the same properties can be found together in 'groups'.


Elements in the same group have the same number of electrons in their outer shell (these are known as valance electrons).


Elements in the same period have the same number of electron shells.


Group 1 (Alkali Metals)

React with water to form an alkaline solution:

Lithium + water —> lithium hydroxide + hydrogen

React vigorously when heated with chlorine gas:

Sodium + chlorine —> sodium chloride


Reactivity increases down the group – the outer negative electron is further from the positive nucleus so more easily lost.


Lower melting and boiling points down the group.


Transition Metals

  • strong
  • dense
  • shiny
  • good conductors of heat and electricity
  • often have more than one ion, forming coloured compounds

Group 7 (Halogens)

Exist as pairs of atoms.


Less reactive down the group – the outer shell is further from the nucleus so harder to gain an electron.


Higher melting and boiling points down the group.


A more reactive halogen will displace a less reactive halogen.


Group 0 (Noble Gases)

Full outer shells so are inert.


Boiling point increases down the group as atoms have more electrons so stronger intermolecular forces form between molecules.


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Page last updated: 14/04/2017