Patterns in the Periodic Table

Patterns in the Periodic Table

1. Densities, melting points and boiling points

   • usually metals are more dense
     • Carbon is most dense element (diamond)
   • mercury and bromine are the only elements liquid at room temperature (25"c)

2. covalent radius

   • half the distance between the nuclei of 2 bonded atoms
   • Increases down the group \
     • the number of occupied electron shells/energy levels increases
   • decreases across the period (left>right)
     • the nuclear charge is increasing which exerts an increasing attraction on the outer electrons, resulting in a decrease in covalent radius

3. ionisation energy

   • ''the first ionisation energy of an element is the energy required to remove one mole of electrons from one mole of gaseous atoms of an element''
   • ionisation increases across a period (left to right)>>
     • positive nuclear charge is increasing so it is more difficult for the negative electron to be pulled away
     • the atoms are getting smaller and so the negative electron is closer to the positive nucleus and attracted more strongly
   • ionisation energy increases down a group
     • the outer electron to be removed is in a shell further away from the nucleus so the atoms are getting bigger and so the negative electron is further away from the positive nucleus and attracted more strongly
     • there will be a greater screening effect due to more shells of inner electrons between the outer shell and the nucleus. These inner electrons are said to 'screen' or 'shield' the electrons in the outer shell from the positive nucleus

4. electronegitivity

   • ''electronegativity is a measure of the attraction an atom has for the electrons in a bond''
   • electronegativity values increase along a period
   • decreases down a group
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