Periodicity

• Periodicity


  The repeating pattern of physical or chemical properties going across the periods
• Classification of elements


  • s block
  • p block
  • d block
• Elements classified into s, p or d block

  • According to which orbitals the highest energy electrons are in
• Atomic radius decreases from left to right across a period


  Due to the increased number of protons creating more positive charge attraction for electrons which are in the same shell with similar shielding
• 1st Ionisation Energy generally increases from left to right across a period


  Due to the increasing number of protons as the electrons are being added to the same shell
• Small drop in 1st Ionisation Energy between Mg and Al

  Mg has its outer electrons in the 3s sub shell, whereas Al is starting to fill the 3p subshell. Al's electron is slightly easier to remove because the 3p electrons are higher in energy
• Small drop in 1st Ionisation Energy between phosphorous and sulfur

  Sulfur's outer electron is being paired up with another electron in the same 3p orbital. When the second electron is added to an orbital there is a slight repulsion between the two negatively charged electrons which makes the second electron easier to remove
• Melting and boiling points of Na, Mg, Al


  Metallic bonding: strong bonding - gets stronger the more electrons there are in the outer shell that are released to the sea of electrons. A smaller sized ion with a greater positive charge also makes the bonding stronger. Higher energy is needed to break bonds
• Melting and boiling points of Si


  Macromolecular: many strong covalent bonds between atoms, high energy needed to break covalent bonds - very high melting and boiling points
• Melting and boiling points of Cl2 (g), S8 (s), P4 (s)

  Simple molecular: weak van der Waals between molecules, so little energy is needed to break them - low melting and boiling points
• Melting and boiling point of S8 higher than P4

  S8 has more electrons (128) than P4 (60) so has stronger van der Waals between molecules
• Melting and boiling point of Ar

  Monoatomic: weak van der Waals between atoms
• Exactly the same trend in period 2 as period 3
• Period 2 = Li, Be, B, C, N, O, F, Ne
• Period 3 = Na, Mg, Al, Si, P, S, Cl, Ar
• Similar trend in period 2 as period 3 with drops between Be & B and N to O for same reasons
• Li, Be have metallic bonding (high melting points)
• B, C are macromolecular (very high melting points)
• N2, O2 are molecular (gases! Low melting points as small van der Waals)
• Ne is monoatomic gas (very low melting point)