Periodicity

• • The Periodic Table is a list of all known elements arranged in order of increasing atomic number, from 1 to 118.
• The elements are arranged so that atoms with the same number of shells are placed together, and atoms with similar electronic configurations in the outer shell are also placed together. 
• • The elements are arranged in rows and columns.
• • Elements with one shell are placed in the first row (i.e. H and He)
• • A row of elements thus arranged is called a period. The period number, n, is the outer energy level occupied by electrons.
• The outer electrons are known as the valence electrons.
• Elements are aligned vertically (in columns) with other elements in different rows, if they share a similar outer-shell electronic configuration.
• • As atomic number increases, the properties of the elements show trends that repeat themselves in each period of the periodic table
  • These trends are known as periodic trends, and the study of these trends is known as periodicity
• • The s-block elements are all those with only s electrons in the outer shell.
• • The p-block elements are all those with at least one p-electron in the outer shell.
• • The d-block elements are all those with at least one d-electron and at least one s-electron but no f or p electrons in the outer shell (up to 5d).
• • The f-block elements are all those with at least one f-electron and at least one s-electron but no d or p electrons in the outer shell.
• Trends across a period

  • Each period starts with an electron in a new highest energy shell.
  • Across period 2, the 2s sub-shell fills with two electrons, followed by the 2p sub-shell with six electrons.
  • Across period 3,  the same pattern of filling is repeated for the 3s and 3p sub-shells,
  • For each period, the s and p sub-shells are filled in the same way - a periodic pattern.
• The first ionisation energy (IE1) is the energy required to remove one mole of electrons from one mole of atoms of an element in the gaseous state to form one mole of gaseous ions.
• • E.g. the first ionisation energy of Na is:

  Na (g) → Na+ (g) + e-          First ionisation energy = +496 kJ mol-1
• • Ionisation energies show periodicity.