Periodicity

• Orbitals

  • S-orbitals are spherical, p-orbitals are dumbbell-shaped, and there are five d-orbitals
• Classification

  An element is classified as s, p, d or f block according to its position in the Periodic Table, which is determined by its proton number
• Properties of elements in the Periodic Table

  • Atomic radius
  • First ionisation energy
  • Melting point
• First ionisation energy

  The enthalpy change for the removal of one mole of electrons from one mole of atoms of an element in gaseous state
• Trends in the elements

  • Atomic radius decreases as you move from left to right across a period
  • First ionisation energy increases as you move from left to right across a period
  • Melting point increases as you move from left to right across a period
• Reasons for trends

  • Atomic radius decreases as the number of protons in the nucleus increases
  • First ionisation energy increases as the number of protons in the nucleus increases
  • Melting point increases as the number of delocalised electrons increases
• Atomic radius

  Distance from the nucleus to the outer electron
• Aufbau principle

  States that the lowest energy sub-levels are occupied first
• Pauli exclusion principle

  States that each orbitals may contain no more than two electrons
• Hund's rule

  States that single electrons occupy all empty orbitals within a sub-level before they start to form pairs in orbitals
• Periodicity

  Describes a repeating pattern in properties of elements across periods of the periodic table
• Atomic radius in period 3

  • Atomic radius decreases across periods 3 as proton number increases across the period but shielding remains constant
  • More protons increases nuclear charge leading to a greater attraction between the nucleus and the outer electrons
  • This pulls the electrons closer to the nucleus and results in a smaller atomic radius
• First ionisation energies down a group

  Going down a group, ionisation energy decreases due to increasing principle energy levels (number of shells) meaning less energy is required to remove the electron due to more shielding
• 1st ionisation of period 3 elements pt1

  • From Na to Ar the first ionisation energy increases since even though they have the same amount of shielding, they also have an increase number of protons which means a stronger electrostatic attraction between the outer electrons and the nucleus so more energy is needed to remove an outer electron
• 1st ionisation energy of period 3 elements pt2

  • However, for sulfur it has a lower ionisation energy then phosphorus because it has two electrons in one of its 3p orbitals. So those electrons would repel from each other which makes it easier for one of them to be removed so less energy is needed to remove it
  • For aluminium its ionisation energy is also lower than Mg because there's only one electron in the 3p orbital so it's more easier to remove
• Electron orbitals

  S-orbitals are spherical

  
• Electron orbitals pt2

  p-orbitals are dumbbell shaped

  
• Electron orbitals pt3

  d orbitals are clovers

  
• 1st ionisation energy of period 3 pt3

  • Also Al has a lower ionisation energy then Mg because it has a single outer electron in the 3p orbital which makes it easier for it to be removed so less energy is needed to remove the electron