3.1.1 Periodicity

• The periodic table has evolved over time. Back in the early 1800s they ordered the elements according to atomic mass
• In 1869 - Mendeleev created a periodic table similar to the one we have today.
• Mendeleev took Newland's work and adapted it. He still ordered the elements by atomic radius
• Mendeleev left gaps where elements didn't fit Newland's theory; this means elements were now grouped in terms of similar chemical properties.
• Mendeleev left gaps as a prediction of the properties of undiscovered elements.
• Modern periodic table; the elements are ordered by atomic number(proton number) not mass number
• groups are columns, periods are rows
• elements in the same group have the same number of electrons in the outer shell.
• Group number states the number of electrons in the outer shell
• Elements in the same group have similar properties
• Elements in the same period have the same number of electron shells
• Ionisation energy is the minimum amount of energy required to remove one mole of electrons from 1 mole of atoms in the gaseous state.
• Equation for ionisation energy:

  Na(g) ---> Na+(g) + e- 1st IE energy =+495.8 kJ mol-1
• Ionisation requires energy so they are always an endothermic processes and have a positive value
• Shielding:

  • The more electron shells between the positive nucleus and negative electron that is being removed
  • The means less energy required
  • There is a weaker attraction
• Atomic size:

  • The bigger the atom size
  • The further away the outer electrons are from the nucleus.
  • The attractive force between the nucleus and outer electrons reduces.
  • so it's easier to remove electrons
• Nuclear charge:

  • The more protons in the nucleus
  • The bigger the attraction between nucleus and outer electrons
  • This means more energy required to remove the electron
• Ionisation energy decreases as we go down a group:

  • The atomic radius increases as we go down the group
  • Outer electrons further from the nucleus.
  • Attractive force is weaker
  • Energy required to remove an electron decreases
• • Shielding increases as we go down the group
  • More shells between nucleus and outer shell.
  • Attractive force is weaker
  • Energy required to remove an electron decreases