Periodicity

• Define Atomic Radius

  Distance between the centre of the nucleus and the outer electron shell
• What is the trend in atomic radius across a period?

  Decreases
• Why does atomic radius decrease across a period?

  • Nuclear charge increases across a period
  • Similar shielding (outer electron is in the same energy level for each element)
  • So outer electron is more closely attracted to the nucleus
• Define Electronegativity

  The power of an atom to attract a shared pair of electrons in a covalent bond
• What is the trend in electronegativity across a period?

  Increases
• Why does electronegativity increase across a period?

  • Nuclear charge increases across the period
  • Similar shielding
  • So ability to attract a shared pair of electrons in a covalent bond increases
• Define first ionisation energy

  Energy needed to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of 1+ ions
• What is the general trend in first ionisation energy across a period?

  Increases
• Why does first ionisation energy generally increase across a period?

  • Nuclear charge increases
  • Similar shielding
  • So more energy needed to remove 1 mole of electrons from 1 mole of gaseous atoms
  • The outer electron is more strongly attracted to the nucleus
• What are the exceptions in the trend of first ionisation energy?Aluminium and Sulfur
• Why is aluminium exceptions in the trend of first ionisation energy?

  • Electrons moved from 3p not the 3s
  • 3p is higher in energy so less energy required to remove it
  • It has pair of electrons in p orbital so electrons repel so less energy is needed to remove one
• Define melting point

  The temperature at which a substance changes state from solid to liquid
• What is the trend in melting point across a period?

  • Sodium, Magnesium and Aluminium have high melting points
  • Silicon has a very high melting point
  • Phosphorus, sulphur, chlorine and argon have low melting points
• Why do Sodium, Magnesium and Aluminium have high melting points?

  • Giant lattice structures
  • Have metallic bonding
  • So strong electrostatic attraction between positive metal ions and negative delocalised electrons
  • High energy is needed to break the bonds
• Why does Silicon have a very high melting point?

  • It is macromolecular
  • So has lots of strong covalent bonds between atoms in a molecule
  • High energy is needed to break the bonds
• Why do Phosphorus, Sulphur, Chlorine and Argon have low melting points?

  • They are covalent bonds
  • They are molecular
  • So have weak van der Waals forces between molecules and don't require much energy to break the bonds
• Order of melting points between Phosphorous, Sulphur, Chlorine and Argon

  Highest

  • Sulphur (S8)
  • Phosphorus (P4)
  • Chlorine (Cl2)
  • Argon (Ar)

  Lowest
• Why does Ar have the lower melting point than sulphur, phosphorus and chlorine?

  • Monoatomic - single atom (small)
  • Very weak van der Waals forces so little energy to break the bonds
• Why does Sulphur have a higher melting point phosphorus, chlorine and argon?

  • Larger molecule (S8)
  • Van der Waals forces are stronger
  • Higher energy needed to break the bonds
• State and Explain the trend in melting point of the group II elements Ca-Ba (3 marks)

  • Decreases
  • Atomic radius increases
  • So weaker attraction to nucleus for delocalised electron
• By referring to structure and bonding explain why silicon has a high melting point (3 marks)

  • Silicon is macromolecular
  • Covalent bonds need to be broken
  • Strong covalent bonds need high energy to break
• State and explain the trend in atomic radius of the period 3 elements from sodium to chlorine (3 marks)

  • Deceases
  • Nuclear charge increases
  • Similar shielding
• Explain why the value of 1st ionisation energy of sulfur is less than the value of the first ionisation energy of phosphorus (2 marks)

  • Paired electrons in (3)p orbital
  • Repel each other
• Define Periodicity ?

  The repeating patterns of trends in physical and chemical properties in the periodic table