Periodicity

• Periodic table


  A systematic pattern for the classification of chemical elements

  
• Historical development of the periodic table

  1. Lavoisier classified elements into metals and non-metals

  2. John Dalton, Jöns Jacob Berzelius, Dobereiner, John A. Newlands, Lothar Meyer worked on systematic classification

  3. In 1869, Dmitri Mendeleev produced the first periodic table

  
• Mendeleev's periodic law


  When elements are arranged in order of their relative atomic mass, similar properties recur at regular intervals

  
• Mendeleev's periodic table had many gaps, he predicted that there were undiscovered elements which could fill the gaps
• Later elements such as scandium, gallium and germanium were discovered, which not only fitted exactly into Mendeleev's table but also had the properties he predicted
• Modern periodic law


  The properties of the elements are a periodic function of their atomic number
• Periodicity


  The repetition of properties of elements after a certain interval when they are arranged in increasing order of their atomic number
• Law of periodicity


  The physical and chemical properties of the elements recur in a systematic and predictable way when the elements are arranged in order of increasing atomic number
• Modern periodic table


  • Divided into eight vertical columns (groups) and seven horizontal rows (periods)
  • Groups 1-8(0) constitute the chemical families
  • Elements in the same group have the same number of valence electrons
  • Elements between alkaline earth metals and boron family are transition elements
  • Elements in groups 3-7 are representative elements
  • Elements in periods 6 and 7 are lanthanides and actinides (inner transition elements)

  
• Groups


  Vertical columns in the periodic table

  
• Group names


  • Group 1 - Alkali metals
  • Group 2 - Alkaline earth metals
  • Group 3 - Boron family
  • Group 4 - Carbon family
  • Group 5 - Nitrogen family
  • Group 6 - Oxygen family
  • Group 7 - Halogens
  • Group 0 - Rare gases or inert gases

  
• Periods


  Horizontal rows in the periodic table
• Elements in the same period have the same number of electron shells

  
• The number of valence electrons of the elements in the same period increases progressively by one across the period from left to right
• Period 1 contains only two elements: Hydrogen and Helium
• Periods 6 and 7 contain the lanthanide and actinide series (inner transition elements)
• Metals


  Groups 1-3, donate their valence electrons
• Non-metals


  Groups 4-8, accept or share electrons
• Group 0


  Noble gases, their outer electrons are filled up
• Metalloids


  Elements along the boundary line between metals and non-metals
• S-block

  Groups 1 and 2, alkali and alkaline earth metals
• P-block

  Groups 3 to 7 and 0
• D-block

  Transition elements between groups 2 and 3
• 1. block


  Lanthanides and actinides, inner transition elements
• Electronic configuration of first 20 elements


  • 1H - 1s1
  • 2He - 1s2
  • 3Li - 1s2 2s1
  • 4Be - 1s2 2s2
  • 5B - 1s2 2s2 2p1
  • 6C - 1s2 2s2 2p2
  • 7N - 1s2 2s2 2p3
  • 8O - 1s2 2s2 2p4
  • 9F - 1s2 2s2 2p5
  • 10Ne - 1s2 2s2 2p6
  • 11Na - 1s2 2s2 2p6 3s1
  • 12Mg - 1s2 2s2 2p6 3s2
  • 13Al - 1s2 2s2 2p6 3s2 3p1
  • 14Si - 1s2 2s2 2p6 3s2 3p2
  • 15P - 1s2 2s2 2p6 3s2 3p3
  • 16S - 1s2 2s2 2p6 3s2 3p4
  • 17Cl - 1s2 2s2 2p6 3s2 3p5
  • 18Ar - 1s2 2s2 2p6 3s2 3p6
  • 19K - 1s2 2s2 2p6 3s2 3p6 4s1
  • 20Ca - 1s2 2s2 2p6 3s2 3p6 4s2
• Periodicity


  The variation of the properties of elements in a regular pattern both down the group and across the period
• Periodic properties


  • Melting and boiling point
  • Electrical and thermal conductivity
  • Atomic and ionic size [atomic and ionic radius]
  • Atomic volume
  • Ionization energy
  • Electron affinity
  • Electronegativity
  • Electropositivity

  
• Melting and boiling point


  • Increase across the period for groups 1, 2, and 3 due to strong metallic bonds
  • Very high for group 4 due to covalent bonds forming a giant crystalline lattice
  • Low for groups 5, 6, 7 due to simple covalent molecules held by weak intermolecular forces

  
• Across the period


  Melting and boiling points of metallic elements increase, while those of non-metals decrease

  
• Down the group


  Melting and boiling points of metallic elements decrease, while those of non-metallic elements increase

  
• Electrical and thermal conductivities

  • Decrease across the period as metallic character decreases and non-metallic character increases
  • Increase down the group as more electron shells are occupied
• Atomic radius


  Distance between the centre of the nucleus and the outermost electronic shell

  
• Covalent radius


  Half the distance between two identical atoms which are covalently bonded
• Vanderwaals radius


  Half the distance of two identical atoms which are not chemically bonded
• Across the period


  Atomic radius decreases due to increasing attraction of electrons by the nucleus
• Down the group


  Atomic radius increases due to the progressive increase in the number of shells occupied by the electrons
• Values of atomic radii of the first 20 elements
• Ionic radius


  Measure of the space occupied by an ion in the crystal lattice
• Ions formed through electron loss

  Smaller than the parent atom
• Ions formed through electron gain

  Larger than the parent atom