Periodicity

• in the periodic table, elements are arranged in order of

  increasing atomic number
• What is a period?

  a horizontal row of elements in the periodic table
• What is a group?

  a vertical column. Elements have similar chemical properties because they have the same number of outer shell electrons
• Periodicity is?


  The repeating pattern of properties of elements in the periodic table.
• Trends across period 3

  First ie increases across the period with a dip between group 2 and 3 and between group 5 and 6
• Why does first ie increase across period 3?

  Across a period, the e- being removed is in the same shell with similar shielding. There are more protons in the nucleus so the nuclear attraction to the outer e- increases and more energy is needed to remove it
• Why does atomic radius decrease across a period?


  There are same number of shells but more protons in nucleus so nuclear attraction is greater and e- are held more tightly
• Ionic radius across then period

  Positive ions are smaller than their atoms because same number of protons attracting fewer e- so held more tightly

  Negative ions are larger than their atoms as same number of protons attracting more e- so held less tightly
• Electronegativity across a period

  Increases across a period. More protons in nucleus, same number of shells so similar shielding therefore the pair of e- in the covalent bond are more strongly attracted.
• Melting and boiling point across a period

  Period 3 (Na to Ar). There are variations in boiling and melting point due to changes in structure and bonding. From Na to Al, the structure is giant metallic and metallic bonding, so melting point increases as the charge on the ion increases and the number of delocalised e- increases. Therefore the metallic bond is stronger and so requires more energy to break.

  Si: giant covalent, covalent bonds, very strong, high melting and boiling point

  P4,S8,Cl2: simple molecular, weak VDW S8 highest as largest more e-

  Ar: simple atomic, weak vdw