The Periodic Table (Cambridge O Level Chemistry)

• There are over 100 chemical elements which have been isolated and identified
  • Each element has one proton more than the element preceding it
  • This is done so that elements end up in columns with other elements which have similar properties
  • Elements are arranged on the periodic table in order of increasing atomic number
  • The table is arranged in vertical columns called groups and in rows called periods

• Period: These are the horizontal rows that show the number of shells of electrons an atom has and are numbered from 1 - 7
  • E.g. elements in period 2 have two electron shells, elements in period 3 have three electron shells

• Group: These are the vertical columns that show how many outer electrons (also known as valency electrons) each atom has and are numbered from I – VII, with a final group called Group 0 (instead of Group VIII)
  • E.g. Group IV elements have atoms with 4 electrons in the outermost shell, Group VI elements have atoms with 6 electrons in the outermost shell and so on

• The group number can help determine the charge that metal and non-metal ions form
• For metals, the group number corresponds to the number of electrons it will lose to achieve a full outer shell and the charge of the metal ion
  • E.g. sodium is in Group I, it will lose 1 electron and form an ion with a 1+ charge
  • Magnesium is in Group II, it will lose 2 electrons and form an ion with a 2+ charge

• For non-metals in Group VII and VI, they will gain 1 and 2 electrons respectively to gain a full outer shell 
  • E.g. non-metals in Group VII gain 1 electron to form ions with a 1- charge
  • Non-metals in Group VI gain 2 electrons to form ions with a 2- charge

All elements are arranged in the order of increasing atomic number from left to right

Valency

• Valency (or combining power) tells you how many bonds an atom can make with another atom or how many electrons its atoms lose, gain or share, to form a compound
  • E.g. carbon has a valancy of 4 as it is in Group IV so a single carbon atom can share 4 electrons to make 4 single bonds or 2 double bonds
• The following valencies apply to elements in each group: