The following is a table of the flame colours observed in the group 1 and 2 elements: The reaction with oxygen to form a metal oxide occurs spontaneously in air and when the metals are heated by flame, showing a distinct colour.The heavier, less dense and less polarizing ions like Sr 2 and Ba 2 cannot do this, so the metal peroxide is stable. The peroxide ion becomes a stable O 2- oxide ion and the weak O-O covalent bond breaks. These are small with a very high charge density which draws the 2- negative charge in the peroxide ion towards it with great strength (they polarize the ion). ![]() It does not form in this reaction with the smaller group 2 metals such as Be or Mg because they form highly polarizing ions. The peroxide ion ( -O-O -) has two oxygen atoms, each with a weak covalent bond to the other, and a 1- charge (O 2 2- overall). In the reaction with heavier group two metals, a metal peroxide can form, such as with barium and strontium: This can be written generally as:Īgain, the stoichiometry is slightly different in the two reactions. ![]()
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