Redox

• Oxidation number is always 0 for elements

  In a pure element any bonding is with atoms of the same element so in $H_2, O_2, P_4, S_8, Na, Fe$ the oxidation number of each atom in the element is 0.
• Each atom in a compound has an oxidation number

  The oxidation number has a sign that is placed infront of the number.

  If the atom gains electrons i.e. is reduced then its oxidation number is negative.

  If it is oxidised then the oxidation number is positive.
• The sum of oxidation numbers in a molecule= the total charge
• Roman numerals are used in the names of compounds of elements that form ions with different charges.

  The roman numeral shows the oxidation state of the element without a sign

  The sign of the oxidation number can be figured out from the overall charge
• Special cases of oxidation numbers:

  • Hydrogen in metal hydrides has oxidation number of -1 e.g. in NaH and CaH_2
  • Oxygen in peroxides = -1 e.g. in h_2O_2
  • Oxygen when bonded to fluorine = +2 F_2O
• Reduction is a decrease in oxidation number and oxidation is an increase in oxidation number is oxidation
• Redox reactions involve reduction and oxidation. If something is reduced, something else must be oxidised.
• Dilute acids undergo redox reactions with some metals to produce salts and hydrogen gas