O Level Chemistry: Tips on assigning Oxidation Number (or States)

Halo students,

Do you find assigning Oxidation Number (or Oxidation States) to elements and then getting the correct chemical formula of compounds very challenging, and often lost marks on it. Fret Not! In fact, assigning Oxidation Number to the elements and obtaining the subsequent chemical formula of the compounds is easy if you observe the following rules:

  • The oxidation numbers of an atom or the atoms in a neutral molecule must add up to zero.
  • If an atom or molecule is ionic its oxidation number must add up to its overall charge.
  • Alkali metal atoms (Group I) have an oxidation number equal to +1 within compounds.
  • Alkali earth atoms (Group II) have an oxidation number of +2 within compounds.
  • Halogens have -1 oxidation number within compounds.
  • Hydrogen is always assigned a +1 oxidation number in compounds, except in metal hydrides (e.g. LiH) whereby the Li has a + 1 oxidation number while H adopts a -1 oxidation number.
  • Oxygen is assigned an oxidation number of -2 in compounds except when It is found in Hydrogen Peroxide whereby each 0 atom has a -1 oxidation state.
  • The sum of oxidation states of all the atoms in a molecule is equal to zero.
  • The sum of oxidation states of all the atoms in a radical is equal to the charge on the ion.

Examples To Note:

N2O      +1 state for N, -2 state for O

LiH         +1 state for Li, -1 state for H

H2O2 +1 state for H, -1 state for O

O2 0 state for O

SO42- +6 state for S, -2 state for O

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Many people talked about teaching Chemistry, I simply LOVE it. I am a passionate Chemistry Coach based in Singapore, Southeast-Asia and aspire to be one of the most dynamic, powerful and humorous speakers in Asia. My 16+ years of coaching experience has equipped me to understand the true reasons why students are not able to perform well in Chemistry, and allow me to structure my teaching methodology to cater to different levels of learners. If you have found this post useful, please share it with your friends. I would really appreciate it! Sean Chua

2 Responses to O Level Chemistry: Tips on assigning Oxidation Number (or States)

  1. Sean, I have to say that this was the best interactive presentation that I have seen so far. I was very confused about this Balancing Chemical equations and now I am able to finish the prob not only efficiently but in timely manner. I want to thank you for such support.

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    thanks once again
    Mitu Singh

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