There are two methods to determine oxidation numbers of atoms in molecules or ions.
Using lewis structure
Using algebraic equation
Find oxidation numbers of atoms of carbon tetrachloride molecule using lewis structure
First, lewis structure of carbon tetrachloride molecule molecule should be drawn.
Mark direction of electrons attracting of bonds due to electronegativity difference
Atoms which have higher electronegativity values, can attract electrons of
bonds towards them.
Electronegativity values of carbon and Chlorine - Pauling Scale
Now, we can look how electrons are attracted between carbon and chlorine atoms.
Among carbon and chlorine atoms
Chlorine is the most electrogetive element. Due to higher electronegativity, electrons of C-Cl bonds are attracted
towards chlorine atoms as following image.
Oxidation number of chlorine atoms: According to the lewis structure of CCl4, electrons of the C-Cl bond tend to attract towards
chlorine atom. Therefore, carbon loses its electron
partially and chlorine partially receives one more excess electron. Due to that excess electron (partially received), chlorine's
oxidation number is -1. So, each chlorine atom's oxidation number is -1.
Oxidation number of carbon atom: Because carbon has more bonds around it, oxidation number should be calculated by taking the summation of
individual oxidation numbers caused due to the bonds.
Due to C-Cl bonds: Because carbon partially loses an electron due to a C-Cl bond, carbon receives +1 oxidation state.
Because, there are four C-Cl bonds around carbon atom, add individual oxidation numbers to find the final oxidation state.
Oxidation state of carbon atom = (+1) + (+1) + (+1) + (+1) = +4
Find oxidation numbers of atoms of CCl4 using algebraic equation
This method use an mathematical algebraic equation to find the oxidation number of an atom. In this method, you can find the unknown
oxidation number of an atom by using known oxidation numbers of other atoms in the molecule or ion.
This method is not 100% accurate for all cases, but can be used in lot of applications.
Oxidation numbers of chlorine
Chlorine has multiple oxidation numbers such as -1, 0, +1, +3, +5, +7. Because, chlorine is more electronegative than carbon, chlorine's
oxidation number should be a negative one. So, chlorine;s only negative oxidation number is -1 and we can substitute that value in our
following algebraic equation.
Summation of individual oxidation numbers of each atom = overall charge of molecule or ion
For CCl4 molecule, overall charge is zero and we can write following algebraic equation.
Oxidation number of carbon atom + oxidation number of chlorine atoms = overall charge of CCl4 molecule
Because oxidation number of carbon atom is taken as unknown, take it as x.
Apply known values for the equation.
x + (-1)*4 = 0
x = +4
We have found that, oxidation number of carbon in CCl4 molecule is +4.