Chemistry · Redox Equilibrium · NEET
Carbon belongs to group 14 and has 4 valence electrons (2s2 2p2). Oxidation state measures how many electrons a carbon has gained or lost compared to a neutral atom. It can gain at most 4 electrons (giving -4) or lose at most 4 electrons (giving +4). It has no more than 4 to lose and no room to take more than 4 into its octet, so +4 and -4 are the hard limits.
In CH4 carbon is more electronegative than hydrogen, so carbon is assigned both bonding electrons in each C-H bond. Set up: C + 4(+1) = 0, so C = -4. This is the most reduced form of carbon (fully gained electrons).
In CO2: C + 2(-2) = 0, so C = +4. In CCl4: C + 4(-1) = 0, so C = +4. In both, carbon is bonded to more electronegative atoms (O or Cl) and loses its share of electrons, giving the maximum +4. These are the most oxidised forms of carbon.
Yes. Carbon takes intermediate values in many compounds. In CH3Cl carbon is -2, in CH2Cl2 it is 0, in CHCl3 it is +2, and in HCHO (formaldehyde) it is 0. In organic molecules an average (fractional) oxidation number is common because different carbons in the molecule differ. But every value still stays between -4 and +4.
No. There is no stable compound where carbon exceeds +4 or drops below -4, because carbon only has 4 valence electrons to trade. This is why in the reaction CH4 to CCl4, carbon swings from its lowest value -4 all the way to its highest value +4, a full 8-unit change.
What is the change in oxidation number of carbon in the following reaction? CH4(g) + 4Cl2(g) -> CCl4(l) + 4HCl(g)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Highest is +4 (as in CO2, CCl4, CF4) and lowest is -4 (as in CH4). This comes directly from carbon having 4 valence electrons.
Oxidation number is decided by electronegativity. Hydrogen is less electronegative than carbon, so carbon pulls the electrons and becomes -4. Oxygen is more electronegative than carbon, so carbon loses the electrons and becomes +4.
For a single carbon it is a whole number, but in organic molecules with several carbons the average can be a fraction, for example carbon is -8/3 in propane (C3H8). The value still stays within -4 to +4.
It changes by 8 units, from -4 to +4. This is the largest possible oxidation-state swing for carbon.
Carbon has only 4 valence electrons. It cannot lose more than 4, so it cannot go above +4, and it cannot gain more than 4 into its octet, so it cannot go below -4.
