Oxidation Numbers and Molecular Composition

Oxidation Numbers and Molecules

Definition of Molecules and Compounds

  • When two or more elements combine, the resulting substance is called a molecule or a compound.

    • Example: HCl (Hydrogen and Chlorine).

Oxidation Numbers

  • All molecules need to have an overall oxidation number of zero.

HCl Example

  • HCl consists of Hydrogen (H) and Chlorine (Cl).

  • To achieve an oxidation number of zero, H and Cl must have oxidation numbers that balance each other out.

  • Hydrogen (H) comes first and typically has a positive oxidation number (e.g., +1).

  • Chlorine (Cl) comes second and typically has a negative oxidation number (e.g., -1).

MgCl₂ Example

  • In MgCl₂ (Magnesium Chloride), Magnesium (Mg) has an oxidation number of +2.

  • Chlorine (Cl) has an oxidation number of -1.

  • Since there are two chlorine atoms (Cl₂), the total negative oxidation is -2.

  • Therefore, +2(Mg)+(−1×2)(Cl2)=0+2 (Mg) + (-1 \times 2) (Cl₂) = 0, resulting in a net oxidation number of zero for the molecule.

Balancing Equations

  • If an equation is unbalanced, coefficients are added to balance it.

  • For instance, if Mg has +2 oxidation and Cl has -1, you need two Cl atoms to balance Mg's charge.

  • Thus, the formula becomes MgCl₂ to ensure the overall oxidation number is zero.