The oxidation number of lithium is +1. The atomic number of Lithium is 3 and its electron configuration is 1S22S1. Hence, Lithium prefers to lose an electron during a chemical reaction and it forms the Lithium positive ion or Li+.
The oxidation number is the number of electrons that an atom of an element gains or loses during a chemical reaction. If it gains an electron it forms a negative ion such as the chloride ion Cl- or the bromide ion Br-. If it loses an electron it forms a positive ion such as the sodium ion Na+ or the potassium ion K+.
If an atom loses two electrons, its oxidation number is 2, such as the Barium ion Ba2+ or the calcium ion Ca2+. If an atom gains two electrons, it forms divalent negative ions such as O2- or S2- ion. Similarly, Aluminium has an oxidation number of 3.