The Spin Quantum Number (\(m_s\)) describes the angular momentum of an electron. An electron spins around an axis and has both angular momentum and orbital angular momentum. Because angular momentum is a vector, the Spin Quantum Number (s) has both a magnitude (1/2) and direction (+ or -). Each orbital can only hold two electrons. One electron will have a +1/2 spin and the other will have a -1/2 spin. Electrons like to fill orbitals before they start to pair up. Therefore the first electron in an orbital will have a spin of +1/2. After all the orbitals are half filled, the electrons start to pair up. This second electron in the orbital will have a spin of -1/2. If there are two electrons in the same orbital, it will spin in opposite directions. Combinations of Quantum Numbers The three quantum numbers ( n , l , and m ) that describe an orbital are integers: 0, 1, 2, 3. The principal quantum number ( n ) cannot be zero. The allowed values of n are therefore 1, 2, 3, 4... The angular quantum number ( l ) can be any integer between 0 and n - 1. If n = 3, l can be either 0, 1, or 2. The magnetic quantum number ( m ) can be any integer between - l and + l . If l = 2, m can be -2, -1, 0, +1, or +2. Orbitals that have same value of principal quantum number form a Shell(n). Orbitals within the shells are divided into subshell (l) s: l = 0 p : l = 1 d : l = 2 f : l = 3 Exercise \(\PageIndex{1}\): Tungsten What is the spin quantum number for Tungsten (symbol W)? Answer Tungsten has 4 electrons in the 5d orbital. Therefore 1 electron will go into each orbital (no pairing). The 4th electron will have a +1/2 spin. Exercise \(\PageIndex{2}\): Gold What is the spin quantum number for Gold (symbol Au)? Answer Gold has 9 electrons in the 5d orbital. Therefore the electrons will start to pair up, which means the 9th electron will pair up, giving it a -1/2 spin. Exercise \(\PageIndex{3}\): Sulfur What is the spin quantum number for Sulfur (symbol S)? Answer Sulfur has 4 electrons in the 3p orbitals. The 4th electron in this orbital will be the first one to pair up with another electron, therefore giving it a -1/2 spin. References Housecroft, Catherine E., and Alan G. Sharpe. Inorganic Chemistry . 3rd ed. Harlow: Pearson Education, 2008. Print. (pg 15). Nostrand, Van. Encyclopedia of Chemistry. 5th ed. John Wiley and Sons, Inc., 2005. Print. (pg 1396).