Electron Configuration

Pauli’s Exclusion Principle No two electrons have the same quantum number. (opposite spin) Maximum electrons in any orbital is two () Pauli Exclusion Principle

The Pauli exclusion principle summarizes experimental observations that no two electrons in one atom can have the same four quantum numbers. That means that within one orbital, electrons must have opposite spin. It also means that one orbital can hold a maximum of two electrons (with opposite spin).

An s sublevel, with one orbital, can hold a maximum of 2 electrons. A p sublevel, with three orbitals, can hold a maximum of 6 electrons. A d sublevel, with five orbitals, can hold a maximum of 10 electrons. An f sublevel, with seven orbitals, can hold a maximum of 14 electrons.

The building-up principle (or aufbau principle) is a scheme used to reproduce the ground-state electron configurations by successively filling sublevels with electrons in a specific order (the building-up order). This order generally corresponds to filling the orbitals from lowest to highest energy. Note that these energies are the total energy of the atom rather than the energy of the sublevels alone.

Hund’s Rule RIGHT WRONG Hund’s Rule When filling degenerate orbital's, electrons will fill an empty orbital before pairing up with another electron. RIGHT WRONG

Electron Configuration Electron configuration for Neon Increasing Energy ______ ______ ______ ______ 2 p 2 s 1s2 2s2 2p6 Electron Spin 1 s Electron Configuration Orbital Diagram

Electron Configuration Aufbau diagram shows each orbital O (atomic number 8) ____ ____ ____ ____ ____ 2s 2p 1s electron configuration 1s2 2s2 2p4

____ ____ ____ ____ ____ ____ ____ ____ 3s 3p S (atomic number 16) Write the electron configuration for Sulfur by using: S (atomic number 16) ____ ____ ____ ____ ____ ____ ____ ____ 3s 3p ____ 2s 2p 1s 1s2 2s2 2p6 3s2 3p4 How many unpaired electrons does sulfur have? Orbital Diagram Electron configuration 2 unpaired electrons!

1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 7s 7p This results in the following order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p

Electron Configuration Let’s Practice P (atomic number 15) 1s2 2s2 2p6 3s2 3p3 Ca (atomic number 20) 1s2 2s2 2p6 3s2 3p6 4s2 As (atomic number 33) 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p3 W (atomic number 74) 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d4

Electron Configuration Your Turn N (atomic number 7) 1s2 2s2 2p3 Na (atomic number 11) 1s2 2s2 2p6 3s1 Sb (atomic number 51) 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p3 Cr (atomic number 24) 1s2 2s2 2p6 3s2 3p6 4s2 3d4

The lowest-energy configuration of an atom is called its ground state. Any other configuration represents an excited state.

Exceptions Expect: 1s2 2s2 2p6 3s2 3p6 4s2 3d4 Copper Expect: 1s2 2s2 2p6 3s2 3p6 4s2 3d9 Actual: 1s2 2s2 2p6 3s2 3p6 4s1 3d10 Silver Expect: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d9 Actual: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 4d10 Chromium Expect: 1s2 2s2 2p6 3s2 3p6 4s2 3d4 Actual: 1s2 2s2 2p6 3s2 3p6 4s1 3d5 Molybdenum Expect: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d4 Actual: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 4d5 Exceptions are explained, but not predicted! Atoms are more stable with half full sublevel

Another way to learn the building-up order is to correlate each sublevel with a position on the periodic table. The principal quantum number, n, correlates with the period number. Groups IA and IIA correspond to the s sublevel; Groups IIIA through VIIIA correspond to the p sublevel; the “B” groups correspond to the d sublevel; and the bottom two rows correspond to the f sublevel. This is shown on the next slide.

Energy level equals period number 1 2 3 4 5 6 7 Energy level equals period number minus one Energy level equals period number minus two

Electron Filling in Periodic Table s s p 1 2 d 3 4 5 6 * 7 W f * W

Periodic Table and Electron Configuration Using the periodic table for the filling order of orbitals, by going in atomic number sequence until you use all the needed electrons in the element