Extracting electron structure from the quantum numbers All atoms have the same set of atomic orbitals The lowest energy orbitals are filled first

Learning objectives Describe the rules for orbital filling: aufbau principle, Pauli principle and Hund’s rule Use the rules to predict electron configurations for elements Write complete and condensed electron configurations

How many electrons can be added to the subshells? 1s, 2s, 3s etc. 2 electrons 2p, 3p, 4p etc. 6 electrons 3d, 4d etc. 10 electrons 4f, 5f etc. 14 electrons

Order of filling: the aufbau principle Lower energy orbitals are filled first in order of (n + l) –Where orbitals have same (n + l), lowest n orbital fills first (2p before 3s etc. But 4s before 3d because of (n + l)) Only two electrons per orbital (Pauli principle) Hund’s rule: –If two or more degenerate (same energy) orbitals are available, one electron goes in each till they are all half full

Add electrons to the orbitals – lowest first 2p 3d 3p 4p4s 3s 2s 1s H (z = 1)

2p 3d 3p 4p4s 3s 2s 1s He (z = 2) Fill lowest orbital

2p 3d 3p 4p4s 3s 2s 1s Li (z = 3) Begin next orbital

2p 3d 3p 4p4s 3s 2s 1s Be (z = 4) Fill 2s

2p 3d 3p 4p4s 3s 2s 1s B (z = 5) Begin filling 2p

2p 3d 3p 4p4s 3s 2s 1s C (z = 6) Electrons don’t like to pair

2p 3d 3p 4p4s 3s 2s 1s N (z = 7) Filling 2p orbital according to Hund’s rule

2p 3d 3p 4p4s 3s 2s 1s O (z = 8)

2p 3d 3p 4p4s 3s 2s 1s F (z = 9) Almost filled shell – group 7

2p 3d 3p 4p4s 3s 2s 1s Ne (z = 10) Filled shell – noble gas

2p 3d 3p 4p4s 3s 2s 1s Na (z = 11) Start new shell – group I

2p 3d 3p 4p4s 3s 2s 1s K (z = 19) Aufbau principle: next electron enters 4s not 3d

2p 3d 3p 4p4s 3s 2s 1s Sc (z = 21) Next electron enters 3d not 4p

The aufbau shortcut: follow the red arrows Write the orbitals in columns as shown Draw red diagonal arrows Follow the arrows as you fill the orbitals

Shape of the periodic table explained by orbital picture 2 groups 10 groups 14 groups 6 groups

Electron configurations The complete electron configuration of the atom shows the occupancy and order of the filled orbitals –H – 1s 1 ; He – 1s 2 ; Li – 1s 2 2s 1 ; O – 1s 2 2s 2 2p 4 –K – 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 ; –Ga - 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 1

Configuration shorthand Heavier elements become cumbersome to write the complete configuration Abbreviate the filled shells to the noble gas symbol immediately below the element in question. The partially filled shells are shown completely –Al – [Ne]3s 2 3p 1 –K – [Ar]4s 1 –Rb – [Kr]5s 1 –Zn – [Ar]4s 2 3d 10

Determining electron configurations Exceptions! Not all elements obey the rules exactly –Half-filled shells –Heavier elements orbitals lose clear differentiation in energy –Minimizing electron – electron repulsion Webelements.com provides a guide