Chapter 8: Electron configurations and

periodicity

Chemistry 1061: Principles of Chemistry I

Andy Aspaas, Instructor

Electron spin and configurations

• Electrons may have only one of two possible spins– Quantum number ms = +1/2 or -1/2– Similar to poles of a magnet

• Electron configuration: distribution of electrons among available subshells

– Notation lists one subshell after another, with number of electrons in each given in superscript

– Ex. 1s22s1 means 2 electrons in 1s and 1 electron in 2s• Orbital diagram: drawing of orbitals and their occupations

– Electrons indicated by arrows, either up or down indicates spin

Orbital occupancy

• Pauli exclusion principle: no 2 electrons in the same atom may have the same 4 quantum numbers

– Each orbital may contain two electrons, but they must be opposite spin - same-spin pair in an orbital is forbidden

• Each subshell can hold twice as many electrons as orbitals in the subshell

Subshell occupancies

Subshell Number or orbitals

Maximum number of electrons

s (l = 0) 1 2

p (l = 1) 3 6

d (l = 2) 5 10

f (l = 3) 7 14

Filling orbitals

• Every atom has an infinite number of electron configrations, but the ground state (stablest) electron configuration can be predicted

• Aufbau principle: electron configurations can be predicted by successively filling subshells with electrons in a specific order

• # electrons in a neutral atom = atomic number

Using the periodic table to fill orbitals

• Use the row number to determine n for s and p sublevels

• Section of the periodic table corresponds with the subshell that’s added

– First two groups: s– Last 6 groups: p– Transition metals: d– Inner transition metals: f

Available subshells

• 1s

• 2s, 2p

• 3s, 3p, 3d

• 4s, 4p, 4d, 4f

• 5s, 5p, 5d, 5f

• 6s, 6p, 6d, 6f

• 7s, 7p, 7d, 7f

• These subshells are allowed based on allowed quantum numbers

• But, they are filled based on the order they appear in the periodic table

• 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 etc

Writing electron configurations

• Just follow the periodic table from the start, adding electrons in the appropriate sublevel

• Electron configurations can be abbreviated using a noble-gas core (in brackets)

– Ex. Ca = 1s22s22p63s23p64s2 or just [Ar]4s2

Electron addition to subshells

• Hund’s rule: lowest energy arrangement of electrons in a subshell is obtained by first putting in as many same-spin electrons as possible before pairs are made

• Oxygen: 1s22s22p4 therefore has 2 unpaired electrons in the 2p subshell

– Any atom with unpaired electrons is paramagnetic (attracted to a magnetic field due to unpaired electrons)