Atomic configuration

1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p6 etc.

Atomic shells

• Currently 7 known shells– K, L, M, N, O, P, Q from inside to outside

• Each shell comprises of different kinds of orbitals– 4 known orbitals: sharp, principal, diffuse and fundamental

– There is a proposed g orbital for the 8th period

Shape of atomic orbitals

• No real shape, just a probability density• Possible to define a region in which the

electron can be found >90% of the time• Wavefunction given by:

• Square of wavefunction gives probability of electron at a certain point from the nucleus.

,22

!2

)!1(2 1213

3

lmlln

l

na

Zr

nlm Yna

ZrL

na

Zre

lnn

ln

na

Z

Electrons in orbitals• There are varying kinds of each orbital:• s – 1, p – 3, d – 5, f – 7• Each orbital can hold 2 electrons– Pauli’s exclusion principle states that no 2 particles

can occupy the same state at the same time– Only 2 possible values for spin– Hence only 2 electrons with opposite spins can

occupy the same orbital, else rule will be violated• Maximum number of electrons per type of

orbital:• s – 2, p – 6, d – 10, f – 14

Atomic Configuration• List out all the orbitals that the atom has as

follows:• E.g. Na: 1s 2s 2p 3s• Add the number of electrons in each orbital as a

superscript:• Na: 1s22s22p63s1

• Filled shells up to the previous period can be replaced by square brackets around a noble gas

• E.g. Na: [Ne]3s1

– Sodium’s electron configuration is simply Neon’s + 3s1

A slice of an atomA rough representation

Mg

1s2

2s2

2p2

2p4

2p6

3s2

1s22s22p63s2Low High

Aufbau rule

• Aufbau – German, meaning ‘construction’• Rule for filling in orbitals• List out all the orbitals and shells as follows,

then they are filled in in this order:Shell s p d f

1 / K 1s

2 / L 2s 2p

3 / M 3s 3p 3d

4 / N 4s 4p 4d 4f

5 / O 5s 5p 5d 5f

6 / P 6s 6p 6d 6f

7 / Q 7s 7p 7d 7f

Exceptions

• There a few exceptions to the Aufbau rule starting from the transition metals.

• Chromium (24Cr)– Should be [Ar]4s23d4

– Atomic configuration [Ar]4s13d5

• Copper (29Cu)– Should be [Ar]4s23d9

– Atomic configuration [Ar]4s13d10

• 20 known exceptions in total

The ExceptionsH He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba * Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra **

* La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

** Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lw

Exceptions are shown in red text.Elements whose atomic configuration have not been determined are not shown.

Why are there exceptions?

• Aufbau rule assumes that each orbital has different energy levels

• In reality, this may not be true• Hence electrons can be located in another

orbital with similar energy• Half-filled and completely filled orbitals may

also be more stable

Why are there exceptions?

• For the heavier elements, electrons are moving closer to the speed of light

• This causes the mass of the electron to increase and the orbital size to shrink

• Tends to decrease the energy level of the s orbital

• Explains anomalies like mercury, a liquid at room temperature, and the colour of gold

References• IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book").

Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook.

• Electron Configuration. (n.d.). Retrieved January 18, 2011, from http://en.wikipedia.org/wiki/Electron_configuration

• Norrby, L. J. (1991). Why Is Mercury Liquid? Accounts of Chemical Research, pp110-113.

The End

Any questions?