Electron Configurations
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Example A:
Write the electron configuration for silicon. Diagram its orbital notation (Aufbau orbital diagram).
How to Solve: Check the atomic number of silicon – it is number 14. The number of protons in a neutral atom
equals the number of electrons, so silicon has 14 electrons that must be distributed according to the three rules:
Aufbau Principle, Pauli’s exclusion Principle and Hund’s Rule. Use the periodic table to trace the pattern of
addition of electrons until there are 14 electrons total. The 1s orbital fills first, followed by the 2s, then 2p, the 3s
and 3p.
The configuration is :
1s22s22p63s23p2
The orbital notation is:
1s
2s
2p 2p 2p
3s
3p 3p 3p
Example B:
Write the electron configuration for silver.
How to Solve: Check the atomic number of silver – it is number 47 – so it must have 47 electrons in the neutral
atom. Silver is an exception to the Aufbau Principle. The electrons fill the first four energy levels as predicted:
1s22s22p63s23p64s23d104p6. There are now 11 electrons left. Instead of the expected 5s24d9 , one electron is
promoted from the 5s orbital to the 4d orbital, giving 5s14d10. The added stability of having a completely full 4d
orbitals more than makes up for the slight instability of the half-full 5s orbital.
The complete electron configuration of silver is:
1s22s22p63s23p64s23d104p65s14d10 .
Try the following problems:
1. Write the electron configuration and orbital notation for potassium.
2. Write the electron configuration for molybdenum.
3. Write the electron configuration and orbital notation for magnesium.
4. Write the electron configuration for antimony, Sb.
ELECTRON CONFIGURATION PRACTICE
1. Write the electron configuration for each of the following:
a. lithium
d. phosphorus
b. arsenic
e. neon
c. calcium
f. nitrogen
2. Write the electron configuration and orbital notation for the following.
a. carbon
d. sodium
b. silver
e. oxygen
c. scandium
f. iron
3. Write the electron configuration for the following ions:
a. sulfide ion
c. calcium ions
b. fluoride ions
d. aluminum ions