Electron configurations and orbitals
Place electrons into subshells and identify the outermost occupied level.
What you will learn
- Read subshell notation.
- Write simple ground-state configurations.
Shells, subshells and orbitals
A shell is labelled by principal quantum number. A subshell has a type such as s or p and contains orbitals. Each orbital can hold up to two electrons with opposite spins. An s subshell contains one orbital, whereas a p subshell contains three.
The superscript in 2p⁶ gives the number of electrons in that subshell. It is not the number of orbitals or a charge.
Build and check a configuration
For the light atoms used here, fill 1s, then 2s, then 2p, then 3s and 3p. Add the superscripts to check that the electron count matches the neutral atom's atomic number. For ions, adjust the electron total using the charge.
Identify the highest occupied principal shell before describing the outer electrons. Do not confuse the last written subshell with a general rule that would work for every transition-metal configuration.
Worked example
Write the ground-state electron configuration of magnesium, atomic number 12.
Show the worked solution
- Place 2 electrons in 1s and 2 in 2s.
- Place 6 in 2p; the running total is 10.
- The remaining two occupy 3s.
Answer 1s² 2s² 2p⁶ 3s²
Common mistakes
- A p subshell holds six electrons, not two.
- An ion's electron count differs from that of its neutral atom.
What can you explain now?
Which subshell contains the outermost electron of sodium, atomic number 11?
Compare with the explanation
3s
Ten electrons fill 1s²2s²2p⁶; the eleventh is 3s¹.
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