AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/1/1-5)
Unit 1 · Topic 1.5
1.5 Atomic Structure and Electron Configuration
An atom has a small, positive nucleus surrounded by electrons arranged in shells and subshells. You write that arrangement as an electron configuration, and Coulomb's law explains why some electrons are held more tightly than others.
Key terms
- Coulomb's law
- electron configuration
- Aufbau principle
- shells and subshells
- core electrons
- valence electrons
Inside the atom
The nucleus holds protons (charge 1+) and neutrons (no charge). It contains almost all the atom's mass but takes up a tiny fraction of its volume. Electrons (charge 1−) surround it. A neutral atom has as many electrons as protons; an ion has gained or lost electrons.
Coulomb's law
Coulomb's law describes the force between two charged particles: F ∝ q₁q₂ / r². Here q₁ and q₂ are the charges and r is the distance between them.
Bigger charges mean a stronger force. A larger distance means a much weaker force, because r is squared: doubling the distance cuts the force to one fourth. Opposite charges attract; like charges repel.
For an electron in an atom, this means an electron is held more tightly when it's closer to the nucleus, or when the nucleus has more protons pulling on it. The energy needed to remove an electron, its ionization energy, follows the same logic.
Shells, subshells and the Aufbau principle
Electrons sit in shells (energy levels n = 1, 2, 3, …), and each shell has subshells labeled s, p, d and f. An s subshell holds up to 2 electrons, p holds 6, d holds 10 and f holds 14.
In the ground state (the lowest-energy arrangement), electrons fill subshells from lowest energy upward. That rule is the Aufbau principle. The filling order is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p. The periodic table shows this order: the s-block is groups 1–2, the p-block is groups 13–18 and the d-block is the transition metals.
Write each subshell with a superscript for its electrons: oxygen is 1s² 2s² 2p⁴. A shortcut uses the previous noble gas in brackets: iron is [Ar] 4s² 3d⁶.
Core and valence electrons
Valence electrons are in the outermost shell (the highest n); for main-group elements, they're the s and p electrons of that shell. Core electrons are all the others. Chlorine, 1s² 2s² 2p⁶ 3s² 3p⁵, has 7 valence electrons (3s² 3p⁵) and 10 core electrons.
Core electrons sit between the nucleus and the valence electrons and partly cancel the nucleus's pull. This is called shielding. The pull a valence electron actually feels is the effective nuclear charge, roughly the number of protons minus the number of core electrons.
Configurations of ions
For anions, add electrons into the next open spot: S is [Ne] 3s² 3p⁴, so S²⁻ is [Ne] 3s² 3p⁶. For main-group cations, remove electrons from the outermost shell: Na⁺ is 1s² 2s² 2p⁶.
Transition metal cations lose their 4s electrons before any 3d electrons, even though 4s filled first. Fe is [Ar] 4s² 3d⁶, Fe²⁺ is [Ar] 3d⁶, and Fe³⁺ is [Ar] 3d⁵.
You won't be tested on quantum numbers, or on the configurations of exceptions like Cr and Cu.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Configuration of a transition metal ion (classic trap)
Write the ground-state electron configurations of Fe and Fe³⁺.
Show the solutionHide the solution
- Step 1: Iron has 26 electrons. Fill in order: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ (2 + 2 + 6 + 2 + 6 + 2 + 6 = 26). In shorthand, [Ar] 4s² 3d⁶.
- Step 2: Fe³⁺ has lost 3 electrons. Remove the two 4s electrons first, because 4s is the outermost shell (n = 4), then one 3d electron.
- Step 3: The common wrong answer, [Ar] 4s² 3d³, removes from 3d first.
Answer: Fe: [Ar] 4s² 3d⁶; Fe³⁺: [Ar] 3d⁵
- Example 2
Using Coulomb's law to compare electrons
Which takes more energy to remove: a 2p electron from a neon atom or the 3s electron from a sodium atom? Explain using Coulomb's law.
Show the solutionHide the solution
- Step 1: Neon (1s² 2s² 2p⁶): its 2p electron is in shell 2. Shielding by 2 core electrons leaves an effective pull of about 10 − 2 = +8.
- Step 2: Sodium (1s² 2s² 2p⁶ 3s¹): its 3s electron is in shell 3, farther from the nucleus. Ten core electrons shield it, leaving about 11 − 10 = +1.
- Step 3: By Coulomb's law, the force grows with the charge and falls with distance. Neon's 2p electron is closer and feels a much larger effective charge.
Answer: Removing a 2p electron from Ne takes much more energy, because it is closer to the nucleus and feels a much larger effective nuclear charge than Na's 3s electron.
- Example 3
Coulomb's law with changing charge and distance
Two ions attract with force F. If the charge on each ion is doubled and the distance between them is doubled, what is the new force?
Show the solutionHide the solution
- Step 1: F ∝ q₁q₂ / r². Doubling both charges multiplies the top by 2 × 2 = 4.
- Step 2: Doubling r multiplies r² by 4.
- Step 3: New force = F × 4 ÷ 4 = F.
Answer: The force is unchanged: F
Common mistakes
- Removing 3d electrons before 4s electrons when forming transition metal cations. The 4s electrons go first.
- Counting all electrons as valence electrons. For main-group elements, only the outermost s and p electrons count.
- Forgetting that distance is squared in Coulomb's law. Doubling r cuts the force to one fourth, not one half.
- Explaining with 'atoms want a full octet'. Exam answers should use attraction between the nucleus and electrons, charge and distance.
On the exam
- Questions often ask you to write a configuration for an atom or ion, or to identify an element from its configuration. Count total electrons to check your work.
- When you explain why one electron is harder to remove than another, name the factor (number of protons, distance or shielding) and connect it to Coulomb's law. A vague 'it's more stable' earns nothing.
Connected topics
Videos
Check yourself
4 questions on 1.5 Atomic Structure and Electron Configuration. Pick an answer to see if you got it, and why.
Which of the following species has the ground-state electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶?
Removing a 1s electron from a lithium atom requires far more energy than removing its 2s electron. Which of the following best explains this observation?
Which of the following is the ground-state electron configuration of a selenium atom?
What is the ground-state electron configuration of the Fe³⁺ ion?
0 of 4 answered