Topic 1.5 Notes – Atomic Structure and Electron Configuration

1. Atomic Structure and Coulombic Attraction

Every atom contains three subatomic particles:

• Protons
  • Charge = +1
  • Mass ≈ 1 amu
  • Located in the nucleus
• Neutrons
  • Charge = 0
  • Mass ≈ 1 amu
  • Located in the nucleus
• Electrons
  • Charge = -1
  • Mass ≈ 0 amu (very small)
  • Found outside the nucleus

The atomic number equals the number of protons. In a neutral atom, electrons = protons.

The key force holding electrons near the nucleus is electrostatic attraction. Opposite charges attract, and the strength of that attraction follows Coulomb’s law:

$$F \propto \frac{q_{1} q_{2}}{r^{2}}$$

• Larger charges → stronger attraction
• Smaller distance $r$ → much stronger attraction (because of $r^{2}$)

This explains two big ideas you’ll use all year:

• Electrons closer to the nucleus have lower energy.
• Electrons farther away are easier to remove.

When you justify ionization energy on a test, your reasoning almost always traces back to charge and distance.

2. Energy Levels, Sublevels, and Electron Organization

Electrons don’t sit randomly. They occupy specific energy levels.

Shells (Principal Energy Levels)

• Labeled $n = 1, 2, 3, 4...$
• As $n$ increases:
  • Distance from nucleus increases
  • Energy increases

The outermost shell is the valence shell.

Subshells (Sublevels)

Each shell contains subshells:

• s → max 2 electrons
• p → max 6
• d → max 10
• f → max 14

These subshells correspond directly to the blocks of the periodic table.

Periodic table divided into s, p, d, and f blocks

The left two columns are the s-block, the right six columns are the p-block, the middle transition metals are the d-block, and the bottom rows are the f-block. If you can read these blocks, you can write electron configurations without memorizing a long energy chart.

Core vs Valence Electrons

• Valence electrons = electrons in the highest $n$ level (for main-group elements, only s and p)
• Core electrons = inner electrons

Example:
Phosphorus → $1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{3}$
Valence electrons are in $n=3$: $3s^{2} 3p^{3}$ → 5 valence electrons.

3. The Rules for Writing Ground-State Electron Configurations

Three rules control how electrons fill orbitals.

Aufbau Principle

Electrons fill orbitals in order of increasing energy.

Order (follow periodic table layout):
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p

Notice 4s fills before 3d. That detail matters later for ions.

Pauli Exclusion Principle

• Max 2 electrons per orbital
• Must have opposite spins

No orbital can hold more than two electrons.

Hund’s Rule

In equal-energy orbitals (like the three p orbitals):

• Fill singly first
• Then pair

Orbital box diagrams illustrating Hund’s rule (N → Ne)

In the 2p orbitals for N, O, F, and Ne, notice how each box gets one electron before any pairing happens.

This reduces electron-electron repulsion.

Quantum numbers exist behind all this, but assigning them is not tested on the AP exam.

4. How to Write Electron Configurations

Example: Sulfur (atomic number 16)

1. 16 electrons total
2. Fill in order:

$1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{4}$

Check: 2 + 2 + 6 + 2 + 4 = 16 ✔

Noble Gas Shortcut

Use the previous noble gas in brackets.

Sulfur becomes:
$[Ne] 3s^{2} 3p^{4}$

This saves time and reduces mistakes.

Writing Ions

Cations: remove electrons from the highest $n$ level first.

Example:
Fe = $[Ar] 4s^{2} 3d^{6}$

Fe²⁺: remove 4s electrons first →
$[Ar] 3d^{6}$

That 4s-before-3d removal shows up constantly on tests.

Anions: add electrons following Aufbau order.

5. Electron Configuration and Ionization Energy

Ionization energy = energy required to remove an electron.

Think Coulomb’s law every time.

Ionization energy increases when:

• Nuclear charge increases (more protons)
• Atomic radius decreases (smaller $r$)
• Effective nuclear charge increases (less shielding)

Ionization energy decreases when:

• Electron is farther from nucleus
• More core electrons shield the attraction

When comparing two atoms, your explanation should include words like:

• “greater nuclear charge”
• “smaller radius”
• “stronger attraction between nucleus and valence electron”

If those ideas aren’t in your answer, you’re probably not scoring full credit.