Atoms and Electron Configuration

Learning Objective 1.5.A Represent the electron configuration of an element or ions of an element using the Aufbau principle.

Quick Notes

Atoms are made of a positively charged nucleus (protons and neutrons) and negatively charged electrons.
Electrons occupy energy levels (shells) and sublevels (subshells) according to the Aufbau principle.
Aufbau principle: electrons fill the lowest‑energy orbitals first.
Electron configurations show how electrons are arranged in an atom or ion.
Core electrons = inner shells; valence electrons = outermost electrons.
Coulomb’s law (qualitative): F ∝ (q₁ × q₂) / r² — stronger with greater charge; weaker with greater distance.
Ionization energy depends on distance from the nucleus and effective nuclear charge (shielding).

Full Notes

Structure of the Atom

Atoms consist of a small, dense, positively charged nucleus containing protons and neutrons, surrounded by negatively charged electrons.

AP Chemistry diagram of a helium atom: nucleus with protons and neutrons, electrons shown around the nucleus.

Protons and neutrons are in the nucleus. Electrons are distributed around the nucleus in regions called orbitals, which are grouped into shells and subshells.

Coulomb’s Law and Atomic Structure

Coulomb’s Law describes the electrostatic force between two charged particles:

AP Chemistry banner of Coulomb’s law showing F is proportional to (q1 × q2) divided by r squared, with definitions of variables.

F ∝ (q₁ × q₂) / r²

F = force of attraction or repulsion
q₁ and q₂ = charges on the particles
r = distance between them

This law helps explain trends such as ionization energy — the energy required to remove an electron from an atom. Smaller r (electron closer to nucleus) → stronger attraction; greater nuclear charge → stronger attraction; more shielding by inner electrons → weaker attraction on valence electrons.

Shells, Sub‑shells, Orbitals and the Principal Quantum Number (n)

AP Chemistry diagram showing shells (n = 1, 2, 3), subshells s, p, d, and their orbitals.

Electrons in an atom are arranged in energy levels (shells) labelled by the principal quantum number n (n = 1, 2, 3…). Each shell contains subshells (s, p, d; and f from n = 4). Each subshell consists of orbitals — regions of space with high probability of finding an electron — and each orbital holds a maximum of 2 electrons with opposite spins.

Orbitals in Sub‑shells and Their Electron Capacity

s subshell → 1 orbital → holds 2 electrons
p subshell → 3 orbitals → holds 6 electrons
d subshell → 5 orbitals → holds 10 electrons

AP Chemistry diagram of shells and subshells showing s, p, d labels and example orbital shapes with electron capacities 2, 6, and 10.

Electrons fill subshells in order of increasing energy (the Aufbau principle).

The order up to krypton (Z = 36): 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p

AP Chemistry energy‑level diagram illustrating the Aufbau filling order up to 4p.

Note: The 4s subshell is filled before 3d because it is lower in energy.

Electron Configurations (Full and Shorthand)

Electron configurations show how electrons are arranged within atoms or ions.

AP Chemistry diagram showing electron configuration notation with shell number, subshell letter, and superscript electrons (e.g., 1s2 2s2 2p6).

The notation uses: a number for the shell (n), a letter for the subshell (s, p, d), and a superscript for the number of electrons.

Examples Full configuration:
O (Z = 8) → 1s² 2s² 2p⁴
Ca (Z = 20) → 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

Shorthand notation uses the nearest noble gas to represent inner (core) electrons:
Ca = [Ar] 4s²
Fe = [Ar] 3d⁶ 4s²

Core and Valence Electrons

Core electrons are in the inner, filled shells. Valence electrons are in the outermost shell and determine chemical reactivity.

Example Oxygen has 2 inner electrons in the first shell and 6 valence electrons in the second (outermost) shell.

AP Chemistry diagram highlighting core versus valence electrons with configuration 1s2 2s2 2p4.

Electron Configuration of Ions

When atoms become ions, they lose or gain electrons:

Cations (positive ions): Electrons are removed from the highest principal energy level first.
Na: 1s² 2s² 2p⁶ 3s¹ → Na⁺: 1s² 2s² 2p⁶
Anions (negative ions): Electrons are added to the next available orbital.
Cl: 1s² 2s² 2p⁶ 3s² 3p⁵ → Cl⁻: 1s² 2s² 2p⁶ 3s² 3p⁶
Transition metals: remove electrons from 4s before 3d when forming ions.
Fe: [Ar] 4s² 3d⁶ → Fe²⁺ = [Ar] 3d⁶

Matt’s exam tip

When writing electron configurations, double‑check that you’ve filled subshells in the correct order and that your total number of electrons matches the element or ion’s charge. For ions, remove electrons from the outermost shell (highest principal energy level).

Ionization Energy and Electron Removal

Ionization energy is the energy required to remove an electron from an atom or ion.

Distance from nucleus: Electrons further from the nucleus are easier to remove (less attraction).
Nuclear charge: More protons = stronger attraction = harder to remove electrons.
Shielding: Core electrons block the attraction between the nucleus and outer electrons, making them easier to remove.

Trend:

Ionization energy increases across a period (left to right).
Ionization energy decreases down a group (top to bottom).

Summary

Atoms have a nucleus and surrounding electrons that occupy specific energy levels and subshells.
The Aufbau principle guides orbital filling, enabling us to write electron configurations.
Configurations explain chemical reactivity, ion formation, and periodic trends like ionization energy.
Coulomb’s law (charges and distance) explains why electrons closer to the nucleus are harder to remove and how shielding reduces attraction.
Key points: Aufbau = lowest to highest energy; configuration = list of occupied orbitals; core vs valence; ionization energy depends on nuclear charge, distance, and shielding.