1.3Structure 1 — Models of the particulate nature of matter

Electronic Configurations — IB Chemistry SL Revision Notes & Topical Questions

15 min read 23 parts Lesson 6 of 6 1 quiz

📚 IB Chemistry SL Hub / Topical Revision

0%read

Lesson summary

1.3 Electronic Configurations — IB SL Notes — IB CHEMISTRY SL notes: Relate frequency, wavelength and energy, use the hydrogen spectrum as evidence for energy levels, and write electron configurations up to Z = 36.Speed — all electromagnetic waves travel at c = 3.00 × 108 m s−1 i…

Photon energy
Relationship
Order of increasing energy
Visible range
Excitation
Line spectrum

Part 1 of 9Electromagnetic radiation

Electromagnetic radiation

Speed

Speed all electromagnetic waves travel at c = 3.00 × 10⁸ m s⁻¹ in a vacuum, with c = λν.

Photon energy

E = hν, with h = 6.63 × 10⁻³⁴ J s.

Relationship

Relationship frequency and wavelength are inversely proportional, and photon energy increases with frequency.

Order of increasing energy

Order of increasing energy radio < microwave < IR < visible < UV < X-ray < gamma.

Visible range

Visible range from about 400 nm to 700 nm (red, lower energy).
- (violet, higher energy)

Part 2 of 9Emission spectra

Excitation

Excitation heat or an electric discharge promotes electrons to higher levels.

Emission

Emission when electrons fall back they emit photons with energy equal to the level difference, ΔE = hν.

Line spectrum

Line spectrum only certain frequencies are emitted because levels have fixed energies, and the pattern is unique to each element.

Absorption comparison

Absorption comparison an absorption spectrum has dark lines on a continuous background at the same frequencies as the element's emission lines.

Part 3 of 9Hydrogen spectrum as evidence for energy levels

Series	Electron falls to	Region
Lyman	n = 1	Ultraviolet
Balmer	n = 2	Visible
Paschen	n = 3	Infrared

Convergence

Convergence energy levels get closer together as n increases, so lines converge at high frequency.

Convergence limit

Convergence limit corresponds to an electron escaping the atom (n = ∞).

Conclusion

Conclusion the discrete lines show that electrons occupy fixed energy levels.

Part 4 of 9Energy levels, sublevels and orbitals

Sublevel	Orbitals	Max electrons	Shape
s	1	2	Spherical
p	3	6	Dumbbell (p_x, p_y, p_z)
d	5	10	Mostly four-lobed
f	7	14	Complex

Level n

Level n contains n sublevels and holds up to 2n² electrons (2, 8, 18, 32).

Sublevels by level

Sublevels by level n = 1: 1s; n = 2: 2s, 2p; n = 3: 3s, 3p, 3d; n = 4: 4s, 4p, 4d, 4f.

Orbital capacity

Orbital capacity each orbital holds two electrons of opposite spin.

Shapes

Shapes s orbitals are spherical and p orbitals are dumbbell-shaped along the x, y and z axes at 90°, and both grow larger as n increases.

Degenerate orbitals

Degenerate orbitals orbitals in the same sublevel, such as 2p_x, 2p_y and 2p_z, have equal energy.

Part 5 of 9Filling rules

Filling rules

Aufbau principle

Aufbau principle electrons fill the lowest energy orbitals first, in the order 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p.

Pauli exclusion principle

Pauli exclusion principle an orbital holds a maximum of two electrons with opposite spins.

Hund's rule

Hund's rule electrons occupy degenerate orbitals singly with parallel spins before pairing.

4s before 3d

4s is lower in energy before 3d is occupied, so it fills first.

Orbital diagrams

Orbital diagrams show each orbital as a box and each electron as an arrow;
- nitrogen has 2p with three unpaired electrons.

Part 6 of 9Writing configurations

Find the total number of electrons from the atomic number, adjusted for any charge.

Fill the sublevels in Aufbau order with maximum occupancy s 2, p 6, d 10, f 14.

Check that the superscripts add up to the electron total.

Write the condensed form using the previous noble gas core.

Iron (Z

26) — 1s²2s²2p⁶3s²3p⁶4s²3d⁶, condensed [Ar]4s²3d⁶.

After the d-block

Ga is [Ar]3d¹⁰4s²4p¹ and Br is [Ar]3d¹⁰4s²4p⁵.

Part 7 of 9Exceptions and ions

Exceptions and ions

Chromium

[Ar]3d⁵4s¹ because a half-filled d sublevel is extra stable.

Copper

[Ar]3d¹⁰4s¹ because a full d sublevel is extra stable.

Transition metal cations

4s electrons are removed before 3d, so Fe³⁺ is [Ar]3d⁵.

Main-group ions

Main-group ions lose or gain outer electrons, so Ca²⁺ is [Ar].

Part 8 of 9Blocks of the periodic table

Rule
- elements are grouped by the sublevel that receives the last, highest-energy electron.

Block	Groups
s-block	1–2 (plus He)
p-block	13–18
d-block	3–12
f-block	Lanthanoids and actinoids

Part 9 of 9More from the specification

Key terms

Term	Meaning
Electromagnetic spectrum	The full range of radiation ordered by wavelength or frequency, from radio waves to gamma rays.
Line spectrum	A series of discrete lines produced when excited electrons fall to lower energy levels.
Energy level	A main shell of fixed energy labelled by principal quantum number n.
Sublevel	A subdivision of a main level (s, p, d or f) containing orbitals of equal energy.
Atomic orbital	A region around the nucleus with a high probability of finding an electron, holding at most two.
Aufbau principle	Electrons fill orbitals in order of increasing energy.
Hund's rule	Degenerate orbitals fill singly with parallel spins before pairing.

Avoid these mistakes

Writing 4s before 3d for the electrons removed in ions — 4s electrons are lost first.

Placing paired electrons in a p sublevel before every orbital has one — Hund's rule requires singly occupied orbitals first.

Giving Cr as [Ar]3d⁴4s² — it is [Ar]3d⁵4s¹.

Using lines evenly spaced for the hydrogen spectrum — they converge at high frequency.

Last updated: 15 October 2026