Light, Electrons & the Electromagnetic Spectrum

Example: Sodium's core electrons are the 10 electrons in the 1s, 2s, and 2p orbitals.

Key constants to know:

The Nucleus – Identity & Stability

Quantum Numbers – The Electron's Complete Address

Think of quantum numbers as an electron's postal address. Just like a full address tells you the country, city, street, and apartment number, four quantum numbers tell you everything about where an electron lives and how it behaves. No two electrons in an atom can share the exact same four numbers – this is the Pauli Exclusion Principle.

Atomic Orbitals vs Orbits – The Modern Truth

Electron Configuration – The Seating Chart

Electron configuration tells you exactly where every electron in an atom lives. Think of it as a seating chart assigning each electron to its proper energy level and subshell.

The Three Rules

Subshell Capacities

The Diagonal Rule – A Memory Trick

Write out the subshells in a grid and draw diagonal arrows:

Follow the diagonals from top-right to bottom-left: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d

Worked Examples

Special Stability – The Cr and Cu Exceptions

Some atoms "cheat" because a half-filled ($d^5$) or fully filled ($d^{10}$) d subshell is extra stable. The symmetry of having every d orbital equally occupied provides a stability bonus.

Electron Configurations of Ions (Cations)

When a metal atom forms a positive ion, electrons are removed from the outermost shell first (highest $n$ value).

Valence Electrons – The Important Ones

The Electromagnetic Spectrum

Now we shift from the world of electrons to the world of light – and we'll soon see they're deeply connected.

The Full Spectrum – From Gentle to Deadly

Visible Light – The Rainbow Slice

Within the visible spectrum, colours range from red (longest wavelength, ~700 nm) to violet (shortest wavelength, ~400 nm):

$\text{Red} \rightarrow \text{Orange} \rightarrow \text{Yellow} \rightarrow \text{Green} \rightarrow \text{Blue} \rightarrow \text{Indigo} \rightarrow \text{Violet}$

Remember: ROYGBIV – or the mnemonic "Richard Of York Gave Battle In Vain"

Ionising vs Non-ionising Radiation

Wave Properties & Key Equations

Light behaves as a wave, and understanding wave properties is essential for solving problems.

Wave Vocabulary

The Three Fundamental Relationships

Summary of Relationships

Additional Important Equations

Photons & the Quantum Nature of Light

Here's where physics got weird – and beautiful. Classical physics said light was purely a wave. But experiments in the early 1900s proved that light also behaves like a stream of particles called photons. This wave-particle duality is one of the most profound discoveries in all of science.

The Photoelectric Effect – Einstein's Nobel Prize

Where:

• $KE$ = kinetic energy of the ejected electron
• $hf$ = energy of the incoming photon
• $h\nu_0 = \phi$ = work function (minimum energy needed to free an electron from the metal)
• $\nu_0$ = threshold frequency (minimum frequency needed)

Hydrogen Emission Spectrum – The Atomic Barcode

If electrons could have any energy value, heating hydrogen would produce a continuous rainbow of all colours. Instead, we see something remarkable – only specific, discrete lines of colour appear, like a barcode.

Ground State, Excited State & Electron Transitions

This is where everything comes together – atomic structure meets light emission.

The Absorption-Emission Cycle

Flame Tests – The Colourful Fingerprints

Why Flame Tests Work

The process is exactly the absorption-emission cycle we just described:

1. Flame provides energy → electrons get excited (jump to higher levels)
2. Electrons are unstable in excited state → quickly fall back
3. As they fall, they emit photons with specific wavelengths → we see a specific colour
4. Each metal has unique energy level spacings → unique colour → identification!

Flame Test Colours – Memorise These!

Summary of Key Equations

Vocabulary

Key Takeaways

Test Yourself

Concepts 1–3: Electron Configuration, Valence Numbers & Quantum Numbers

Concepts 4–12: EM Spectrum, Frequency, Wavelength, Photons, Emission & Flame Tests

The bigger the drop, the more energy the photon carries → shorter wavelength → bluer/more violet light. Smaller drops → less energy → longer wavelength → redder light.