Atomic Structure and Electron Configuration

Atomic structure

An atom has a nucleus of protons and neutrons, surrounded by electrons. The atomic number (protons) defines the element; the mass number = protons + neutrons. Isotopes have the same protons but different neutrons.

Mass spectrometry

A time-of-flight (TOF) mass spectrometer measures the mass and abundance of isotopes/ions:

1. Ionisation (electron impact or electrospray) — sample is turned into +1 ions.

2. Acceleration — ions accelerated by an electric field to the same kinetic energy.

3. Ion drift — lighter ions travel faster through the flight tube.

4. Detection — time of flight gives mass/charge (m/z); the detector records abundance.

Relative atomic mass is calculated from isotope abundances:

Ar = Σ(isotope mass × % abundance) ÷ 100

Electron configuration

Electrons occupy energy levels (shells) divided into sub-shells (s, p, d, f) made of orbitals (each holding 2 electrons with opposite spins):

• s sub-shell: 1 orbital (2 electrons)
• p: 3 orbitals (6 electrons)
• d: 5 orbitals (10 electrons)

Fill in order of increasing energy: 1s 2s 2p 3s 3p 4s 3d 4p… (4s fills before 3d, and empties first when ionising).

Example: iron (Z = 26): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶.

Ionisation energies

The first ionisation energy is the energy to remove one mole of electrons from one mole of gaseous atoms. It provides evidence for shells and sub-shells:

• Successive ionisation energies increase (harder to remove from an increasingly positive ion); big jumps reveal a new, closer shell — evidence for electron shells.
• Across a period, ionisation energy generally rises (increasing nuclear charge), with small dips (e.g. between s and p sub-shells, and due to electron-pair repulsion).
• Down a group, it falls (outer electron further from the nucleus, more shielding).

Worked example

Write the electron configuration of a calcium ion, Ca²⁺ (Ca is Z = 20).

• Ca: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s². Remove the two 4s electrons → Ca²⁺ = 1s² 2s² 2p⁶ 3s² 3p⁶. ✓

Common mistakes

• Filling 3d before 4s — 4s fills first (but is removed first on ionisation).
• Forgetting orbitals hold 2 electrons (opposite spin).
• Muddling the periodic trend explanations (nuclear charge, shielding, distance).

Exam tips

• Learn the four TOF stages in order.
• Practise electron configurations, including ions (remove from the outermost shell, 4s before 3d).
• Explain ionisation-energy trends using nuclear charge, shielding and distance, and successive-IE jumps as evidence for shells.

Key facts to remember

• TOF mass spec: ionisation → acceleration → drift → detection; Ar = Σ(mass × abundance)/100.
• Sub-shells s/p/d hold 2/6/10 electrons; 4s fills before 3d (removed first).
• First ionisation energy trends: up a period, down a group; successive IE jumps are evidence for shells.