Atomic structure: particles in the atom, isotopes, mass spectrometer, electronic configuration

Particles in the Atom

Think of an atom as a tiny solar system: 🔴 protons (positive charge), ⚪ neutrons (neutral), and ⚡ electrons (negative charge) orbit around the nucleus.

• Protons determine the atomic number (Z) – the element’s identity.
• Neutrons add mass but no charge; they make isotopes.
• Electrons are arranged in shells and decide chemical behaviour.

Isotopes

Isotopes are atoms of the same element that differ in the number of neutrons.

Mass Spectrometer

Imagine a carnival ride that separates balls by weight – that’s what a mass spectrometer does with ions.

1. ⚡ Ionisation: Atoms lose or gain electrons to become ions.
2. 🌀 Acceleration: Ions are accelerated by an electric field.
3. 🧭 Deflection: A magnetic field bends the path; heavier ions bend less.
4. 📊 Detection: The detector records the number of ions at each mass-to-charge ratio.

Result: A spectrum showing peaks at different m/z values, revealing the element’s isotopic composition.

Electronic Configuration

Electrons fill shells (energy levels) around the nucleus. Each shell can hold a maximum of 2n² electrons.

Notation: 1s² 2s² 2p⁴ for oxygen.

Rule of thumb: “Shells fill before they spread” – electrons fill lower energy levels first.

Exam Tips

• Remember that Z = number of protons and A = Z + number of neutrons.
• When drawing electronic configurations, use the Aufbau principle (1s, 2s, 2p, 3s, ...).
• For mass spectrometry questions, sketch the ion path: ionisation → acceleration → deflection → detection.
• Use the periodic trend that atomic radius decreases across a period and increases down a group.
• Practice converting between mass number (A) and atomic mass using isotopic abundances.