Sub-atomic particles, isotopes, mass spectrometry and relative atomic mass, electron configurations in s, p and d sub-shells, and successive ionisation energies.

What this dot point is asking

WJEC wants you to describe the three sub-atomic particles and isotopes, explain how a time-of-flight mass spectrometer works, calculate relative atomic mass from abundance data, write electron configurations using $s$, $p$ and $d$ sub-shells, and interpret successive ionisation energy patterns as evidence for shell structure.

The answer

Sub-atomic particles and isotopes

An atom has a dense central nucleus of protons (relative charge $+1$, relative mass $1$) and neutrons (charge $0$, mass $1$), surrounded by electrons (charge $-1$, mass $1/1836$). The atomic number $Z$ is the number of protons; the mass number $A$ is protons plus neutrons.

For example, chlorine exists as $\text{Cl}^{35}$ and $\text{Cl}^{37}$, which react identically but have different masses.

The mass spectrometer and relative atomic mass

A time-of-flight (TOF) mass spectrometer measures isotopic masses and abundances in four stages: ionisation (the sample is vaporised and ionised, often by electron impact, forming $\text{X}^+$), acceleration (ions are accelerated by an electric field to constant kinetic energy), drift (ions travel a field-free region, lighter ions arriving first), and detection (each ion gains an electron at the detector, generating a current proportional to abundance).

Electron configurations

Electrons occupy sub-shells in order of increasing energy: $1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p$. The $4s$ sub-shell fills before $3d$ because it is slightly lower in energy.

Successive ionisation energies

The first ionisation energy is the energy to remove one mole of electrons from one mole of gaseous atoms: $\text{X}_{(g)} \rightarrow \text{X}^+_{(g)} + e^-$. Successive ionisation energies always rise because each electron is pulled from an increasingly positive ion. A large jump appears when the next electron comes from a shell closer to the nucleus, which reveals how many electrons are in each shell, and so which group the element is in.

Examples in context

Mass spectrometry in dating and forensics. TOF mass spectrometers identify trace isotopes in archaeology (carbon dating) and detect doping agents in sport, applying exactly the abundance-to-mass-spectrum logic above. Ionisation energy and the periodic table. The pattern of successive ionisation energies for sodium (one easy electron, then a huge jump) was historical evidence that placed it in Group 1, confirming the shell model that underpins the modern periodic table.

Try this

Q1. Write the full electron configuration of an iron atom. [1 mark]

• Cue. $1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 4s^2$.

Q2. A sample of boron is 20.0 percent boron-10 and 80.0 percent boron-11. Calculate its relative atomic mass. [2 marks]

• Cue. $A_r = (0.200 \times 10) + (0.800 \times 11) = 2.0 + 8.8 = 10.8$.

Q3. Outline the four stages by which a time-of-flight mass spectrometer separates ions. [4 marks]

• Cue. Ionisation, acceleration to constant kinetic energy, drift through a field-free region (lighter ions faster), detection generating a current proportional to abundance.