Subatomic particles and isotopes, relative atomic mass from mass spectra, the principles of time-of-flight mass spectrometry, and electron configuration in shells, sub-shells and orbitals including ionisation energy evidence.

What this topic is asking

Eduqas topic C1.2 covers the structure of the atom, isotopes, how mass spectrometry measures relative masses, and how electrons are arranged in shells, sub-shells and orbitals. Ionisation energies provide the experimental evidence for that arrangement, so the topic links the model of the atom to measurable data.

Subatomic particles and isotopes

Relative masses

The relative atomic mass $A_r$ is the weighted mean mass of an atom relative to one twelfth the mass of a carbon-12 atom. It is calculated from the abundances of the isotopes.

Time-of-flight mass spectrometry

Eduqas requires the principle of the modern time-of-flight (TOF) mass spectrometer. A sample is ionised (usually to $1+$ ions), the ions are accelerated by an electric field to the same kinetic energy, they drift along a flight tube, and lighter ions arrive at the detector first. The time of flight gives the mass-to-charge ratio $m/z$, and the relative size of each peak gives the abundance of that isotope.

Electron configuration

Electrons occupy shells (principal quantum number $n$) divided into sub-shells ($\text{s}, \text{p}, \text{d}$) made of orbitals, each holding two electrons of opposite spin. They fill in order of increasing energy, which puts $4\text{s}$ below $3\text{d}$:

So iron is $1\text{s}^2 2\text{s}^2 2\text{p}^6 3\text{s}^2 3\text{p}^6 3\text{d}^6 4\text{s}^2$, often written $[\text{Ar}]3\text{d}^6 4\text{s}^2$. Chromium and copper are exceptions, taking $3\text{d}^5 4\text{s}^1$ and $3\text{d}^{10} 4\text{s}^1$ to gain a more stable half-full or full $3\text{d}$ sub-shell.

Ionisation energy as evidence

The first ionisation energy is the energy to remove one mole of electrons from one mole of gaseous atoms: $\text{X}(\text{g}) \rightarrow \text{X}^+(\text{g}) + \text{e}^-$. Successive ionisation energies always increase, because each electron is pulled from an increasingly positive ion. Large jumps appear when removal moves to a shell closer to the nucleus, revealing the shell structure and the group number.

Examples in context

Example 1. Radioactive dating uses isotope ratios. Carbon-14 and carbon-12 are chemically identical, so living tissue takes them up in the same proportion as the atmosphere; after death the carbon-14 decays, and the changed isotope ratio (a mass-spectrometry measurement) dates the sample.

Example 2. The successive-ionisation pattern for sodium. Sodium shows a small first ionisation energy, then a jump to the second (removing from the full $n = 2$ shell), then a much larger jump after the ninth electron (removing from the $n = 1$ shell). The pattern $2, 8, 1$ falls straight out of the data, confirming sodium is in Group 1.

Try this

Q1. Write the full electron configuration of a sulfur atom (atomic number 16). [1 mark]

• Cue. $1\text{s}^2 2\text{s}^2 2\text{p}^6 3\text{s}^2 3\text{p}^4$.

Q2. Explain why the first ionisation energy of magnesium is higher than that of sodium. [2 marks]

• Cue. Magnesium has one more proton, so a greater nuclear charge attracts the outer electrons more strongly, and its atomic radius is slightly smaller, so more energy is needed to remove the outer electron.