Sub-atomic particles, isotopes and relative masses, the mass spectrometer and relative atomic mass calculations, electron configuration in s, p and d sub-shells, and ionisation energy evidence.

What this dot point is asking

CCEA wants you to describe the three sub-atomic particles and their relative masses and charges, define isotopes and relative atomic mass, explain how a mass spectrometer works and use spectra to calculate relative atomic mass, write electron configurations using s, p and d sub-shells, and use successive ionisation energies as evidence for shells and sub-shells.

Sub-atomic particles and isotopes

The relative atomic mass ($A_r$) is the weighted mean mass of an atom of an element relative to $\frac{1}{12}$ the mass of a carbon-12 atom.

The mass spectrometer

A modern time-of-flight mass spectrometer works in four stages: ionisation (the sample is ionised, for example by electron impact or electrospray), acceleration (ions are accelerated by an electric field to the same kinetic energy), flight or deflection (ions are separated by their mass-to-charge ratio $m/z$), and detection (ions strike a detector, producing a signal proportional to abundance).

To find relative atomic mass from a spectrum, multiply each isotope mass by its percentage abundance, add the products, and divide by 100:

For chlorine with $75\%$ chlorine-35 and $25\%$ chlorine-37, $A_r = \frac{(35 \times 75) + (37 \times 25)}{100} = 35.5$.

Note that the molecular ion in a spectrum can carry a $2+$ charge, so a peak at $m/z = 31.5$ would correspond to a $\text{Cu}^{2+}$ species of mass 63. The detector reads the mass-to-charge ratio, not mass directly.

Electron configuration

Electrons occupy shells divided into sub-shells (s, p, d) that hold a maximum of 2, 6 and 10 electrons. Sub-shells fill in order of increasing energy: $1s\,2s\,2p\,3s\,3p\,4s\,3d\,4p$, because the $4s$ sub-shell is slightly lower in energy than $3d$. For example, iron is $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^6\,4s^2$.

Two stability quirks are tested at CCEA: chromium is $[\text{Ar}]\,3d^5\,4s^1$ and copper is $[\text{Ar}]\,3d^{10}\,4s^1$, because a half-filled or fully filled $3d$ sub-shell is more stable than the expected $3d^4\,4s^2$ or $3d^9\,4s^2$. When writing ion configurations, remove the $4s$ electrons first: $\text{Fe}^{2+}$ is $[\text{Ar}]\,3d^6$ and $\text{Fe}^{3+}$ is $[\text{Ar}]\,3d^5$.

Common traps

Examples in context

Example 1. Carbon-14 dating and the constancy of chemical behaviour. Carbon-12, carbon-13 and carbon-14 are isotopes with 6 protons but 6, 7 and 8 neutrons. Because they share the same electron configuration, $1s^2\,2s^2\,2p^2$, they react identically, which is why living organisms take up all three in the same ratio as the atmosphere. Only when the organism dies does the radioactive carbon-14 decay, and its lower abundance is read on a mass spectrometer to date the sample. This is a direct application of the CCEA point that isotopes differ in mass but not in chemistry.

Example 2. Chlorine in a mass spectrum of a chloroalkane. When chloromethane $\text{CH}_3\text{Cl}$ is analysed, the molecular ion region shows two peaks two mass units apart in a 3:1 ratio, mirroring the $75\%$ chlorine-35 to $25\%$ chlorine-37 abundance. Recognising this 3:1 doublet is how chemists confirm a single chlorine atom is present, and a 9:6:1 triplet would signal two chlorines. The same weighted-mean idea that gives $A_r(\text{Cl}) = 35.5$ explains the relative peak heights.

Try this

Q1. Define the term isotopes. [2 marks]

• Cue. Atoms of the same element with the same number of protons but different numbers of neutrons.

Q2. A sample of boron is $20\%$ boron-10 and $80\%$ boron-11. Calculate its relative atomic mass. [2 marks]

• Cue. $\frac{(10 \times 20) + (11 \times 80)}{100} = 10.8$.