Fundamental particles (protons, neutrons, electrons), mass number and atomic number, isotopes, the time of flight (TOF) mass spectrometer, relative atomic mass, electron configuration in sub-shells, and the trends in ionisation energy across periods and down groups.

What this dot point is asking

AQA wants you to know the properties of the fundamental particles, define mass number, atomic number and isotopes, describe how a time of flight mass spectrometer works, calculate relative atomic mass from isotopic data, write electron configurations in sub-shells, and explain ionisation energy trends.

The fundamental particles

The time of flight (TOF) mass spectrometer

Four stages:

1. Ionisation. In electrospray ionisation the sample is dissolved in a volatile solvent and pushed through a fine needle at high voltage; each molecule gains a proton to form $\text{X}\text{H}^+$ (so the recorded mass is $M_r + 1$). In electron impact ionisation the sample is vaporised and bombarded with high-energy electrons that knock one electron out to form $\text{X}^+$ (often fragmenting larger molecules).
2. Acceleration. The positive ions are accelerated through an electric field so that every ion of charge $+1$ gains the same kinetic energy. Because $KE = \tfrac{1}{2}mv^2$ is fixed, heavier ions end up moving more slowly.
3. Ion drift. The ions pass through a field-free flight tube of fixed length. Lighter ions reach the detector first; the time of flight $t$ is proportional to $\sqrt{m}$.
4. Detection. Each ion gains an electron at the detector, generating a current; the size of the current measures abundance, and the flight time gives the mass-to-charge ratio $\frac{m}{z}$.

Because all ions are accelerated to the same kinetic energy, $\tfrac{1}{2}mv^2 = \text{constant}$, so the flight time over a fixed distance directly encodes the mass. This is why a TOF instrument can resolve isotopes that differ by a single mass unit.

Relative atomic mass

The relative atomic mass is the weighted mean of the isotope masses, weighted by abundance. The same idea gives relative isotopic abundance from a mass spectrum, where each peak height is proportional to the abundance of that isotope.

Electron configuration and ionisation energy

Electrons fill sub-shells in order of increasing energy: $1s\ 2s\ 2p\ 3s\ 3p\ 4s\ 3d\ 4p$. The $4s$ sub-shell is slightly lower in energy than $3d$, so it fills before $3d$ but, because it is the outermost, it empties first when the atom ionises (so $\text{Fe}^{2+}$ is $[\text{Ar}]3d^6$, not $[\text{Ar}]4s^2 3d^4$). Each $s$ sub-shell holds 2 electrons, each $p$ holds 6 and each $d$ holds 10, and within a sub-shell orbitals are singly filled before pairing (Hund's rule).

The first ionisation energy is the energy to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous $+1$ ions: $\text{X(g)} \rightarrow \text{X}^+\text{(g)} + \text{e}^-$. It depends on nuclear charge, atomic radius and shielding. It rises across a period (greater nuclear charge pulls the same shell in, smaller radius, similar shielding) and falls down a group (extra inner shells mean more shielding and a larger radius, so the outer electron is held less tightly). Successive ionisation energies always increase, and a large jump between two successive values reveals a change of shell, which is how the group of an element can be deduced.