Quick questions on Basic ideas about atoms - WJEC A-Level Chemistry

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.

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.

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.

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

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

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