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Atomic number counts protons; mass number counts protons plus neutrons. Isotopes have the same proton number and different neutron numbers. An ion forms by gaining or losing electrons, so an ordinary chemical reaction does not change the element. Relative atomic mass is a weighted mean compared with one twelfth of a carbon-12 atom. Multiply each isotopic mass by its fractional abundance and add the contributions; do not take an unweighted mean unless abundances are equal.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Explain electron configurations of atoms and ions
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which electrons leave iron first when forming Fe2+?
4s electrons. The neutral filling order is not the ionisation order.
After watching 2: A large jump after the second ionisation energy suggests how many outer-shell electrons?
Two. The third electron is removed from an inner shell.
What you'll be able to do
Atomic number counts protons; mass number counts protons plus neutrons. Isotopes have the same proton number and different neutron numbers. An ion forms by gaining or losing electrons, so an ordinary chemical reaction does not change the element. Relative atomic mass is a weighted mean compared with one twelfth of a carbon-12 atom. Multiply each isotopic mass by its fractional abundance and add the contributions; do not take an unweighted mean unless abundances are equal.
An orbital holds at most two electrons with opposite spins. The s, p and d subshells contain one, three and five orbitals respectively. Fill equal-energy orbitals singly before pairing. For neutral first-row transition atoms 4s fills before 3d, but electrons are removed from 4s first when forming their ions. Chromium and copper have the familiar half-filled or filled d-subshell exceptions. Fe is [Ar]3d6 4s2; Fe2+ is [Ar]3d6.
First ionisation energy removes one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. It is endothermic. Across a period, increasing nuclear charge usually strengthens attraction, but a higher-energy p electron and paired-electron repulsion explain small decreases. Down a group, extra shells and shielding usually dominate increased nuclear charge. A large jump in successive energies indicates removal from an inner shell after the outer-shell electrons have gone.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Why is the first ionisation energy of oxygen lower than nitrogen?
Paired-electron repulsion offsets the increase in nuclear charge.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.