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Across a period, proton number rises while added electrons enter the same principal shell. Shielding changes relatively little, so attraction generally increases and atomic radius decreases. First ionisation energy generally increases, with subshell and paired-electron exceptions. Down a group, extra occupied shells increase radius and shielding. An explanation must compare the relevant factors; stating that there are more protons is insufficient when extra shells change the distance and shielding substantially.
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 periodic trends in atomic radius and ionisation energy
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What usually happens to atomic radius across Period 3?
It decreases. Increasing nuclear charge attracts the same-shell electrons more strongly.
After watching 2: Which Period 3 element forms a giant covalent structure?
Silicon. Silicon is an extended covalent network.
What you'll be able to do
Across a period, proton number rises while added electrons enter the same principal shell. Shielding changes relatively little, so attraction generally increases and atomic radius decreases. First ionisation energy generally increases, with subshell and paired-electron exceptions. Down a group, extra occupied shells increase radius and shielding. An explanation must compare the relevant factors; stating that there are more protons is insufficient when extra shells change the distance and shielding substantially.
Across Period 3, sodium, magnesium and aluminium have giant metallic structures. Greater ionic charge and more delocalised electrons generally strengthen metallic bonding. Silicon forms a giant covalent network, so many strong bonds must be overcome. Phosphorus, sulfur and chlorine are molecular; melting mainly overcomes intermolecular forces. Sulfur’s larger S8 molecules have stronger London forces than P4 or Cl2. Argon is monatomic with weak London attractions. The change of structure makes a simple smooth trend impossible.
Successive ionisation energies can reveal an outer-shell electron count, while oxide chemistry and physical properties give complementary clues. An unusually high melting point does not alone prove a metal: giant covalent silicon is a counterexample. Distinguish atomic radius from ionic radius. Forming a positive ion removes outer electrons and can remove an occupied shell; a negative ion has increased electron repulsion. For isoelectronic ions, greater proton number usually produces a smaller radius.
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: Compare radii of Na+ and Mg2+.
Mg2+ is smaller.
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.