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The modern table is arranged by increasing atomic number. Elements in a group have related outer-electron arrangements and similar chemical behaviour. Mendeleev left gaps and predicted properties of undiscovered elements; later discoveries supported his arrangement. Modern atomic-number ordering resolves problems from ordering by mass alone. Metals tend to lose electrons, while many non-metals gain or share them.
Video by Cognito. 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-table organisation
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What orders the modern table?
Atomic number. Atomic number is proton number.
After watching 2: How does Group 7 reactivity change down the group?
It decreases. Incoming electrons are attracted less strongly.
What you'll be able to do
The modern table is arranged by increasing atomic number. Elements in a group have related outer-electron arrangements and similar chemical behaviour. Mendeleev left gaps and predicted properties of undiscovered elements; later discoveries supported his arrangement. Modern atomic-number ordering resolves problems from ordering by mass alone. Metals tend to lose electrons, while many non-metals gain or share them.
Group 1 metals lose one outer electron. Down the group that electron is farther from the nucleus and more shielded, so it is lost more easily and reactivity increases. Group 7 halogens gain an electron; increased distance and shielding make this harder down the group, so reactivity decreases. A more reactive halogen displaces a less reactive halogen from its halide solution.
Group 0 elements have complete outer shells and are relatively unreactive. Their boiling points rise down the group because larger atoms have stronger attractions between them. Halogens also have rising boiling points down the group. A boiling-point explanation concerns attractions between particles, while a reactivity explanation concerns electron transfer. Use the right model for the property being asked about.
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 potassium more reactive than lithium?
Potassium is more readily oxidised.
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.