The Periodic Table Explained: Trends That Predict Everything
May 9, 2026 · 6 min · periodic table · chemistry help · atomic structure
The periodic table looks like a memorisation nightmare. It's actually a brilliant map of how electrons behave. Once you know how to read it, you can predict reactions you've never seen.
The two big patterns
- Going down a group — properties change because atoms get bigger
- Going across a period — properties change because protons increase but electrons stay in the same shell
Both patterns make the same physics — atomic radius, ionisation energy, electronegativity, metallic character — change in predictable directions.
Atomic radius
- Larger going down (more shells)
- Smaller going across (more protons pull electrons in)
Ionisation energy
The energy to remove the outermost electron.
- Lower going down (electron is farther from nucleus)
- Higher going across (electron is held more tightly)
Electronegativity
How strongly an atom pulls electrons in a bond.
- Lower going down
- Higher going across (peaks at fluorine)
Fluorine is the most electronegative element. Caesium is one of the least.
Reactivity
- Group 1 (alkali metals): more reactive going down (Cs > Rb > K > Na > Li)
- Group 17 (halogens): more reactive going up (F > Cl > Br > I)
The reasons are opposite — alkali metals lose an electron more easily as they get bigger, halogens gain an electron more easily as they get smaller.
What examiners want you to predict
- Ionic vs covalent bonding (large electronegativity difference → ionic)
- Whether a reaction will displace another (more reactive metal displaces less reactive)
- The nature of the oxide (metal oxide → basic, non-metal oxide → acidic)
- The trend in melting points within a group
Common pitfalls
- Treating noble gases as inert (they used to be called inert gases — modern chemistry has noble gas compounds)
- Confusing first ionisation energy with electron affinity
- Forgetting that hydrogen sits in group 1 but is not a metal