Periodic Table Trends
How atomic radius, electronegativity and ionisation energy change as you move across a period and down a group, with the real numbers for periods 2 and 3.
| Trend | Across a period → | Down a group ↓ | Pourquoi |
|---|---|---|---|
| Atomic radius | Decreases | Increases | More protons pull the same shell in; going down adds a whole new shell. |
| Electronegativity | Increases | Decreases | A bigger nuclear charge at a shorter distance attracts bonding electrons more strongly. |
| First ionisation energy | Increases (with small dips) | Decreases | Outer electrons are held more tightly across a period and further from the nucleus down a group. |
| Metallic character | Decreases | Increases | Metals lose electrons easily, which is easiest for big atoms on the left and bottom. |
Periods 2 and 3: the numbers
| Élément | Atomic radius (pm) | Electronegativity | 1st ionisation energy (eV) |
|---|---|---|---|
| Li | 167 | 0.98 | 5.39 |
| Be | 112 | 1.57 | 9.32 |
| B | 87 | 2.04 | 8.3 |
| C | 67 | 2.55 | 11.26 |
| N | 56 | 3.04 | 14.53 |
| O | 48 | 3.44 | 13.62 |
| F | 42 | 3.98 | 17.42 |
| Ne | 38 | — | 21.56 |
| Na | 190 | 0.93 | 5.14 |
| Mg | 145 | 1.31 | 7.65 |
| Al | 118 | 1.61 | 5.99 |
| Si | 111 | 1.9 | 8.15 |
| P | 98 | 2.19 | 10.49 |
| S | 88 | 2.58 | 10.36 |
| Cl | 79 | 3.16 | 12.97 |
| Ar | 71 | — | 15.76 |
How to predict a trend by hand
- Find both elements on the periodic table and note whether they share a period (row) or a group (column).
- Same period: the one further right has more protons pulling on the same shell, so it is smaller, more electronegative and harder to ionise.
- Same group: the one further down has more shells, so it is bigger, less electronegative and easier to ionise.
- Neither: compare each with an element in between, or use the corner rule (fluorine top right is the extreme for electronegativity, caesium bottom left for size).
Exemple résolu
Which is bigger, sodium or chlorine? Both are in period 3, and chlorine has 6 more protons pulling on the third shell, so sodium is bigger: 190 pm vs 79 pm. Down group 1 the radius grows from Li (167 pm) to Na (190 pm) to K (243 pm) while the first ionisation energy falls from 5.39 eV to 5.14 eV to 4.34 eV — which is why potassium reacts with water more violently than lithium.
See how many outer electrons each group has on the valence electrons periodic table, pull electrons off real atoms in the Laboratoire d'Électronique, or colour the interactive periodic table by any of these trends.
Foire aux questions
What are the main periodic table trends?
Atomic radius, electronegativity, ionisation energy and metallic character. Radius and metallic character grow towards the bottom left; electronegativity and ionisation energy grow towards the top right (fluorine is the most electronegative element).
Why does atomic radius decrease across a period?
Each step to the right adds one proton but the new electron goes into the same shell. The stronger nuclear charge pulls that shell closer, so the atom gets smaller.
Why does ionisation energy drop from Be to B and from N to O?
Boron’s outer electron is in a 2p orbital, slightly higher in energy than 2s, so it is easier to remove. Oxygen has one paired 2p electron, and the repulsion between the pair makes it easier to remove than nitrogen’s unpaired one.
Why do noble gases have no electronegativity value?
Electronegativity measures pull on electrons in a bond. Helium, neon and argon form no stable compounds, so on the Pauling scale they have no value.
