The periodic table isn't just a list: where an element sits tells you how it behaves.
Rows and columns
- Groups are columns. Elements in a group behave alike, because (for the main groups) they have the same number of valence electrons.
- Periods are rows. Going across a period, each element has one more proton.
The families
| Group | Family | Valence e⁻ | Usual ion |
|---|---|---|---|
| 1 | alkali metals | 1 | +1 |
| 2 | alkaline earth metals | 2 | +2 |
| 3–12 | transition metals | varies | varies |
| 17 | halogens | 7 | −1 |
| 18 | noble gases | 8 (He: 2) | none |
This is the pattern from Unit 1's ions deck: the group gives the charge.
Metals, nonmetals and in between
Metals fill the left and middle; nonmetals sit at the upper right. The metalloids run along the zigzag line between them.
Two trends
- Atomic radius: bigger as you go down (more shells), smaller as you go across (more protons pull the electrons in).
- Electronegativity: the opposite. It rises across and falls down, peaking at fluorine.
Why Mendeleev's table was clever
When Dmitri Mendeleev arranged the elements in 1869, he left gaps and predicted the properties of elements nobody had found yet. Gallium and germanium were later discovered and fit his predictions.