- The modern periodic table
- Protons and electrons were discovered at the start of the 20th century. They were then arranged in the periodic table in order of their atomic numbers
- The groups have similar properties because they have the same number of electrons in their outer shell.
- In a group, the reactivity of the elements depends on the total number of electrons. Going down the group; there are more occupied energy levels and the atoms are larger. As the atoms get larger, they are less attracted to the nucleus.
- When metals react they loose electrons, so the reactivity of metals in a group increases going down.
- When non-metals react they gain electrons, so the reactivity of non-metals decreases going down a group