Elements are ordered by atomic number.
The number of electrons in the outer shell of an atom is equal to its group number.
Elements with the same number of electrons in their outer shells have similar chemical properties.
Reactivity:
- The bigger the distance between outer electrons and nucleus, the weaker the force of attraction and the more likely the electrons are to be lost
- Metals loose an electron when they react, so reactivity increases down the group
- Non metals gain an electron when they react, so reactivity decreases down the group
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