Key points
1. The group that an element is in is determined by its atomic (proton) number
2. The number of electrons in the highest energy level of the atom determines its chemical properties
3. We can explain trends in reactivity as we go down a group in terms of the number of energy levels in the atoms
4. Elements in the same group have similar properties because their atoms have the same number of electrons in the highest occupied shell
5. Larger atoms (atoms with more energy levels between the outer shell and nucleus) :
- Lose electrons more easily
- Gain electrons less easily
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