Elements in the same group of the periodic table have similar properties. Thats because their atoms have the same number of electrons in the highest occupied energy level.
Within a group, the properties of the elements are affected by the number of lower energy levels underneath the outer level. As we go down a group the number of occupied energy levels increases, the atoms get bigger. This means that the outer electrons are further from the positive nucleus. This has 2 effects:
- The larger atoms lose electrons more easily
- The larger atoms gain electrons less easily
In both cases, this happens because the outer electrons are further away from the nucleus. Not only that, he inner energy levels 'screen' or 'shield' the outer electrons from the positive charge in the nucleus. We can see this effect with the alkali metals and halogens.
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