You can explain trends in reactivity as you go down the groups in terms of the attraction between the electrons in the outermost shell and the nucleus.
This electrostatic attraction depends on:
- the distance between the outermost electron shell and the nucleus
- the number of occupied inner shells (energy levels) of electrons which provide a shielding effect
- the size of the positive charge on the nucleus (the nuclear charge)
In deciding how easy it is for atoms to gain or lose electrons from their outermost shell, the above three factors must be taken into account. The increased nuclear charge caused by extra protons in the nucleus going down a group is outweighed by the other two factors.
Therefore electrons are easier for the larger atoms to lose going down a group and harder for them to gain going down a group.
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