Shielding effect

Shielding is when an electron becomes less attracted to other atoms the further it is away from the nucleus. This is because the forces holding atoms together become weaker as distance increases.
In group 1 elements.
In a reactive metal like sodium, the single electron on an electron shell furthest from the nucleus is more likely to be lost in a reaction with another element. This is because less energy is needed to pull them away. Other elements with similar properties like potassium that are lower down on the periodic table than sodium will be even more likely to lose their furthest electron in a reaction. This is because of the addition of more electron shells between the nucleus and the furthest electron, increasing distance.
In group 17 elements.
The shielding effect also happens with elements that have many electrons on their furthest electron shell, for example chlorine. In this case however, more energy is needed to give the atom an electron. The amount of energy needed is increased with the number electron shells around the nucleus of the atom, for example in elements further down the periodic table like bromine.