Why does electronegativity increase across a period but decrease down a group?
Electronegativity increases across a period because the effective nuclear charge increases while atomic radius decreases. As you move from left to right, atoms gain protons in the nucleus without adding new electron shells. This stronger nuclear charge pulls bonding electrons closer, making the atom more effective at attracting electrons from another atom. At the same time, the shrinking atomic radius places bonding electrons nearer to the nucleus, strengthening attraction even further. These two effects—higher effective nuclear charge and smaller atomic size—combine to raise electronegativity steadily across a period.
In contrast, electronegativity decreases down a group because atoms gain new electron shells. Each additional shell increases the distance between valence electrons and the nucleus, significantly weakening the nuclear attraction. This increased distance is amplified by enhanced electron shielding, as inner electrons repel the outer electrons and cancel part of the nuclear pull. As a result, even though nuclear charge increases down the group, the rise in shielding and atomic radius outweighs the additional protons. The atom becomes less effective at attracting electrons, causing electronegativity to drop.
These competing effects explain why elements like fluorine are extremely electronegative while elements like cesium are not. Fluorine’s compact size and high effective nuclear charge give it a strong pull on electrons, making it highly reactive and likely to form polar bonds. Cesium, on the other hand, has a loosely held valence electron positioned far from the nucleus, giving it very low electronegativity and encouraging ionic bond formation.
Ultimately, electronegativity trends are governed by the balance between nuclear charge, electron shielding and atomic radius—three fundamental drivers of periodic behavior.
Frequently Asked Questions
Why doesn’t increased nuclear charge make atoms more electronegative down a group?
The increase in shielding and atomic radius grows faster than the nuclear charge, weakening attraction.
Why is fluorine more electronegative than oxygen?
Fluorine has a smaller radius and higher effective nuclear charge, giving it a stronger pull on shared electrons.
Does bonding type depend on electronegativity?
Yes. Large electronegativity differences lead to ionic bonds, while smaller differences form polar covalent bonds.
RevisionDojo Call to Action
Want periodic trends explained in a way that actually sticks? RevisionDojo helps IB students master the “why” of chemistry so you can learn faster and feel confident in class and exams.
