Why Does Band Gap Energy Increase Higher Up the Periodic Table?

[amm508]

Hi,

Which of ZnS and CdSe will have the larger band gap energy, explain.

The answer is given as Zinc sulfide will have a larger band gap energy than cadmium selenide. Both
are II-VI compounds, and Zn and S are both higher vertically in the periodic table than Cd and Se. In moving from bottom to top up the periodic table, band gap energy increases.

I don't understand why moving from bottom up in the periodic table increases bang gap energy. Is it because electronegativity increases?

Would really appreciate if anyone could explain this!

Thank you v much in advance.

 

[TeethWhitener]

I don't understand why moving from bottom up in the periodic table increases bang gap energy. Is it because electronegativity increases?

This is one way of thinking about it. More precisely, the difference in electronegativities increases going from CdSe to ZnS. What this means is that ZnS is more ionic in character, so the electrons in the crystal tend to stay more localized to one center than spreading out over multiple centers.