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In my book(NCERT Chemistry Part I, Textbook for Class XI[1]) a list of values of electron gain enthalpies of various elements is given. According to that list, oxygen has less negative electron gain enthalpy than selenium, tellurium and polonium. Shouldn't it be the opposite because electron gain enthalpy becomes less negative down a group?

Electron Gain Enthalpy of common elements

Reference:

[1]: Rajput, Anup, Kumar. Classification of Elements and Periodicity in Properties. NCERT Chemistry Part I, Textbook for Class XI, First Edition(October 2019 Reprint); Uppal, Shveta, Banerjee, Binoy, Eds.; National Council of Educational Research and Training: New Delhi, India, 2006; pp 90.

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  • $\begingroup$ It's due to extremely small size of oxygen as compared to others in the group. $\endgroup$ May 18 at 14:33
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Electron Gain Enthalpy becomes less negative down the group.

This is true, as a general trend. But the 2nd period is an exception to this trend.

In p block, it is observed that the EGE of some 2nd period elements is actually less negative then their 3rd period counterparts.

In the case of oxygen, it’s EGE is even less than selenium, tellurium and polonium. It’s because oxygen has very small radius and thus have high electron density. Due to this, the incoming electron faces a lot of repulsion from the already present electrons.

So, it takes more energy to add an electron to an oxygen atom i.e. oxygen has less negative EGE. On the other hand, selenium and tellurium have much larger radius and thus the electron density isn’t very high. Hence, their EGE is more negative as its easier to add an electron to them.

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  • $\begingroup$ Please take into account that my answer applies to Po only because your data supports it. I myself have not checked if it is true or not. $\endgroup$
    – WhySee
    May 18 at 17:17

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