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In page Inert pair effect:

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In the case of Groups 13 to 15 the inert pair effect has been further attributed to, "the decrease in bond energy with the increase in size from Al to Tl so that the energy required to involve the s electron in bonding is not compensated by the energy released in forming the two additional bonds."[1] That said, the authors note several factors are at play, including relativistic effects in the case of gold, and that, "a quantitative rationalisation of all the data has not been achieved."[1]