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The less valence electrons an atom has, the least likely it will gain electrons. Electron affinity increases the further right and up that you go on a periodic table. So these elements in order from top to bottom: Be, Se, Br.
The trend for electron affinity on the periodic table is that it increases from left to right and it decreases from top to bottom. Those with high electron affinity are also more likely to release energy when placing an electron while those with low electron affinity will absorb energy. Therefore, Be has the highest electron affinity because it is higher than Br and and Se. Then, because Se and Br are on the same level height wise, you would determine electron affinity by how directionally right it is on the periodic table. Br is more right than Se, thus it has more electron affinity. So the order should be Be, Br, then Se. I hope this helps :)
I'd like to also mention that noble gases have little to no electron affinity which is why Ar comes last in the order and is has the most energy absorbed by gaining an electron compared to others. This is because Ar does not want an electron; it is already stable.
Electron affinity is defined as the change in energy (in kJ/mole) of a neutral atom (in the gaseous phase) when an electron is added to the atom to form a negative ion. That is why this question in involving the concept. It is basically asking for increasing electron affinity.
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