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The question 94 is clear what is the answer, just think about the atomic radius of the two elements. The question 93 may be explained Defining second electron affinity The second electron affinity is the energy required to add an electron to each ion in 1 mole of gaseous 1- ions to produce 1 mole of gaseous 2- ions. This is more easily seen in symbol terms. X- (g) + 1e- -> X2-(g) It is the energy needed to carry out this change per mole of X-. Why is energy needed to do this? You are forcing an electron into an already negative ion. It's not going to go in willingly! O (g) + 1e- -> O-(g) 1st EA = -142 kJ mol-1 O- (g) + 1e- -> O2-(g) 2nd EA = +844 kJ mol-1 The positive sign shows that you have to put in energy to perform this change. The second electron affinity of oxygen is particularly high because the electron is being forced into a small, very electron-dense space. http://www.chemguide.co.uk/atoms/properties/eas.html http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Atomic_and_Molecular_Properties/Electron_Affinity#Second_electron_affinity
@Cuanchi I always get confused with the electron affinity although people have explained it to me before, like how it changes across the periodic table. from your explanation it seems like it's just adding and electron to a neutral atom, and then adding another electron to an atom that has a (-) charge. I need to read through those links though.
like based on that second reaction you're adding an electron to an atom that already has a negative charge.
I guess sodium and chlorine, I thought about it again.. had to look this up as well like the electrons in sodium would be less tightly held than in chlorine, and as you go from left to right your adding more protons and the Zeff goes up. the increasing pull would pull the electrons in closer to nucleus. I think it would be farther from the nucleus and have a lower Zeff choice B.
Yea, you are all right!! the electron affinity is not that easy concept to understand and the second electron affinity I never hear before. Besides always are confusion about the sign and the value, when the question ask greater (-44 kJ > or more positive than -73 kJ) Most groups (columns) of the periodic table do not exhibit any definite trend in electron affinity. Among the group 1A metals, however, electron affinity becomes more positive as we move down the column. Electron affinity generally becomes more negative as we move to the right across a period in the periodic table.