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The difference between 3.11 and 3.9 is that 3.11 is talking about a 3+ cation whereas 3.9 is talking about a 2+ cation. For 3.11A, we need to find an element with the configuration [Ar]3d74s2. A parent atom loses 3 valence electrons to form a cation, so going from a cation to a parent atom means that we must add 3 electrons. Therefore, the parent must have the configuration [Ar]3d74s2. The same process can be said about 3.11B.
[Ar] 3d6 for an 3+ cation means that 2 4s valence electrons plus one d electron were taken away. This makes sense for Co, which normally is 3d7 4s2. Two 4s electrons and one d electron was taken away from Co3+. For Fe 3+, two 4s electrons and 1 d electron were taken away leaving 3d5, so this must be Fe, which normally is 3d6 4s2.
Electrons are removed from the outer most energy sub shell. In Co, the electron configuration would be [Ar]3d74s2. Because the cobalt is now Co 3+, 3 electrons are removed. The first 2 are removed from 4s2. The next outermost sub shell is 3d7. So now its just 3d6. In number 9, note that we are only removing 2 electrons not 3.
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