The number of electrons in the outermost shell tells you how reactive the atom is because it tells you how many it needs to feel full, or how many it can lose. Valence electrons are the highest energy and furthest out electrons. In general, elements are less reactive when their outermost shell is full.
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When considering the d block (which contains iron (Fe)), the period numbers can be a little confusing. If we look at the number on the left, it might seem that iron should be 4d, because it is in the 4th period.
However, the rows for d block are lettered differently. Notice that there are four rows in the d block. Starting from the top, they are 3d, 4d, 5d and 6d. Iron is in the first row of d block, so it is 3d.
Neutral iron has 26 electrons.
We start with [Ar] as the base because that brings us to 18 electrons.
Then we move down to the forth s block (which starts with potassium (K)). That gives us 4s2. So now we are up to 20 electrons.
We continue into d block on the same row which, as we mentioned is 3d (this starts with scandium (Sc)) we move over to iron, which is 3d6. That gives us the remaining 6 electrons…for a total of 26.
So, iron is [Ar] 4s2 3d6
Why isn’t Fe noted as [Ar] 4S2, 4D6? It was 3D6 in the video. Thanks!