Why do different text books say there are different numbers of electrons in the atom's shells?

Question

One text book says the first shell holds two electrons, the second 8 and the third 8 as well, but the other book says the first holds 2, second 8 and the third is 18. What the??? Which one is correct? Why is it different?

Answer 1

It depends on the element that you're looking at. It's true that every element can hold only 2 in first shell, up to 8 in second shell, and third holds the rest.

Answer 2

It sort of depends. Electrons fill up shells in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p..... If book 1 is only looking through element 18, he is right. If book 2 is looking for the total number that are in the 3d level, he is right.