Why does the fourth orbit have 18 electrons?
why is that? when the the 2 before only have 8 electrons?
Answers:
The 1st orbit contains only an s-subshell
The 2nd orbit contains both an s-subshell and 3 p-subshells (4 subshells = eight e-)
The third orbit actually contain an s-subshell, 3 p-subshells and 5 d-subshells (9 subshells = 18 e-)
The reason you have written 4th orbit is because the 4s-subshell is filled before the 3d-subshell and this is why the transition elements start in the 4th period not the third.
The number of types of subshell is equal to the period number. So the fourth orbit can contain 32 electrons in one s-subshell, 3 p-subshells, 5 d-subshells and 7 f-subshells. These f-subshells are not filled until much later on (in the 6th period I think - but still the fourth orbit).
electrons are social creatures (witness how they love to make currents) but they also like their space
there's a lot more space in the outer orbits of atoms (as we are talking about a nominal sphere, a small increase in radius means a huge amount more "surface area") so you can have more electrons in stable orbits without them interfering with one another
of course atoms are not really miniature solar systems and electrons are fuzzy wave-particles, but you get the picture
why 18?
all this talk about shells and stuff is really just primitive modelling - equivalent to Newton's model of gravity, so 18 just seems to work until we find out more
God knows
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