Question

According to the 2-8-8 rule, the 3rd orbital has only 8 electrons in its valence orbital, but from the formula 2n^2, the 3rd shell would have 18 electrons in its valence orbital. Why is it so?​

Answer 1

Answer:

There is something called shielding. In this concept, inner electrons shield the outer electrons from Seeing the nucleus. Kind of like being a few rows back in the mosh pit from a concert. The people in front of you sometimes block your view. If you cannot see then you will not feel the same attraction.

So, if we compare electrons in orbitals like 3s, 3p, 3d, it turns out that the electrons in the 3s orbital have the ability to sneak in close to the nucleus so that it is not so shielded by the inner electrons (1s , 2s, and 2p). The 3p orbital cannot sneak in quite so close so it is shielded a bit more than the 3s. Finally, the electron in 3d is so bad at sneaking in close to the nucleus that it is shielded most from the nucleus. The closer the electron (in the orbital) can get to the nucleus, the lower its energy. So, in any n level, the s is lowest, the p is higher and the d is higher still in energy. The 4s (which, like the 3s, can sneak all the way into the nucleus) is actually lower in energy than the 3d (they didn’t start all that much apart anyway). See the figure below.

Thus, as you go across the rows in the periodic table, you get to the 4s before you get to the 3d because 4s is lower, it’s first. In the third row, the 3s and then the 3p orbitals get filled up but then the next in line is the 4s because it is actually less shielded than the 3d orbital electrons. The energy level diagram for potassium shows what I’m saying. The 4s electrons are lower than the 3d electrons. That means that ground state atoms will fill the 4s orbital before filling the 3d orbitals. Thus, The rows in the periodic table are as follows

1s

2s 2p

3s 3p

4s 3d 4p.

So, the first three rows are finished in the periodic table before you get to the 4s orbitals, and then the 3d orbitals.

Answer 2

Answer:

Due to octet rule

Explanation:

This bcoz of octet rule. In the last decade of nineteenth century,after the discovery of noble gases by Rayleigh and Ramsey , Kossel proposed that since the noble gases don't normally react by transfering or sharing of electrons there outermost shell configuration is a stable configuration of eight electrons(octet).

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