Why is 3d higher in energy than 4s?

Answer:

The n+l value for 3d orbital is higher than 4s orbital.That is why 3d has higher energy than 4s orbital.

Explanation:

The sum of" n+ l "value determines the energy of an orbital.Here, n is the principal quantum number and l is the azimuthal quantum number.

For l=0,we get "s" orbital.

For l=1,we get "p" orbital.

For l=2, we get "d" orbital.

Now determine the energy of 4s and 3d orbitals by applying n+l formula.

For 4s ,n=4 and l=0

So,n+l is 4+0 =4

For 3d ,n=3 and l=2

So,n+l is 3+2 = 5

Here,we see that the n+l value for 3d is higher than 4s,so 3d has higher energy than 4s orbital.

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