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Yeah, it has to do with shielding. In hydrogen (one electron), 2s and 2p have the same energy when the electron is excited. However, when there are multiple electrons, the 2s sub-shell has a slightly lower energy than 2p.
Even thought they are in the same shell, n = 2, they are in different subshells. The 2s subshell provides slight electron shielding to the 2p subshell which means that the effective nuclear charge is not felt by the 2p electrons as strongly.
705198479 wrote:how do I know how many bonds or "dots" go on the ion in the lewis structures
The number of dots on an atom is the same as the number of valence electrons that the atom has. So for carbon, for example, you would draw four dots as it has four valence electrons.
Can someone explain what a shell is vs a subshell?
The shell is the n quantum number (broadest division of electrons in an atom). Shells correspond to the rows in the periodic table.
The subshell is the s-p-d-f classification of orbital groups (quantum number l.) Shells are divided into subshells (3s, 3p, 3d).
As "l" (the number corresponding to the orbital) increases, the orbitals become increasingly less likely to penetrate the nucleus. Since the electrons spend more time further from the nucleus as l increases, they also experience more repulsion from other electrons. This means that the energy level will increase slightly as the value of l goes up.
705198479 wrote:is there a rule that restricts a sub shell ? is it important to remember for this class?
you should know how many electrons each shell can hold; the s shell can only hold 2, p holds 6, d holds 10 and f holds 14
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