Is there a difference between energy levels and electron shells?

Question

Is there any difference between energy levels and electron shells, or do they mean the same thing? Does the number of energy levels remain constant when progressing across a period?

Answer 1

Introduction

The absolute energy levels of all electrons in an atom are different, and will shift subtly when electrons are added or taken away. However, it is true that, generally speaking, sets of electrons in an atom will tend to have energy levels very closely clustered together, with big differences in energy to other sets of electrons in the atoms. We say that these electrons are all in the same energy level. On the other hand, election shells have to do with the mathematics of the electrons 'orbitals', and are closely related, but are not the same. If you want to avoid 'hair-splitting' detail at this point, the difference is this -

Energy levels

Each electron in an atom belongs to an energy level in which all the electrons in that level have nearly exactly the same energy, but there are big energy differences between the levels. If you were to strip away all the electrons from an atom and then return them one by one, you would find that you could only put 2 in the lowest level and then 8 in the next. The capacity for the first five levels are 2, 8, 8, 18, 18 in that order. This corresponds to the number of atoms in each of the first five periods of the periodic table and helps explain the properties of elements in the periodic table. This makes 'energy levels' a very useful idea. So energy levels go from 1 to 5 for the first 5 periods of the periodic table, and in simple terms, full energy levels tend to be 'desired' by atoms.

Electron shells

Electron shells are slightly different, but closely related. They are numbered, like the energy levels. The first shell, 'is' the first energy level. Likewise, the second shell, is the second energy level. But after this, it becomes more complicated, and this is where the confusion can come in. With the exception of the first shell, each shell is made up of sub-shells. These sub-shells make up each energy level, but sub-shells can belong to different numbered energy level from shell 3 onwards. To add some detail. There are 3 kinds of shell, 3 ways of satisfying the quantum maths for bound electrons, in the first 5 periods. There are labelled, 's' shells, 'p' shells and 'd' shells. In the mathematics for these, there are 'shell numbers'. For shell number '1', the mathematics only allows for 's'-shells. For shell number 2, 's'-shells and 'p'-shells are allowed, and for shell number 3, s, p and d-shells are allowed. S-shells can take 2 electrons, p-shells 6 electrons and d-shells 10. So shell level 1 has 2 electrons, level 2, 8 (2+6=8) electrons, and level 3, 18 (2+6+10=18) electrons, giving the pattern 2, 8, 18. This is clearly different from the energy levels, and to know why, we have to know more about the sub-shells. The sub-shells are written down with the shell number first, the shell type second, and the number of electrons actually in that shell, third, (usually as a superscript). So Hydrogen's electrons would be 1s1, and Helium's 1s2. For the second energy level, Li would be 1s2 2s1, and further across the period, Carbon, 1s2 2s2 2p2, having six electrons. At the end of the period, Neon would have two full energy levels, written as 1s2 2s2 2p6. (Often lower full shells are ignored, so Neon would be written 2s2 2p6.) So far, electron 'shell numbers' coincide with energy levels. However, the third energy level has only 3s and 3p shells, not 3d. This is because the 3d shell has an energy level between 4s and 4p. So the 3d sub-shell belongs to energy level 4. Similarly, the 4d sub-shell has an energy level between the 5s and 5p sub-shells. These extra 10 electrons in the energy levels 4 and 5 produce the transition metal block in the periodic table.

Conclusion

Shells and energy levels come from the properties of atomic electrons, but one comes from the mathematics of electon orbitals, and the other from the energy that the electrons take in these sub-shells. This means that referring to energy levels as ‘shells’ will produce confusion, when an understanding of ‘sub-shells’ is needed for a better understanding of the periodic table chemistry.

Answer 2

They are different. Take iron as an example. If we using the term shell, the outermost shell should have 16 electrons, according to the "2 8 18" sequence. As for the energy level, the electron configuration of $\ce{Fe}$ should be $\mathrm{(1s)^{2}(2s)^{2}(2p)^{6}(3s)^{2}(3p)^{6}(4s)^{2}(3d)^{6}}$, as the $\mathrm{4s}$ sublevel (subshell) is the furthest from the nucleus, the outermost energy level should have 2 electrons. Shells are more likely to used from a mathematical view since it don't consider the actual energy of each subshell.