How many electrons fit in the 3rd energy level?
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How many total electrons fit in the 3rd energy level including the 1st and second?
18
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How many electrons fit in just the 3rd energy level, the d subshell?
10
Electron shell
https://en.wikipedia.org/wiki/Electron%20shell
Energy level
https://en.wikipedia.org/wiki/Energy%20level
In chemistry and atomic physics, an electron shell, or principal energy level, may be thought of as the orbit of one or more electrons around an atom's nucleus. 
Energy level
Each shell is composed of one or more subshells, which are themselves composed of atomic orbitals. For example, the first (K) shell has one subshell, called 1s; the second (L) shell has two subshells, called 2s and 2p; the third shell has 3s, 3p, and 3d; the fourth shell has 4s, 4p, 4d and 4f; the fifth shell has 5s, 5p, 5d, and 5f and can theoretically hold more in the 5g subshell that is not occupied in the ground-state electron configuration of any known element.
Electron shell Subshells
In chemistry and atomic physics, an electron shell, or principal energy level, can be thought of as the orbit of one or more electrons around an atom's nucleus. Each shell is composed of one or more subshells, which are themselves composed of atomic orbitals. For example, the first shell known as K shell, has one subshell, called 1s and can hold 2 electrons, second shell called L, has two subshells 2s and 2p, these can hold 6, the third shell has 3s, 3p, and 3d can hold 10. With this formula a total of 18 electrons can fit inside the 3rd energy level. 