Electron Shells and Subshells
* Electron Shells: Think of these as concentric circles around the nucleus of an atom. They represent different energy levels. The closer the shell is to the nucleus, the lower the energy level.
* Subshells: Within each shell, there are subshells (s, p, d, f) which are slightly different energy levels. Electrons fill these subshells in a specific order.
Helium (He):
* Atomic Number: 2 (meaning it has 2 protons and 2 electrons)
* Electron Configuration: 1s²
* Explanation: Both electrons in helium occupy the first shell (n=1) and the lowest energy subshell within that shell, the 's' subshell. The 's' subshell can hold up to two electrons.
Lithium (Li):
* Atomic Number: 3 (meaning it has 3 protons and 3 electrons)
* Electron Configuration: 1s² 2s¹
* Explanation: Two electrons fill the first shell (1s²) just like in helium. The third electron has to go to the next higher energy level, the second shell (n=2) and its 's' subshell (2s¹).
The Key Takeaway:
Electrons occupy shells and subshells in order of increasing energy. While helium's two electrons fit snugly in the first shell, lithium's third electron must move to the second shell because the first shell is full.
