Here's a breakdown:
* Atomic Number: Calcium has an atomic number of 20, meaning it has 20 protons and 20 electrons.
* Electron Configuration: The electronic configuration of calcium is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s².
* Shell Filling:
* The first shell (n=1) can hold a maximum of 2 electrons (1s²).
* The second shell (n=2) can hold a maximum of 8 electrons (2s² 2p⁶).
* The third shell (n=3) can hold a maximum of 18 electrons, but it's not fully filled in calcium.
* Why not 10 electrons in the 3rd shell?
* The 4s subshell (which is part of the 4th shell) has lower energy than the 3d subshell. This is due to the shielding effect of inner electrons, which makes the 4s subshell more stable.
* Therefore, the 4s subshell is filled first, holding 2 electrons (4s²) before any electrons go into the 3d subshell.
In summary: Calcium has 8 electrons in its 3rd shell (3s² 3p⁶) because the 4s subshell is filled first, leaving the 3d subshell empty.
