Hi,
Iodine has 53 electrons and configuration of: 2, 8, 18, 18, 7.
Why does the fourth shell not reach maximum and therefore it picks up 7 electrons and begins a new shell.
Thank you.
It has to do with the order in which the subshells fill; i.e., after 18 electrons are in the 4th shell, (that's the 1s2(K shell 2) 2s2 2p6(Lshell 8) 3s2 3p6 3d10(M shell 18) 4s2 4p6 4d10(N shell 18) 5s2 5p5), the 5s and 5p subshells are lower energy and fill next. Here is a diagram from Wikipedia that shows how the filling takes place.
http://en.wikipedia.org/wiki/Electron_configuration#/media/File:Electron_orbitals.svg
To understand why the fourth shell in iodine's electron configuration does not reach its maximum capacity and fills up the seven remaining electrons in a new shell, we need to understand the principles of electron filling and the rules governing it.
The electron filling pattern in atoms follows a specific order known as the Aufbau principle, which states that electrons fill into the lowest available energy level first. According to the Aufbau principle, the electron configuration of iodine (I) can be written as follows:
1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 5s^2 4d^10 5p^5
Each shell can hold a specific number of electrons, which is given by the formula 2n^2, where 'n' represents the energy level (shell number). Here's a breakdown of the maximum electron capacity for each shell:
- First shell: 2 electrons (2 x 1^2 = 2)
- Second shell: 8 electrons (2 x 2^2 = 8)
- Third shell: 18 electrons (2 x 3^2 = 18)
Now, when we move to the fourth shell, according to the formula, it should hold a maximum of 32 electrons (2 x 4^2 = 32). However, the actual electron configuration of iodine only fills up to 18 electrons in the third shell (3s^2 3p^6), and the remaining 14 electrons are not filled in this shell.
The reason for this deviation from the expected electron filling pattern lies in the energy distribution of orbitals. The 4s orbital is actually at a slightly lower energy level than the 3d orbital, creating an exception to the Aufbau principle. So, the 4s orbital gets filled before the 3d orbital.
In the electron configuration of iodine, the 4s orbital is completely filled with 2 electrons (4s^2), but the 3d orbital is still vacant. Then, the 5s orbital gets filled with 2 electrons (5s^2), following which, the remaining 14 electrons finally fill the 4d and 5p orbitals (4d^10 5p^5).
In summary, the electron configuration of iodine deviates from the maximum capacity of the fourth shell because the 4s orbital fills before the 3d orbital due to a slight difference in energy levels. This leads to the 4d and 5p orbitals being filled with the remaining 14 electrons, divided between them (4d^10 5p^5).