Shell model of Atom
The above electron configuration shows how electrons are arranged in an atom or molecule.
Shell model:
The shell model of the atom describes the arrangement of electrons around the nucleus of an atom in terms of energy levels or shells.
Three basics points will help to understand the arrangement of electrons around the nucleus of an atom.
1. The Atomic Number (number of protons) = The number of electrons in an atom.
2. The maximum number of electrons in the outer shell is eight for neon atom.
3. The maximum number of electrons in the inner shells is 2n2 where n = the shell number.
| Shell number, n | 1 | 2 | 3 | 4 | 5 |
| Maximum number of electrons in shell, 2n2 | 2 | 8 | 18 | 32 | 50 |
Every element on the last column of the Periodic table is chemically inert. In this model, this phenomenon is explained by shell–filling. Successive atoms become smaller because they are filling orbits of the same size, until the orbit is full, at which point the next atom in the table has a loosely bound outer electron, causing it to expand. The first Bohr orbit is filled when it has two electrons, and this explains why helium is inert. The second orbit allows eight electrons, and when it is full the atom is neon, again inert.