Let us now consider taking a bare nucleus, charge Z , and adding Z electrons to it one by one. From the Pauli exclusion principle, each electron must be in a different state . Thus He has two electrons in the 1 s state. Li must have two electrons in 1 s , and one electron in 2 s . This means that 10 electrons fill closed shells.
Atoms interact chemically by sharing or partially transferring electrons.
Not surprisingly, atoms with filled shells only, like He and Ne, are chemically unreactive. To some extent, valency can vary depending on the strength of attraction of other atoms in the chemical environment. Filling a Box with Electrons When many Li atoms are put together to form a solid, it is found that the loosely attached outer electrons leave their original atoms and wander freely throughout the metal. Each such plane wave state in the box can be represented by three numbers n x , n y and n z , representing the number of nodes of the standing wave in the x , y and z directions respectively. Extending slightly our analysis of an electron in a two-dimensional box, the energy of such a state will be .