What is Electron Cloud Model ?

Electron Cloud Model :  In recent models of the atom, electrons form electron clouds, in which electrons travel in spherical and dumbbell-shaped orbitals around the nuclei. Those closest to the nucleus have a spherical orbital called the 1s orbital. The 1s orbital contains up to two electrons. In the second energy level, the first two electrons also travel in a larger spherical orbital called the 2s orbital. Up to six more electrons occupy dumbbell-shaped 2p orbitals, each of which contains two electrons. The three 2p orbitals are arranged at right angles to each other. Additional electrons form the more complicated d and f orbitals.

The orbitals are grouped around the nucleus in shells, each with a letter designation. The innermost, or K-shell, consists of only one orbital, an s orbital. This level has the least energy and is filled first, because atoms and molecules are more stable when they contain the least energy. The next shell, the L-shell, has four orbitals, an s orbital and three p orbitals. Thus, it holds up to eight electrons. The M-, N-, O-, P-, and Q- shells have different numbers of orbitals.

The outermost shell determines the chemical reactivity of the atom. A few elements such as helium, neon, and argon have completely filled outer shells and so almost never react chemically with other atoms.