Explain the Bohr's model?

Bohr's Model :  Electrons move around the nucleus at tremendous speeds and occupy most of the space in an atom. The exact position or location of an electron at any given moment can only be predicted on the basis of probability.
In a widely accepted model of the atom originally proposed by Neils Bohr, electrons move in spherical spaces called orbitals, or shells, which correspond to different energy levels. Electrons are distributed according to their energy levels, with the higher energy electrons residing in the outer shells.
The innermost shell contains only 2 electrons. In common elements, the next outer shells contain 8 electrons each.

Atoms either gain, lose, or share electrons in the outer shells. Because the outer shells of many atoms are incomplete, most atoms will interact with other atoms during chemical reactions to achieve stable outer shells.
The number of electrons that an atom must either gain, lose, or share to complete the outer shell is known as its valence, or oxidation number. For example, carbon has six electrons, two in its first energy level and four in the outer level. Thus, it can form a stable outer shell by gaining, losing, or sharing four electrons to complete its outer shell when it joins, or bonds, with another atom or atoms to form a compound.
The following table lists the oxidation numbers of some important ions frequently used in Biology.