.THE BOHR MODEL
•When atoms are heated they give off electromagnetic radiation (light). This light is passed through a prism and a number of bands are observed- called emission spectra.
•Niels Bohr developed an new model in 1913 that explained the emission spectra of atoms.
The Bohr model proposes:
•Electrons are found in fixed circular orbits around the nucleus
•These orbits correspond to different energy levels in the atom
•Electrons are notfound between energy levels
•Larger radii correspond to higher energy levels.
·Electrons move in energy levels called shells. Electrons move around the nucleus in these shells in pathways called orbitals.
•The arrangement of electrons in the shells is called the atoms electronic configuration.
The maximum number of electrons which each shell can hold is 2n^2 where n is the shell number or energy level
•The Valence electrons (outer shell electrons) are the electrons involved in chemical reactions.
•The outer shell of elements never have more than 8 electrons.
•During chemical reactions atoms bond in such a way to achieve a stable outer shell of 8 electrons- octet rule.
•In 1926, the German scientist Erwin Schrodinger proposed that electrons behaved as waves.
•He proposed that the electrons occupy a three- dimensional space called an orbital around the nucleus.
•Schrodinger found that:
•There are major energy levels- shells
•Shells contain separate energy levels of similar energy- called subshells- s,p,d,f.
Electron configurations - s,p,d, f model
Electron Configuration using Sub Shells:
•Shells consist of subshells (called s, p, d and f) with slightly different energies.
•Orbitals are regions of space in which electrons of a particular energy orbit.
•Orbitals may be empty or may contain a maximum of 2 electrons.
•Electrons always tend to occupy the level of lowest energy.
•Orbitals in shells are filled in order of increasing energy.
1s < 2s < 2p < 3s < 3p < 4s < 3d etc…