<P> Additionally, an electron always tends to fall to the lowest possible energy state . It is possible for it to occupy any orbital so long as it does not violate the Pauli exclusion principle, but if lower - energy orbitals are available, this condition is unstable . The electron will eventually lose energy (by releasing a photon) and drop into the lower orbital . Thus, electrons fill orbitals in the order specified by the energy sequence given above . </P> <P> This behavior is responsible for the structure of the periodic table . The table may be divided into several rows (called' periods'), numbered starting with 1 at the top . The presently known elements occupy seven periods . If a certain period has number i, it consists of elements whose outermost electrons fall in the ith shell . Niels Bohr was the first to propose (1923) that the periodicity in the properties of the elements might be explained by the periodic filling of the electron energy levels, resulting in the electronic structure of the atom . </P> <P> The periodic table may also be divided into several numbered rectangular' blocks' . The elements belonging to a given block have this common feature: their highest - energy electrons all belong to the same l - state (but the n associated with that l - state depends upon the period). For instance, the leftmost two columns constitute the' s - block' . The outermost electrons of Li and Be respectively belong to the 2s subshell, and those of Na and Mg to the 3s subshell . </P> <P> The following is the order for filling the "subshell" orbitals, which also gives the order of the "blocks" in the periodic table: </P>

Who proposed the quantum mechanical model of an atom