Explain Sommerfeld theory

Sommerfeld theory: The theory was originally proposed in 1900 to define and correlate the electrical and thermal properties of metals. Later, quantum mechanics became the basis for the theory of most of the common properties of simple metals like as sodium, with one free   electron per atom, magnesium with two, and aluminum with three. Transition metals, like as iron, have partially filled electronic d states and are not treated by the free-electron model