which key element contained in the Baroque music that classical music left out

the return to "classical" thought

Hydrogen. In classical atomic theory, it contains only a proton.

an elemnt can form several bonding depending on the elctronegativity. Types of Bonding: Covalent (QM) Ionic (classical) Dipole-dipole (classical) Ion-dipole (classical) Van der Waals forces (QM and classical)

The "fire element" has many varied meanings, depending on which philosophical, religious or cultural thought it is applied to. Several explanations of the phrase are given in the Wikipedia entry for "Fire (classical element)", which can be found below.