physical property boiling point melting point atomic number electron shell atomic mass noble gas alkali metal states of matter physical properties of matter Periodic Table of Elements kinetic energy potential energy energy transformation balanced/unbalanced forces inclined plane Science Density Matter Mass Atom Flammability Reactivity Nucleus proton electron subatomic neutron metalloid ductile malleable hypothesis observation atom density mixture compound electromagnetic wave electromagnetism conduction convection radiation velocity acceleration reflection diffraction gravity inertia work Amplitude Conductivity