Assertion (A): The magnetic moment of an atom is due to both, the orbital and the spin motion of every electron. 

Reason(R): A charged particle produces a magnetic field. 

A is true but R is false

In an atom, the electrons revolve around the nucleus and can be considered as small current loops. Along with orbital motion, an electron also has a spin motion. So, the total magnetic moment of an electron is a vector sum of its magnetic moments due to orbital and spin motions of the electron. A charged particle in motion produces a magnetic field.