The stability of matter compared with energy

[newtonian]

How stable are the different relatively stable atoms?<br /><br />I know K-40 (radioactive Potassium) has a half-life a few billion years long.<br /><br />Do all atoms eventually decay (as in radioactive decay, but much slower)?<br /><br />At the present time, is energy or matter increasing relative to the other (assuming our universe is not having input of energy from beyond our universe - compare dark energy)?<br /><br />This also would involve virtual particles appearing from "empty" space - as in quantum effects.<br /><br />How stable are quarks - can they last forever?<br /><br />Are neutrons, for example, more stable than photons (I know it depends on the environment)?<br /><br />Can a photon last forever?<br /><br />Can any forms of matter or energy convert to dark energy?<br /><br />Or vice versa?