Warning! Spoilers for Ant-man!!!
So if you watched Ant-man you should already know how the Pym Particles work and that Scott shrank down to subatomic size. This article states that that is impossible using the "physic" laws in the movie.
What does subatomic mean?
It basically means smaller than an atom. For an example protons, neutrons and electrons are subatomic.
How do the Pym Particles work?
In the movie Hank Pym aka the original Ant-man says that the Particles reduce the space between atoms which leads to shrinkage of the person or thing, keeping its mass and energy.
Why is it impossible to shrink to subatomic size?
Now that you know these two things you can work out that the smallest size something can shrink to using the Pym Particles is the size of all the atoms in your body plus a tiny amount off space between them (You don't want any nuclear reactions do you?). Hence that size is still going to be bigger than one atom which means you can't shrink anything to subatomic size using Pym Particles.
Is there any way of shrinking stuff into subatomic size?
I personally think it's impossible but for the sake of the movie it would be more believable if the Particles reduced the size of the atoms not the space between the atoms.
Thanks for reading!