Answer ↓
Protons, neutrons, electrons, quarks and gluons
Explanation ↓
An atom isn't the smallest particle in the Universe - there are many particles much smaller than atoms .
Inside atoms , in the nucleus , are protons and neutrons.
Inside protons and neutrons are quarks , which are "glued" together by gluons.
Around the nucleus are electrons .
So as you can see, atoms are not the smallest particles in the Universe .
