How are protons and neutrons combined to create the nucleus of an atom?
Protons and neutrons are not a fundamental particles, rather they made of three particles known as quarks bound together.
In addition to electrical forces from their charges, quarks also feel the strong force, another of the fundamental forces. It is this force that binds together quarks into neutrons and protons and also holds the protons and neutrons together in the nucleus of an atom. As the name suggests, this is a very strong force - it is able to hold together protons in a nucleus despite their like charges repelling each other and remarkably, the further apart you move the particles, the stronger the force gets between them!