[kapital]

I have a basic question abaut a hydrogen atom which is is one proton and one electron. Electric force beetwen this to is atractive so which force prevent them from coming completly together?

[Corribus]

This was a question that confounded many scientists who studied the hydrogen atom early on. The answer required a whole new physics paradigm known as quantum mechanics.  There's no classical force that prevents the Coulombic collapse of a hydrogen atom. Rather, collapse would violate the fundamental principles of quantum mechanics, which require that as the location of small particle is further confined, it's momentum must increase. If the electron were to collapse into the essentially stationary proton, it's position would be known with infinite precision, and this violates the uncertainty principle. These principles are brought about because small particles are also waves and thus to not behave like baseballs or other macroscopic particles.  Thus the "force" that prevents collapse might be viewed as similar to what is often called a quantum degeneracy pressure. (A similar idea applied on a large scale impacts the fate of stars at the end of their life: in very heavy stars that are no longer sustained by nuclear fusion energy, the force of gravity is so strong that the electron degeneracy pressure is overcome and protons and electrons do collapse into neutrons, overcoming the energy barriers created by the Pauli exclusion principle and forming neutron stars.)

[Enthalpy]

Corribus' answer is just too clear, I have to add my mess...

A particle like 19^th century physicists imagined them would fall on the proton. The electron doesn't because it's a wave, and waves occupy some volume. This problem of the atom's size was the reason to claim "electrons are waves" and introduce the corresponding theory of quantum mechanics.

The kinetic energy of a particle is bigger when the wave changes over a shorter distance. This is natural for an electron moving at uniform speed, it is less if applying the formula when the electron is "immobile", when the wave's amplitude doesn't depend on the time. Quantum mechanics says "stationary" instead, a more differentiated notion, where the electron can occupy all the time the same volume, hence radiate no light, but have a kinetic energy, an orbital momentum, an orbital magnetic momentum.

The kinetic energy maintains an optimum volume for the electron in the atom. If the wave occupies less volume, it changes over a smaller distance, and the kinetic energy increases. Below a certain volume, the kinetic energy increases more quickly than the energy of the electrostatic attraction decreases: this volume gives the smallest energy to the atom, and it is the volume that the electron occupies, it is the atom's volume.

"Volume" would deserve a definition, because the wave extends without limit, it only decreases to a negligible amplitude, so wording like "the amplitude falls below this arbitrary limit outside the volume" would be better.