Why are electrons revolving around the nucleus?

What provides the centripetal force in the following cases electron revolving around the nucleus

An atom has mass number A and atomic number Z.) How many electrons are revolving around the nucleus ?

Calculate the speed of the electron revolving around the nucleus of a hydrogen atom in order that it may not be pulled into the nuclues by electrostatic attraction.

Show that Bohr's second postulate 'the electron revolves around the nucleus only in certain fixed orbits without radiating energy can be explained on the basis of de-Broglie hypothesis of wave nature of electron.

Estimate the ratio of the wavelength associated with the electron orbiting around the nucleus in the ground and first excited state of hydrogen atom.

In hydrogen atom, electron revolves around the nucelus along a path of radius 0.51overset(@)A making 6.8 xx 10^15 revolutions per second. Calculate the equivalent current. Given that charge on electrno =1.6 xx 10^-19C

Show that circumference of the Bohr orbit for the hydrogen atom is an integral multiple of the de-Broglie wavelength associated with the electron revolving around the orbit.