What's the source of centripetal force?

It can be anything.

-- In a planet's orbit, it's gravitation, between the planet
and the sun.

-- In an atom, it's electrostatic, between the positive nucleus
and the negative electron.

-- In a yo-yo, it's the tension in the string. 

-- Inside a car that's turning a corner, it's the friction between
the seat of your pants and the upholstery.

-- On a merry-go-round, it's your grip on the horse you're sitting on.

An  atom  (ours, not carroll's) consists of a positively charged  nucleus, surrounded by one or more negatively charged particles called  electrons. the positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral. most of an atom's mass is in its nucleus; the mass of an electron is only 1/1836 the mass of the lightest nucleus, that of hydrogen. although the nucleus is heavy, it is quite small compared with the overall size of an atom. the radius of a typical atom is around 0.1 to 0.25 nanometres (nm), whereas the radius of a nucleus is about 10-6  nm.  *  if an atom were enlarged to the size of the earth, its nucleus would be only 200 feet in diameter and could easily rest inside a small football stadium.read more on -