The Four Fundamental Forces

• There are four fundamental forces in the universe. The strong nuclear force (or, simply "strong force"), weak nuclear force ("weak force"), electromagnetic force and gravity. Gravity is so weak as to be virtually irrelevant at an atomic or subatomic level, though it is of immense significance on a macroscopic level. The weak nuclear force, while important in advanced particle physics, is not generally studied in introductory studies of atomic forces. The strong nuclear force and the electromagnetic force are, in contrast, essential to understanding what holds atoms together.
  
  

The Electromagnetic Force in the Nucleus

• The nucleus contains positively charged protons, and neutrons, which have no electromagnetic charge. There is no electromagnetic force holding a proton and neutron together and the attraction due to gravity is virtually insignificant. Two protons are strongly repelled by one another. If the distance between two objects attracted or repelled by electromagnetic forces is increased, the force is decreased by a square of that factor. So if the distance is doubled, the force between them is decreased by a factor of four. If the distance is halved, it is increased by a factor of four. Protons in a nucleus are very close and strongly repel one another.
  
  

The Strong Force

• The electromagnetic repulsion between protons in a nucleus is extremely strong. The force holding them together in spite of this electromagnetic repulsion is called the strong nuclear force, or simply the strong force. Like the electromagnetic force, it diminishes with distance, but the strong force diminishes far more rapidly with increasing distance, and becomes much stronger at short distances. If you were to bring two protons close to one another, the electromagnetic repulsion between them would increase exponentially until they were extremely close together, at which point the attraction due to the strong force, incredibly strong at short distances and extremely weak at greater distances, would overcome the electromagnetic repulsion.
  
  

The Electromagnetic Force Between the Nucleus and the Electrons

• The planetary model of the atom popularized by Niels Bohr depicts the electrons orbiting the nucleus in a manner analogous to planets orbiting the sun. Instead of gravity, the force holding the satellites in place is the electromagnetic force. This is a dramatically oversimplified description, and the forces governing the electron's "orbit" of the nucleus are far more complicated than those governing a planet's motion around a sun. The negatively charged electrons are attracted through the electromagnetic force to the positively charged nucleus in a manner roughly similar to the motion of planets held in orbit by gravitational attraction. This description will need to be revised and expanded as you delve more into the mysteries of the atom.