In the early 20th century, an English scientist, Ernest Rutherford, and a Danish scientist, Niels Bohr, developed a way of thinking about the structure of an atom that described an atom as looking very much like our solar system. At the center of every atom was a nucleus, which is comparable to the sun in our solar system. Electrons moved around the nucleus in 'orbits' similar to the way planets move around the sun. (While scientists now know that atomic structure is more complex, the Rutherford-Bohr model is still a useful approximation to begin understanding about atomic structure.)
Opposite electrical charges of the protons and electrons do the work of holding the nucleus and its electrons together. Electrons closer to the nucleus are bound more tightly than the outer electrons because of their distance from the protons in the nucleus. The electrons in the outer orbits, or shells, are more loosely bound and affect an atom's chemical properties. A delicate balance of forces among nuclear particles keeps the nucleus stable. Any change in the number, the arrangement, or energy of the nucleons can upset this balance and cause the nucleus to become unstable or radioactive. (Disruption of electrons in the inner orbits can also cause an atom to emit radiation.) The amount of energy required to break up the nucleus into its parts is called the binding energy; it is often referred to as 'cosmic glue'. This is the same amount of energy given off when the nucleus formed.
'The strength and weakness of physicists is that we believe in what we can measure. And if we can't measure it, then we say it probably doesn't exist. And that closes us off to an enormous amount of phenomena that we may not be able to measure because they only happened once. For example, the Big Bang. ... That's one reason why they scoffed at higher dimensions for so many years. Now we realize that there's no alternative... '