The most important properties of atomic and molecular structure may be exemplified using a simplified picture of an atom that is called the Bohr Model.
This model was proposed by Niels Bohr in 1915;
It was revised later on as it is not completely correct, but it has many features that are approximately correct and they were used in the later models.
The correct theory of the atom is called quantum mechanics;
The Bohr Model is an approximation to quantum mechanics that has the virtue of being much simpler.
A Planetary Model of the Atom
The Bohr Model is probably familar as the "planetary model" of the atom. In the Bohr Model the neutrons and protons occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun (but the orbits are not confined to a plane).
This similarity between a planetary model and the Bohr Model of the atom ultimately arises because the attractive gravitational force in a solar system and the attractive Coulomb (electrical) force between the positively charged nucleus and the negatively charged electrons in an atom are mathematically of the same form. (The form is the same, but the intrinsic strength of the Coulomb interaction is much larger than that of the gravitational interaction; in addition, there are positive and negative electrical charges so the Coulomb interaction can be either attractive or repulsive, but gravitation is always attractive in our present Universe.)
Orbits Are Quantized
Quantized energy levels in hydrogen
The basic feature of quantum mechanics that is incorporated in the Bohr Model and that is completely different from the analogous planetary model is that the energy of the particles in the Bohr atom is restricted to certain discrete values.
One says that the energy is quantized. This means that only certain orbits with certain radii are allowed; orbits in between simply don't exist.
These levels are labeled by an integer n that is called a quantum number.
The lowest energy state is generally termed the ground state. The states with successively more energy than the ground state are called the first excited state, the second excited state, and so on.
Beyond an energy called the ionization potential the single electron of the hydrogen atom is no longer bound to the atom. Then the energy levels form a continuum.
In the case of hydrogen, this continuum starts at 13.6 eV above the ground state ("eV" stands for "electron-Volt", a common unit of energy in atomic physics).