An electron can be bound to the nucleus of an atom by the
attractive Coulomb force. A system of several electrons bound to a
nucleus is called an atom. If the number of electrons is different from
the nucleus' electrical charge, such an atom is called an ion. The wave-like behavior of a bound electron is described by a function called an atomic orbital.
Each orbital has its own set of quantum numbers such as energy, angular
momentum and projection of angular momentum, and only a discrete set of
these orbitals exist around the nucleus. According to the Pauli
exclusion principal each orbital can be occupied by up to two
electrons, which must differ in their spin quantum number.