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In the modern model of the atomic structure, the nucleus of the atom is surrounded by a "cloud" of electrons which are constrained to move within well defined energy levels. Each principle energy level is a measure of the average energy the electrons have that are in that level due to their distance from the positively charged nucleus.
Each principle energy level is further subdivided into regions of space where the electrons are allowed to exist (that is, areas where the electron could actually be located and areas where they are forbidden to be). The regions of space within the principle energy level that electrons are allowed to occupy are called orbitals. Unlike orbits, which are fixed paths the electrons need to follow, orbitals are simply a region of space where an electron may be found.
Each principle energy level has its own unique number or orbitals that matches the energy level number: that is, energy level 1 has 1 orbital; energy level 2 has 2 orbitals; etc. . .
There are four different kinds of orbitals which are defined by their shapes. "s" orbitals are spherical shells, "p" orbitals are three sets of perpendicular lobes, "d" orbitals are five sets of disks, "f" orbitals are really funky shaped. The different shapes also have slightly different energies as they are not all equal distance from the nucleus. In general "s" is closer than "p", "p" closer than "d", and "d" closer than "f".
The general rule is that as one adds electrons to larger and larger atoms, they fill the energy levels closest to the nucleus first just as filling a glass with water fills the bottom of the glass first. Sometimes one will use an Aufbau chart to help keep track of the order of filling for the electrons, especially as the atoms become larger and have exceptions.
For fluorine, which has 9 electrons, the electron cofiguration would be
1s2 2s2 2p5
This is telling us that there are 2 electrons in the 1 principle energy level within an "s" orbital; 2 electrons in the 2nd principle energy level occupying an "s" orbital; and 5 electrons in the 2nd principle energy level occupying a "p" orbital.
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