Atoms are made up of neutrons and protons located within the nucleus and electrons around the nucleus. Electrons inhabit “orbits” that surround the nucleus. Calling them “orbits” is misleading, they are more like areas of probability where electrons are likely to be found called orbitals.  The concept of orbitals will be discussed at another time. All that needs to be known for this article is that orbitals are found in different energy levels called shells, each shell is a different distance from the nucleus.

Notice in the figure above that the atom is split up into circular paths around the nucleus (the area that shows p and n). These paths are the shells. Notice how the first shell contains only 2 electrons. This is a law that all atoms will follow – up to two and only 2 electrons can be in the first shell. Notice how there are 8 electrons in the second shell; the second shell will contain up to 8 electrons. Chlorine only contains 3 total shells and the 3rd shell contains 7 electrons, but the 3rd shell in any atom can contain up to 18 electrons. The number of electrons that can exist in a shell is given by 2n^2, where n is the energy level (also called principal quantum number, but more on that later). A full and stable outer shell contains 8 electrons for groups IVA (14) through VIIA (17) on the periodic table (this is called the octet rule, developed in 1916 by German chemist Walther Kossel and American chemist Gilbert Newton Lewis) and heavily applies to chlorine. The electrons located in the final shell of an atom are called valence electrons. These are the electrons that allow for chemical reactions between atoms. Because chlorine only has 6 valence electrons, it wants two more in order to have a full eight. This is why atoms come together to form molecules and react with one another, in order to stabilize their valence shell. There are many exceptions to the octet rule, the most notable being hydrogen and helium, which only need 2 valence electrons because they only have 1 electron shell around their nuclei. As you recall this shell only allows up to two electrons within it.

Some atoms, like sodium, only have one valence electron and it is easier for them to give away an electron in order to be stable. Atoms like chlorine have 7 valence electrons so they seek another electron to be stable.

Source: https://www.youtube.com/watch?v=mVNMRkl0H9E

This desire for electrons is called electronegativity. Atoms like chlorine are very electronegative – they want electrons. Whereas atoms like sodium, are not very electronegative; they want to give away an electron. The electronegativity of an atom can be calculated using the Mulliken equation, but since this equation requires an understanding of ionization energy and electron affinity (which is different from electronegativity) and will be discussed at a later time. In the Mulliken equation, the IE is the ionization energy and EA is the electron affinity. 

Source:http://calistry.org/calculate/mullikenScaleElectronegativityCalculator#:~:text=This%20online%20chemistry%20calculator

For this article, we will use an electronegativity chart, which shows the electronegativity number for each atom on the periodic table.

The higher the electronegativity number located below the elemental symbol, the higher the electronegativity of the atom. For example, you will notice that chlorine (Cl) is much more electronegative at a 3.16 than sodium (Na) at a 0.93. Differences in electronegativity between two atoms can give rise to 3 different types of chemical bonds

These 3 bonds are covalent, polar covalent, and ionic. Covalent means that between 2 atoms there is not a lot of difference in electronegativity and electrons are perfectly shared between the 2 atoms. Polar covalent means that there is a decent difference in electronegativity between 2 atoms and the electrons will hang around the more electronegative atom more than the less electronegative atom. Remember that an ion is an atom that has gained or lost an electron (or electrons). If we have 2 atoms: Atom 1 and Atom 2 and atom 1 has a much, much higher electronegativity rating, then atom 1 will completely take away Atom 2’s electron(s) and both atoms will become ions. Atom 1 will become negatively charged, atom 2 will become positively charged and the two will be attracted to each other forming an ionic bond.

You can use the numbers in the table to find the electronegativity difference: subtract the smaller electronegativity rating from the larger electronegativity rating. 

Depending on the difference in electronegativity between the 2 atoms, you can tell what type of bond it is.

• If the difference is less than 0.4 then the bond is covalent and the electrons are shared between the 2 atoms
• If the difference 0.4 and 1.8 then the bond is polar covalent and the electrons will  move more toward or more often be located near the more electronegative atom, but the electrons are still shared
• If the difference is greater than 1.8 then the bond will be ionic and the electrons will be taken completely away from the less electronegative atom to the more electronegative atom. This will create 2 oppositely charged atoms that are then attracted to each other and bonded in this way. 

The only naturally occurring atoms that don’t form bonds are the atoms in the final column of the periodic table. These atoms have full outer shells and therefore don’t react with other atoms. They have been termed the noble gases.

Source: https://scienceaid.net/Noble_Gases:_Trends_and_Patterns

One last property of shells to note is as electrons move into shells further away from the nucleus, they move into higher and higher energy states. In other words, the electrons that reside in the final shells of the atom (the valence electrons) have the highest energy. The electrons that reside in the first shells (the one around the nucleus) are in the lowest energy state. If an electron gains energy in some way, it will move up an shells. This is another way that ions will form, if an electron in a certain atom gets hit with enough energy then it will leave the atom entirely.