A covalent bond is where electrons are shared between two nonmetal atoms.
Let’s start with hydrogen, which is a nonmetal on the left side of the periodic table.
Hydrogen has an atomic number equal to one which means that it has one proton and one electron.
If we take two hydrogen atoms, each with its own electron, we can combine them to form a molecule, where each atom contributes one valence electron to the final molecule.
In other words, these two valence electrons are shared by both atoms in the final molecule.
We can represent this process by putting two dots between the two hydrogen atoms.
These two dots represent one covalent bond.
We can also represent a covalent bond by drawing a straight line between the hydrogen atoms.
One straight line means one covalent bond and represents two valence electrons.
Now, in our hydrogen molecule, opposite charges attract.
So the one electron from the hydrogen atom on the left is attracted to the positively charged nucleus of the hydrogen atom on the right.
At the same time, the negatively charged electron from the hydrogen on the right is attracted to the positively charged nucleus of the hydrogen on the left.
So a mutual attractive force holds together the two atoms in a covalent bond.
At the same time, like charges repel, which keeps the two positively-charged nuclei from getting any closer together.
Another way to represent the hydrogen molecule is by writing H with the subscript two.
This implies that we have two hydrogen atoms in our molecule.
So hydrogen is an example of a diatomic molecule, which is a molecule composed of two atoms.
There’s an easy way to remember a list of the elements that form diatomic molecules.
Imagine a mad scientist who is named Dr. Brinclhof, whose name spells out each of the diatomic molecules: “Br” is bromine, “I” is iodine, “N” is nitrogen, “Cl” is chlorine, “H” is hydrogen, “O” is oxygen, and “F” is fluorine.
Let's try drawing some of the structures of these molecules, starting with the example of the fluorine molecule, F2. Fluorine is in group 7A on the periodic table.
Because the group number is equal to the number of valence electrons, we know that each fluorine atom has seven valence electrons around it.
Fluorine is most stable when it has a full set of eight electrons around it, which is referred to as an “octet.”
For this reason, it will tend to form bonds that allow it to get an octet of electrons.
It gains an electron by sharing one of the seven electrons from another fluorine atom to create a covalent bond.
There are therefore two shared electrons in the covalent bond, one from each atom.
Each fluorine atom in a diatomic fluorine molecule has a full octet, consisting of seven electrons from its original set of electrons, and the eighth electron shared with the other, neighboring fluorine atom.
Now, let’s look at the oxygen molecule, O2. Oxygen is in group 6A on the periodic table, so each oxygen atom has six valence electrons.
We can draw these in around each oxygen atom.