1. Diatomic (two atom) Hydrogen
Since hydrogen has only one electron shell, it only needs two electrons to be stable.
One way it can become stable is to share an electron with another hydrogen atom.
Many other gases share electrons in this way: Nitrogen (N2) and Oxygen (O2) are the most common examples.
As the electrons orbit the two atoms, they hold the atoms together, this is a covalent bond.

2. Fluorine Fluorine has 7 electrons in its outer shell.
It needs just one more to be stable—this makes it very reactive.
It can share electrons with another fluorine atom to become stable.
This is a covalent compound.

3. Fluorine and Carbon Carbon has 4 electrons in its outer shell.
It can share electrons with up 4 fluorine atoms—making all five atoms stable
Like most carbon compounds, carbon tetrafluoride is covalent.

4. Fluorine and Sodium
Fluorine can also borrow an electron from another atom—like sodium.
Since sodium has only one electron in its outer shell, the shell disappears and it becomes stable.
Each atom now has an electric charge—they are now ions.
Because their charges are opposite, the two atoms are attracted to each other, forming an ionic bond.