The atomic orbitals have merged together to form a molecular orbital. Electrons can only par up if they already have opposite spin.
The usual way of representing a bond is to draw a line between two elements H – H, but there are other ways.
Covalent bonding is about two atoms sharing electrons to gain a full octet (or greater).
Looking at hydrogen chloride – using dot and cross diagrams.
Both the hydrogen and the chlorine now have a full outer shell (octet), even though they are sharing electrons.
A quicker method of doing this is to leave out the circles that represent the energy levels. This demands tidiness!
Yes, look at methane, formula CH4, the simplest hydrocarbon.
Notice the electrons are as far apart as possible.
Remember this is a representation of a 3-dimensional structure, and we need to develop special rules for drawing these in 3D.
In 2D, this is how we could draw the structure.
However in 3 dimensions, it looks a little like this:
The hydrogen atoms forms the corners a tetrahedron.
The above image requires Chime, a molecular viewer. Click on the software link above if you don't have it installed (its worth it and its free)
In organic chemistry there are double and triple bonds as well as single.
Eg for Oxygen
So a representation of oxygen:
This occurs in Nitrogen, 5 electrons in the outer sub-shell.
We can represent this as:
H2S, HI, CH3OH, PCl3, SiO2, NH3