Re: What different effects do different compounds with the same elements have?

The short answer is that it's very difficult to predict how a molecule 
will behave.

There are many different ways of arranging atoms with the same formula, 
indeed C20H20 could be a very long chain 20 atoms long, a sphere, or 
something all together more complicated than that, depending on how the 
atoms were arranged. By introducing double bonds throughout the molecule 
the number of hydrogens would change, and where the double bonds are would 
change the properties of the molecule.

There is a whole field of chemistry that tries to engineer "designer" 
molecules, for specific purposes. Sometimes they are successful, and can 
build something just because they want to, but more often than not the 
laws of physics and thermodynamics get in the way.

In general, organic compounds (consisting mostly of carbon) are 
combustible as it will burn to give carbon dioxide and water, regardless 
of structure. Indeed, petrol and diesel are both organic molecules and it 
is because of their combustibility that they are commonly used as fuels.

Carbon monoxide is toxic because it is similar in size and shape as a 
diatomic oxygen molecule (O2). This means it can bind to the haemoglobin 
in your blood, but once bound it won't dettach itself like oxygen does, 
and so you would die due to lack of oxygen. This is on case where the 
structure is very important, as anything that is not similar in size and 
shape would not be toxic in the same way.