When oxygen is connected only to carbon, are then the oxygen and carbon connected with a double bond?

My teacher showed us three molecules: a carbohydrate molecule, a protein molecule, and a lauric acid molecule. I noticed that, when an oxygen atom is connected to one carbon atom and isn't connected to any other atom, then the oxygen and carbon atoms are connected with a double bond. Is my hypothesis correct, and has anyone ever found it before?

• Yes-ish and yes Feb 1 at 1:15
• Carbon monoxide $\ce{C#O}$
– ron
Feb 1 at 2:21
• @ron So it seems my hypothesis is wrong. Feb 1 at 2:42
• @KamalSaleh, not necessarily wrong. Things are just a little more complicated than they at first appear. 4 is the normal valency of Carbon, but that can sometimes be changed. In the case of CO (carbon monoxide), an electron jumps from the oxygen to the carbon atom, changing the valency of carbon from 4 to 3, and that of oxygen from 2 to 3. They then join with a triple bond. And some elements prefer ionic bonds, which are different again. But start with the simple concepts, and work your way up to the next level after you are confident enough. Feb 1 at 3:37
• @RayButterworth The valence of carbon is by definition always four, nothing will change that. It is definitely not as simple as you make it out to be. It has also nothing to do with bonding. Feb 1 at 18:48

Over 150 years ago August Kekulé had a dream about how atoms might form molecules. You might find it interesting to read about it.

Chemistry is largely about valence, the number of connections that each atom is able to make with other atoms.

In the cases you are talking about, with co-valent bonds, you can imagine each carbon atom as having 4 arms reaching out, each oxygen having two arms, and each hydrogen having 1 arm.

If an oxygen atom is connected to only one other atom, then that connection must use both arms, holding hands with 2 of the carbon atom's arms (just as you noticed), forming a double bond.

The carbon's other two arms could be connected to another oxygen atom, forming CO2 (carbon dioxide), or to 2 Hydrogen atoms, forming CH2O (formaldehyde). Or it could double bond to another carbon atom, which in turn has two more arms to connect to something else, such as C2H5OH (drinking alcohol).

Molecules based on carbon are so common and potentially so complicated, that there is a whole field (organic chemistry) based on it.

If you are at all interested in chemistry, the concept of valence will be essential to your understanding.

• There are counter examples that are commonly rationalized with a negative formal charge on oxygen (as in hydroxide ions, $\ce{OH-}$, or deprotonated alcohols, $\ce{R-O-}$,)or a positive formal charge (as in the $\ce{C#O}$ @Ron mentioned in a comment to the question or in the hydronium ion, $\ce{H3O+}$). Feb 1 at 14:09
• I realize you are trying to keep it simple, but there is the possibility of forming a radical too: pubchem.ncbi.nlm.nih.gov/compound/Ethoxy-radical Feb 1 at 15:59