A bigger molecule doesn't mean new elements
- The idea
- Predict
- See it
- Why
- Check
The idea
“When a molecule gets longer or gains double bonds, it picks up new elements.”
A long fatty acid looks more complicated than a short one, so something new seems to have gone in. What went in was more carbon and hydrogen.
The short version
The mistake: When a molecule gets longer or gains double bonds, it picks up new elements.
What's actually true: A longer chain or a new double bond changes how many atoms there are. A new element comes only with a new kind of part.
- An element list names the kinds of atoms in a molecule. How many of each doesn't change it.
- Monomers join by dehydration synthesis, which removes water at each link. A double bond means two fewer hydrogens. Neither adds a new kind of atom.
- A new element arrives with a new kind of part, like phosphate bringing phosphorus or a nitrogenous base bringing nitrogen.
On the exam: A new element needs a new kind of part. Size and double bonds only change how many atoms there are.