- A substance is more radiopaque if it contains atoms of high atomic number (AN) such as calcium, iodine, barium, or lead.
- Bone, which contains calcium (AN 20), is more radiopaque than soft tissue, which is made up mostly of carbon (AN 6), hydrogen (AN 1), and oxygen (AN 8).
- Iodine (AN 53) is the key constituent of radiocontrast material and lead (AN 82) is an effective barrier to x-rays.