 Many times in chemistry, we'll see different molecules that have the same constituent atoms. For example, these two molecules here, they both have four carbons, one, two, three, four. One, two, three, four. So if I were to write their chemical formula, BC four, and then they both have one, two, three, four, five, six, seven, eight, nine, 10, one, two, three, four, five, six, seven, eight, nine, 10 hydrogens. So both of them, both of them have the chemical formula, C four, H 10, C four, H 10, but they're still fundamentally different molecules, and you can see that because they have different bonding. For example, over here we have a carbon that is bonded to three other carbons in a hydrogen. Over here I can't find any carbon that's bonded to three other carbons. I can find ones that are bonded to two other carbons, but not one that's bonded to three other carbons. So how we've put the atoms together is actually different. They're bonded to different things. And so when we have the situation where you have the same constituent atoms, where you have the same chemical formula, but you're still dealing with different molecules because of either how their bonds are made or what their shape is, we call those isomers. So an isomer, isomer, you have the same chemical formula. Same chemical formula. But you could have different bonding, but different, different bonding, or shape, bonding, shape, or orientation, orientation. So over here you have just different bonding. And this type of isomer is called a structural isomer. So these characters are structural isomers. Same constituent atoms, but different bonding. Structural isomers. So that's structural isomers right over there.