Elements, Compounds, and PB&J Sandwiches

You hear the words "element" and "compound", "atoms" and "molecules", but do you know how they are related? Well, Dr. B is here to tell you, using the analogy of a peanut butter-and-jelly sandwich. Let me explain...

Elements are the basic building blocks of matter. Elements are composed of tiny, tiny atoms. Compounds are combinations of elements made up of very, very small molecules. Picture a jar of peanut butter, a jar of jelly, and a loaf of bread sitting on the kitchen counter. Is this a PB&J sandwich? No, not yet. But you can use these "elements" to create a "compound", a delicious PB&J sandwich. Get two slices of bread, spread a little peanut butter on one and a little jelly on the other, put them together, and presto! - a peanut butter-and-jelly sandwich:

In the same way, cadium (gray balls) and sulfur (yellow balls), which are two elements, can be combined to make the compound cadmium sulfide, which is used in photocells:

In a similar way, you can combine:

• the elements hydrogen and oxygen to make water, or
• the elements sodium and chlorine to make table salt, or
• the elements carbon, hydrogen, and oxygen to make sugar, or
• the elements carbon, hydrogen, oxygen, and nitrogen to make ibuprofen, or
• other elements to make thousands and thousands of compounds

So where do atoms and molecules come in? Atoms are simply the smallest bits of an element that are still recognizable as that element, and molecules are the tiniest bits of a compound that are still recognizable as that compound. You can have a whole loaf of bread slices, and you can divide the loaf into half, then into half again, and so on. Eventually you come to just a single slice of bread. To make it any smaller, you'd have to cut the bread, but then it's not a slice any more - it's just part of a slice. If you could keep dividing an iron nail in the same way, after a long time, you'd come down to just a single iron atom.

In a similar way, you could start separating an enormous pile of PB&J sandwiches, but sooner or later, you'd end up with one sandwich, and you couldn't divide any more without cutting it. You can have a whole tankful of methane (natural gas), a compound of carbon and hydrogen, but after dividing and dividing, eventually you'd get to one methane molecule.

Is this discussion making you hungry? Maybe you want to go make a sandwich for yourself (but check with an adult first!)!