Pure elemental mercury is shown being poured from a beaker into a Petri dish. This liquid consists of only mercury atoms. Image from here.
Elements are the building blocks of chemistry. A pure element is made up of identical atoms and can be separated out from other matter. All matter is made up of elements. Some matter consists of just one type of element while other matter consists of many types of elements. For example, water consists of two different elements: hydrogen and oxygen. Sugar, one of our favorite things to consume, and consists of three different elements: carbon, hydrogen, and oxygen. Sweet.
While we are on the topic of food, let's make an analogy between elements and ice cream. There are a handful of basic flavors of ice cream such as the standard vanilla, chocolate, and strawberry flavors. Let's say each of these are "elemental types of ice cream." If we wanted to create new crazy flavors, a là Ben and Jerry's, we would start mixing together some of the standard "elements" like vanilla swirled with strawberry ice cream. These new mixtures are analogous to chemical complexes. Time for a snack break yet? Almost.
Nature does this—the mixing of elements, not the snacking—all the time to create what are called, chemical compounds. Chemical compounds are built with elements. A chemical compound is made of at least two atoms of different elements. While there are only 118 known elements, there are a gazillion chemical compounds that are made from the elements. Okay, maybe that is an exaggeration, but you get the point. One scrumptious chemical compound is sugar.
Chemists represent the compound using the following notation: C12H22O11. For now, take note that there are three letters that represent each one of the three elements that composes a sugar molecule: C is for carbon, H is for hydrogen, and O is for OMG, chemistry is super neat (or oxygen). While chemistry may look like a different language, it isn't too hard to decipher once the rules are laid out. Those little subscripts in the chemical formula represent the number of atoms of each element that come together in a single molecule.
Crystals of sugar shown close-up. On an even smaller level, sugar is made up of carbon, hydrogen, and oxygen atoms stuck, or bonded, together to form molecules.
Now back to the issue of the undiscovered elements. How is it possible to know that something, which has not yet been discovered, still exists?
When we look at the elements that have been discovered, we notice patterns. Upon studying patterns for the discovered elements, scientists noticed that a few elements seem to be missing. It is like doing a jigsaw puzzle and realizing once you've used all the pieces that a few are missing from the completed puzzle. We know that they are out there just waiting to be found. The first place we should look is under the couch.