Basics of Chemistry
What everything is made of, and why some materials are stiff, some stretchy, some heat-proof. It all starts with atoms and the bonds between them.
The smallest piece of an element that still behaves like that element. A carbon atom, an oxygen atom. Everything you touch is built from about 90 kinds of atoms.
Two or more atoms bonded together. Water is two hydrogen atoms and one oxygen (H₂O). The atoms and how they're arranged decide what the molecule does.
The 'glue' between atoms, made by sharing electrons. Strong bonds make hard, heat-resistant materials; weaker attractions between molecules make things soft or melty.
A very long molecule made of the same small unit repeated thousands of times, like beads on a string. Plastics, rubber, DNA, and wood are all polymers. The 'n' means 'many'.
The single small unit that repeats to build a polymer — one bead of the string. Ethylene monomers link up into polyethylene, the plastic in bags and bottles.
The outermost electrons of an atom — the ones that form bonds. Carbon has four, which is why it links in four directions and can build endless chains, rings, and networks.
The single most important split in plastics. A thermoplastic (the plastic of a drinks bottle) melts when heated and hardens when cooled, again and again — so it can be remelted and recycled. A thermoset (epoxy glue) sets once by a chemical reaction and can never be melted again.
How neatly a plastic's long chains are packed. In crystalline regions the chains line up in orderly rows, making the plastic stronger and more chemical-resistant but cloudy. In amorphous regions they are a tangled jumble, making it clear but softer. Most plastics are a mix of both.
Really just a measure of how long the polymer chains are. Short chains make a weak, waxy material; long chains tangle together and make it tough and strong. It is one of the biggest levers on how a plastic behaves, set when the polymer is made.