A-: A Greek prefix meaning "not" or "without."
Tomos: A Greek word meaning "cut" or "slice."
So, "atom" literally means "uncuttable" or "indivisible." (Ancient Greeks thought they couldn't be split further!)
An atom is the smallest unit of ordinary matter that forms a chemical element. Everything around you—the air, the water, your desk, and even you—is made of trillions of atoms bonded together.
Though the Greeks thought atoms were uncuttable, we now know they are made of three smaller parts.
Found in the nucleus. They have a positive electric charge. The number of protons decides what element the atom is (e.g., 6 protons = Carbon).
Found in the nucleus. They have no charge (they are neutral). They act like glue to hold the nucleus together.
Tiny particles that zoom around the nucleus in shells. They have a negative charge. They are involved in chemical bonding.
Mind-Blowing Fact: Atoms are mostly empty space! If the nucleus were the size of a marble in the middle of a football stadium, the electrons would be tiny gnats flying in the top stands.
When you write with a pencil, you aren't actually using "lead." You are using a mineral called graphite. Graphite is made entirely of carbon atoms.
In graphite, these carbon atoms are stacked in sheets, like layers of paper. The atoms within each sheet are bonded strongly together, but the bonds between the sheets are very weak. This is why it works for writing. As you drag the pencil across the paper, the weak bonds break, and whole layers of carbon atoms slide off the tip and stick to the paper fibers. A single dot of a pencil point contains about 4 billion billion carbon atoms!