the basic unit of an element (things like carbon, hydrogen, nitrogen, etc.). It’s the smallest bit of that element that’s still that element (i.e. if you took a hunk of carbon and cut it up into clumps of carbon and kept cutting those clumps in half and in half and in half, the level at which you can’t cut further and still have “carbon” is the atom).
An atom is made up of 3 main subatomic particles: neutrons (which are neutral), protons (which are positively charged), and electrons (which are negatively charged). Protons and neutrons hang out together in a dense central nucleus and electrons whizz around them in an “electron cloud.” The particular number of protons defines a particular element (e.g. carbon *always* has 6 and oxygen *always* has 8) but the number of neutrons can vary (giving you different isotopes) and the number of electrons can vary (giving you different charged forms (ions)). Atoms can give, take, and share electrons with one another to form bonds.