Let's say you have a chunk of chalk just any writing board chalk and one of the chemical ingredients for chalk is [CaSO4] calcium sulfate
1 Calcium atom 1 sulfur atom and 4 oxygen atoms

Now the Atom the atom is the smallest units of matter possible to the known thought of man and eye, anything beyond is somewhat speculation,

The Atom is of course multiple trillions perhaps of different atoms chemically combined at the atomic scale through bonds to even make up a piece of matter entirely such as the chalk
and matter is anything that occupies space and has weight
the measurements for matter are
Mass = [kg] Volume = [mL] and we have density which is D=m/v
mass over volume

Mass is how much space is being occupied by that object and same for ml and in general quantity,

and weight its formula is w=mg
weight is equal to mass times the strength of the gravitational field on that piece of matter

Volume is how much space is being taken up or quantity of matter in the 2d state of matter liquids tent to take up the shape of whatever they are being poured in solids typically don't, until you change their state by means of a physical reaction for example you have a cookie pan now lets say you have about 30 ice cubes on it water right [H20]
heat it about 100 degrees C At the atomic scale a nuclear reaction happens perhaps a re arrangement of the atoms of the water molecules and molecules are just two or more different types of atoms chemically bonded at the atomic scale, now in you first perspective of your own eye a physical change happens the ice cube melts but its chemical makeup is still water [h20] now with the cookie pan full of water pour the liquid into an ice cube tray and put in the freezer now you reversed the physical change and the water is back into a high density of a solid the 1st state of matter