Nanoscience is the study of structures that are between 1 nm and 100 nm in size. A structure this small is only between about 10 and 1000 atoms across. Nanoparticles are smaller than fine particles (PM2.5), which have diameters between 100 nm and 2500 nm. Coarse particles (PM10) are larger still, with diameters between 2500 nm and 10 000 nm. Coarse particles are often called dust.
An atom is far smaller than a nanoparticle. A typical atom has a diameter of about 0.1 nm, so a nanoparticle is between about 10 and 1000 atoms across. Standard form helps with these sizes: 1 nm is 1 x 10-9 m, and 100 nm is 1 x 10-7 m.
The key idea is the surface area to volume ratio. Take a cube with sides of 1 cm. Its surface area is 6 cm2 and its volume is 1 cm3, so the ratio is 6 : 1. Make the side 10 times smaller, 0.1 cm, and the surface area becomes 0.06 cm2 while the volume becomes 0.001 cm3. The ratio is now 60 : 1. Each time the side of a cube decreases by a factor of 10, the surface area to volume ratio increases by a factor of 10.
Nanoparticles therefore have a very high surface area to volume ratio. This means that a nanoparticle can have different properties from the same material in bulk. It also means that smaller quantities of nanoparticles are needed to be effective than of the same material made of normal sized particles.