If you spread your arms out the distance from one finger to the other will be about 1 meter. Now picture slicing that distance into 1,000 equally spaced units. That’s a millimeter or 10 to the minus 3 meters in scientific notation. Imagine slicing your arm length into one million units, that’s called a micron or 10 to the minus 6 meters. One billion units? That’s called a nanometer or 10 to the minus 9. Divide a nanometer by 10 and you’re now at the scale upon which atoms are distinguishable, 10 to the minus 10.
If you shrunk by 10 to the minus 10 meters you could walk up to single atoms. On this scale it would be like finding a billiard ball sized object that was egg-like with a dust-thin outer shell and a few tiny beads in the center representing the protons and neutrons in the atomic nucleus.
The dust sized electrons belong to a class of particle called Leptons, from the Greek word “Leptos” meaning “Light.” As you would expect the Lepton family includes all the lightest particles discovered, including the electron, neutrino, and other electron-like particles. For example there’s a particle that is slightly heavier than the electron which exhibits almost all the same properties as it called the Muon. They’re not only light but the also spin at a rate of 1/2 in a certain unit system.
Our nucleus consists of a different class of particles calle Hadrons, from the Greek word “Hadros” meaning heavy or thick. Hadrons are further subdivided into two families, the Baryons (such as the proton an neutron) and the Mesons (such as pions). Baryons are the heavier particles that make up the nucleus while mesons have weights that are somewhere between that of a proton (Baryon) and electron (Lepton), in other words their in the middle, hence the prefix “Meso” which is Latin for “middle.” These particles can spin at different rates in 1/2 increments. Some Hadrons have a spin of 1, or 1/2, -1, 3/2, etc.
All the particles in nature classified as a Hadron or a Lepton, but due to the mathematics behind the model of these particles it is useful to also reclassify them based on their spin. All particles that have and integer number spin are called Bosons, and all the particles with 1/2 integer spins are called Fermions. So, for example a proton is a Hadron, in the heavy family of Baryons, but has a spin of 1/2 and so can also be called a Fermion. A Pion is in the Hadron family as a Meson but can have spins of -1, 0 , or 1 and is thus a Boson.
Hadrons and Leptons, Fermions and Bosons are the major classification system at the atomic particulate level. Protons and Neutrons are the key Hadrons we see while the electron is the key Lepton in the universe, together these particles make up all chemical reactions, and are the source of this remarkable classification system we use at this scale.