# Atom --- An **atom** is the smallest unit of [[matter]] that retains the properties of its [[element]]. They are made up of three distinct **subatomic particles**, two of which (protons and neutrons) form a **nucleus**, and the final one, (electrons) that live in clouds called **electron shells**. ![[atom models.png]] ## The Nucleus The **nucleus** of the atom is where both the positive protons and the neutral neutrons live. ### Protons & Atomic Number A **proton** is a subatomic particle that lives in the nucleus of an atom. It has a [[charge|positive charge]]. The number of protons an atom has determines it's *atomic number*. #### Free Protons (i.e Hydrogen Protons) In biology **free protons** (essentially hydrogen ions) are a kind of a currency, and they are coming and going all the time. Biology depends 100% on pushing protons into compartments, that form [[concentration gradient|gradients]] in order to harness that potential energy. Areas with a higher concentration of hydrogen prtons are much more [[acids and bases|acidic]] than water. ### Neutrons & Isotopes A **neutron** is a subatomic particle that lives in the neucleus of an atom. It has a [[charge|neutral charge]]. The number of neutrons *plus* protons in an atom has determines it's *atomic weight*. Atoms of the same element with different numbers of neutrons are called **isotopes**. As it is the number of protons that give the atom their fundamental properties #### Radioisotopes Unstable isotopes are those that don't have the "prefered" proton to neutron ratio, and are constantly (if, perhaps slowly) rearranging themselves in order to stabalize. These are called [[radioactivity|radioisotopes]], which are radioactive. ## Electrons & Electron Shells **Electrons** are smallest of the three main subatomic particles. They have a [[charge|negative charge]] and live in cloud-like configurations called **[[electron shell|electron shell]]**. ___