Four types of atoms make up 99% of all matter found in living organisms . Carbon, hydrogen, oxygen, nitrogen (c, h, o, n) Also present in simple forms co2, h2o. Nucleus dense core in centre that consists or protons + neutrons and that is orbited by electrons. # of protons = defining feature of an element = atomic number. # of protons + # of neutrons = mass of an atom = mass number. # of electrons = defining feature of an atom"s reactivity. By default, an atom is neutral when # of protons = # of electrons. Atomic mass unit of measurement used to represent the approximate mass of a neutron or proton: mass = 1 atomic mass unit or 1 dalton (da) Orbitals rings around the nucleus that electrons are situated around; grouped into layers or shells based on how far the electrons in that shell travel from the nucleus.
