The Structure of an Atom
The number of protons inside the nucleus of an atom determines the element. The atomic number of an element is the number of protons in an atom. For example, hydrogen's atomic number is one, and there is one proton in the hydrogen atom. An atom's mass number is a value that is equal to the number of protons and neutrons in an atom. Mass number is not the actual mass of an atom, but an approximation. For example, hydrogen's mass number is 1.00794. Some hydrogen atoms do not have neutrons, while some have more than one, so this number is an average of the mass of all hydrogen atoms.
Ions and Isotopes
Electron Orbits Using Bohr Model
The valence shell is the outermost electron shell of an atom, where valence electrons are found. Valence electrons typically follow the octet rule, which states that atoms react by gaining or losing electrons in order to form the most stable electron valence structure.