1. Structure of Atoms In the mid-1800s scientists discovered that atoms can be broken into pieces after all. The smaller parts that make up atoms are called subatomic particles. Electrons (- charged) Protons (+ charged) Neutrons (neutral)

2. Atoms have a Nucleus The nucleus is the dense, central portion of the atom. The nucleus has all of the positive charge, nearly all of the mass but only a very small fraction of the volume of the atom. If an atom the size of a sewing pin was placed in the middle of the 50 yard line on a football field, the electrons would be orbiting around the goal posts! Most of an atom is empty space!

3. Protons and Neutrons Compose the Nucleus Protons are subatomic particles that have a positive charge and are found in the nucleus. The number of protons in the nucleus is the atomic number, which identifies the element. Neutrons are subatomic particles that have no charge and are located in the nucleus. Neutrons add mass to the atom.

4. Isotopes Isotopes are atoms of an element that have the same number of protons, but a different number of neutrons. Most isotopes are rare. An isotope is identified by its atomic mass number. The atomic mass number is the total number of protons + neutrons in the nucleus of the atom. What is the difference between these 3 atoms? NOTE: All of them are Hydrogen atoms.

5. How to Identify an Isotope Look at the atomic mass of the element on the periodic table. Remember that each proton weighs 1 atomic mass unit (amu). Each neutron also weighs 1 amu. If the atom has the same mass as is shown on the periodic table, it is NOT an isotope. If the atom has a different mass, (but the same number of protons), then it IS an isotope.