1. The Atomic Number

2. Reminder: Elements are different types of atoms (like different LEGO blocks) Elements are defined by the number of protons in the nucleus – This is the atomic number – Locate it on your diagram, & label! The same element may have different #’s of electrons & neutrons  ALWAYS have the same # of protons!

3. Atomic Mass # of protons + # of neutrons = atomic mass Located on bottom of element diagram So, we can calculate the # of neutrons by:  Atomic Mass – Atomic Number = # of Neutrons

4. Isotopes

5. Not all elements are the same! – Number of neutrons can vary! – What has to stay the same? Isotopes: atoms of the same element with different numbers of neutrons

6. What does this mean? These don’t affect the chemical or physical properties BUT there are some isotopes that are unstable  atoms fall apart releasing energy (radioactive decay) Have different atomic masses – Why A.M on periodic table are not whole #’s! they are averages!

7. How designate which one you are talking about? Nuclide notation: mass of isotope in upper left hand side of symbol; place atomic number on bottom left hand side – Example: Hyphen Notation: can refer to it by element name & mass – Example: Hydrogen-2

8. Average Atomic Masses Definition: weighted average of the atomic masses of naturally occurring isotopes of an element See periodic table to see averages Math: (mass of A)(%abundance of A) + (mass of B) (%abundance of B) +(mass of C)(%abundance of C)/100 Do worksheet