1. Subatomic Particles in an atom! 3 main particles in the atom: Neutrons Protons Electrons In physics, we will learn that even these particles are made of smaller particles! SIZE LOCATION

2. Remember, most of an atom is empty space! An atomic model the size of Busch Stadium and parking would contain a pea sized nucleus containing 95.95% of the atoms mass. The pea at the pitcher’s mound would be the nucleus, and an ant crawling on the parking lot outside would be an electron!

3. Electrons JJ Thompson, RA Millikan Located around nucleus Negatively (-) charged particles Mass = essentially zero - only 1/2000 of a proton or neutron Charge = -1 Occupies the majority of space in an atom

4. Protons Eugen Goldstein (1886) Located in…. The tiny, dense nucleus! Mass of 1.67 x 10 -24 grams We call that 1 amu, or a mass of 1 Charge = +1

5. Neutrons James Chadwick (1932) Located in…. The tiny, dense nucleus! Mass of 1.67 x 10 -24 grams We call that 1 amu, or a mass of 1 Charge = 0

6. Nuclear Symbols X = element symbol Z = mass number = # protons + # neutrons A = atomic number = # protons (gives an element its identity!!) So… # neutrons = Z-A X Z A charge

7. Atomic Number Number of protons Number on bottom of symbol Gives an atom its identity! If 6 p + in nucleus, the atom is…. Carbon! It 7 p + in nucleus, the atom is…. Nitrogen! 12 6 C 7 14 N

8. Henry Moseley, and English scientist, bombarded atoms of elements with high speed cathode rays (electrons!) He found that the atoms gave off X-rays He found that there was a pattern between the amount of X-rays given off and the element that gave them off! He theorized that the nucleus of an atom was causing these X-rays to be given off The more protons in the nucleus, the more x-rays that were given off! He determined from this exactly how many protons were in the nucleus of each element! He called the number of protons the atomic number, and he said that this gave an element its identity! He rearranged the periodic table according to atomic number, and not according to atomic mass! This cleared up any irregular patterns that were found in the periodic table from before! How was the atomic number of an element determined?

9. He was a physics professor at Oxford University! Henry Moseley did most of his experimentation when he was only 27 years old! Unfortunately, he died a year later, in 1915, during World War I. He was killed by a sniper in Turkey! His short career lasted less than four years….. Moseley was a great scientist!

10. Mass Number Mass Number = number of protons + number of neutrons Number on the top of the symbol It tells you the mass of atom! Not all atoms of the same element weigh the same! Two atoms of Carbon are shown below. One weighs 12, one weighs 14. If they are both Carbon, the must both have the same number of….. Protons! Each has….. 6 protons! So different numbers of…. Neutrons! How many neutrons does each have? How would we figure that out? Mass Number - Atomic Number = Number of neutrons 14 6 C C 12 6 6 neutrons!8 neutrons!

11. ISOTOPES Atoms with same # Protons, but different # neutrons Example– 35 Cl and 37 Cl are isotopes of chlorine They only differ in their….. Mass! They both have how many protons….? 17! Look at the periodic table, though - the periodic table lists the AVERAGE ATOMIC MASS For chlorine, it is 35.453 amu. This doesn’t look like an average between 35 and 37! That is because they don’t exist in nature 50-50! Chlorine-35 makes up 75.77% of all Chlorine atoms in the universe Chlorine-37 makes up 24.23% of all Chlorine atoms in the universe

12. ISOTOPES - again…. How do we calculate the average atomic mass? Take each atom’s mass number, and multiply by the RELATIVE ABUNDANCE (That is what percentage it is in the universe) You must convert the relative abundance to a decimal Example: (35 amu)(.7577) + (37 amu)(.2423) = 35.4846 amu This is called a weighted average - because you don’t just add and divide by two!

13. Electrons and atom charge If # p + = # e - Then atom is NOT CHARGED Charge = #p + - #e- If #p > #e- Then + charged (cation) If # p < # e- Then – charged (anion) ALL ELEMENTS ARE NEUTRAL, AND NOT CHARGED - AN ELEMENT HAS TO REACT IN ORDER TO BE CHARGED! A CHARGED ATOM IS CALLED AN ION

14. How many p +, n, e - ?? 108 47 Ag 20 Ca +2 41 p + = e - = n = Charge on atom…. p + = e - = n = Charge on atom…. 47 61 20 18 21 0 +2

15. Write the nuclear symbol for… An atom containing 18 e-, 16 P, and 17 N. What element is it?? Atomic number = 16 # p + = 16 Element is…. Sulfur Number of electrons tells us… There are two more electrons than protons The electrons are winning by 2! Charge is -2! Mass Number = #p + + #n S 33 -2 16

16. Which of the following represent isotopes of the same element? 16 8 X 16 7 X 14 7 X 14 6 X 12 6 X Look for same atomic #: (same # protons) 16 7 Xand 14 7 X both have 7 protons, so are isotopes of Nitrogen. 14 6 Xand 12 6 X both have 6 protons, so are isotopes of C.