1. Daily science- pg. 30  Create a table like the following on page 30 in your notebook. Label this page History of the Atom. Leave a few lines for each scientist. Scientist/PhilosopherTheoryYearModel/Experiment Democritus Aristotle John Dalton J. J. Thompson Ernest Rutherford Niels Bohr James Chadwick New Atomic Theory

2. THE STRUCTURE OF ATOMS Pg. 29

3. What makes up an atom?  Atom- smallest unit of an element that still retains that element’s properties  Atoms are made of subatomic particles.  The three subatomic particles are the proton, electron, and neutron.

4. Structure of an atom  The nucleus is located at the center of the atom.  It contains positively charged particles, called protons, and neutral particles, called neutrons.  Most of the mass is located here.

5. Structure of an atom  Electrons, or negatively charged particles, exist outside of the nucleus in the electron cloud.

6. Subatomic Particles chart ParticleChargeMassLocation Proton+ 11.67 x 10 -27 In the nucleus Neutron 01.67 x 10 -27 In the nucleus Electron9.11 X 10 -31 Outside the nucleus

7. Atomic Number  The periodic table can be used to find the number of protons, electrons, and neutrons.  The atomic number of an element (labeled Z) tells you how many protons there are.  On the periodic table, the elements are listed as neutral elements so the number of electrons=the number of protons.

8. Atomic number Z

9. Mass Number  The mass number (labeled A) is the number of neutrons + protons.  Mass number (if # neutrons is unknown)=atomic mass rounded to the nearest whole number  The atomic mass is located under the element symbol.  You can find the number of neutrons by subtracting the Atomic number (Z) from the Mass number (A)  A-Z = # of neutrons

10. Mass Number

11. Writing Atomic number and mass number  There is a way to represent atomic mass and number  Steps:  1-start with the element symbol  2- place the mass number on the top left side of the symbol  3-place the atomic number on the bottom left side of the symbol

12. Symbol Al 27 13

13. Isotopes  An isotope is an element that has the same number of protons but a different number of neutrons.  Ex. Carbon has 2 common isotopes:  Carbon-12 Carbon-14  The numbers represent the mass number

14. Determining the number of neutrons  Ex. Carbon 12 and Carbon 14

15. COUNTING ATOMS Pg. 31

16. Atomic Mass  Atomic mass units (amu) are used to express an atoms mass  Atomic mass units are based off the element Carbon  Found under the element symbol

17. Average atomic mass  Average atomic mass is a weighted average of the isotopes of a given element.  More common isotopes have a greater effect on the average atomic mass than do less common isotopes.  Ex. Chlorine- 37 and Chlorine- 35 (average atomic mass is 35.45)

18. Percent Abundance  The atomic mass can be calculated by summing the products of each isotope’s percent abundance and that isotope’s atomic mass.  Mass contribution tells how much of each isotope contributes to the total atomic mass.  Mass contribution = (% abundance) x (atomic mass)

19. Percent Abundance example  Chlorine 35 has an atomic mass of 34.969 amu and its percent abundance 75.770%. What is its mass contribution?  Chlorine 37 has an atomic mass of 36.966 amu and its percent abundance is 24.230%. What is its mass contribution.  Using the two previous answers, what is the average atomic mass of Chlorine?