1. Atoms: The Building Blocks of Matter Chapter 3 Pages 66 - 87

2. The Atom: From Philosophical Idea to Scientific Theory

3. The law of conservation of mass states that mass is neither created nor destroyed during chemical reactions or physical changes. Which law states that a chemical compound contains the same elements in the same proportions by mass regardless of the size of the sample or the source of the compound? law of definite proportions Which law states that when two or more different compounds are composed of the same two elements, then the ratio of masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers? law of multiple proportions

4. The Atom: From Philosophical Idea to Scientific Theory

5. How many statements does Dalton’s Atomic Theory consist of? 5 We now know that only two of those five are applicable today: 1. all matter is composed of atoms 2. atoms of any one element differ in properties from atoms of a different element

6. The Atom: From Philosophical Idea to Scientific Theory LAWS OF DEFINITE AND MULTIPLE PROPORTIONS

7. The Structure of the Atom All atoms consist of two regions: the nucleus and the electron cloud

8. The Structure of the Atom How were electrons discovered? By using a cathode-ray tube What are cathode rays made of? Identical negatively charged particles called electrons Joseph Thomson proposed the “plum pudding model” for atomic structure. He believed negative electrons were spread evenly throughout a positively charged atom. Ernest Rutherford bombarded a thin piece of gold foil with a narrow beam of alpha particles. 1 in 8000 of particles were deflected. From these observations, he concludes that the volume of a nucleus was very small compared with the total volume of an atom. So the atom is largely empty space with a dense center which Rutherford called the “nucleus”.

9. The Structure of the Atom Niels Bohr contributed to understanding atomic structure by proposing a model in which electrons surrounded a positively charged nucleus much like planets surround the sun. Protons have what electrical charge? positive Electrons have what electrical charge? negative Neutrons have what electrical charge? neutral Why are the above three particles called “subatomic”?

10. The Structure of the Atom Atomic radii (the distance between the center of the nucleus to the outer portion of the electron cloud) are very small so they are measured in picometers, pm. Nuclear forces are short-range proton-proton, proton-neutron or neutron-neutron forces that hold the nuclear particles together. Neutrons and protons have about the same mass. Electrons are much smaller than the other two subatomic particles.

11. Counting Atoms The atomic number of an element is the number of protons of each atom of that element. The atomic mass (mass number) is the total number of protons and neutrons that make up the nucleus of an atom.

12. Counting Atoms An isotope is an atom of the same element (same # of protons) but different masses (different # of neutrons). Isotopes can have more or fewer neutrons than a standard atom of the same element.

13. Counting Atoms There are two ways to designate an isotope: hyphen notation and nuclear symbol.

14. Counting Atoms CALCULATION: How many protons, electrons and neutrons are there in an atom of chlorine-37? atomic number = number of protons = number of electrons mass number = number of neutrons + number of protons mass number of chlorine-37 = 37 atomic number of chlorine-37 = 17 # of protons = 17 # of electrons = 17 # of neutrons = 37 – 17 = 20

15. Counting Atoms Most elements exist in nature as mixtures of isotopes. The % of each isotope in nature is nearly always the same, no matter where the element is found. __________ __________ ________ is the weighted average of the atomic masses of the naturally occurring isotopes of an element. CALCULATION: Naturally occurring copper consists of 69.15% copper-63 which has an atomic mass of 62.929 amu and 30.85% copper-65 which has an atomic mass of 64.927 amu. Calculate the average atomic mass of copper. change the percentages to decimals (0.6915)(62.929) + (0.3085)(64.927) = 63.55 amu

16. Counting Atoms A ________ is the amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12. A mole is a unit of measurement. One mole of sodium (Na) has a mass of 22.98 amu. How much mass does one mole of chlorine (Cl) have? If we know the molar amount of a substance, we can calculate how many particles of that substance are present in a sample by using a conversion factor known as Avogadro’s number. 6.022 X 10 23 particles/mol This is the # of particles in exactly one mole of a pure substance. The molar mass = atomic mass. Molar mass is the mass of one mole of a pure substance. Molar mass is also a conversion factor.

17. Counting Atoms Chemists can convert from mass of elements in grams to amount of elements in moles to number of particles of element by using conversion factors molar mass and avogadro’s number respectively.

18. Counting Atoms CALCULATION: What is the mass in grams of 2.25 mol of iron, Fe? 2.25 mol Fe X 55.8 g Fe = ~126 g Fe 1 1 mol Fe 1 1 mol Fe What is the mass in grams of 16.3 mol of nickel, Ni? How many moles of calcium, Ca, are in 5.00 g of calcium? How many moles of gold, Au, are in 3.60 X 10 -5 g of gold? How many atoms of aluminum, Al, are in 2.75 mol of aluminum? How many moles of tin, Sn, are in 2500 atoms of tin? What is the mass in grams of 7.5x10 15 atoms of nickel, Ni? How many atoms of sulfur, S, are in 4.00 g of sulfur?