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CHAPTER 3 Atoms & Atomic Theory Foundations of Atomic Theory • Element: cannot be broken down by ordinary chemical means – Elements combine to form compounds • Chemical Reaction: transformation of a substance (or substances) into one or more new substances Laws of Matter • Conservation of Mass – Matter can not be created or destroyed in a chemical reaction • Law of Definite Proportions – A chemical compound contains the same elements in the same proportions regardless of the size of the sample • Ex: Salt is NaCl, • its composition is always 39.4 % Na & 60.6% Cl • Law of Multiple Proportions – Atoms combine in whole # ratios • Ex: CO2, H2O, C6H12O6 Conservation of Mass Dalton’s Atomic Theory • In 1808 an English school teacher proposed an atomic theory that he created using the laws of matter and previously known atomic theory 1. All matter is composed of atoms 2. All atoms of a given element are identical in size, mass, and other properties 3. Atoms can not be divided, created or destroyed 4. Atoms of different elements combine in simple whole # ratios to form chemical compounds 5. Chemical reactions cause atoms to combine, separate, and rearrange Ex: Fe + O2 Fe2O3 (iron) (oxygen) (iron oxide aka rust) Modern Atomic Theory Not all aspects of Dalton’s atomic theory have proven to be correct. We now know that: 1) Atoms are divisible into even smaller particles (subatomic particles) 2) A given element can have atoms with different masses (isotopes) Atomic Structure • Atoms- atoms are the smallest particles of an element that retain the properties of the element • Subatomic Particles: – Protons: positively charged(+), located in the nucleus of an atom – Neutrons: neutral particles(o), located in the nucleus of an atom – Electrons: negatively charged(-), located in the electron cloud region surrounding the nucleus Properties of Subatomic Particles Discovery of the Nucleus • A scientist named Ernest Rutherford shot Alpha-Particles at a thin sheet of gold foil Rutherford’s Experiment • 99.9% of the alpha-particles went directly through the gold foil • It was previously believed that atoms were solid dense spheres • The experiment had 3 main conclusions: – 1) Atoms are made mostly of empty space – 2) The nucleus is positively charged – 3) The nucleus is very small and very dense Nuclear Forces • Nuclear forces hold the positively (+) charged protons together in the nucleus Nuclear Numbers • Atomic #- the number of protons in the nucleus in the atom – Identifies the element – Periodic table is organized by increasing atomic # – Also tells you the # of electrons surrounding the nucleus • Mass #- The total number of protons + neutrons in the nucleus of an atom – the mass # that appears on the periodic table is an average of all masses of all isotopes of a given element • Isotopes- atoms of the same element with different numbers of neutrons – Nuclide: an isotope of an element Isotopes and Nuclides Average Atomic Mass • The weighted average mass of all isotopes of an atom – Takes into account the abundance of isotopes – Ex: Copper(Cu) has 2 Isotopes: Cu-63 & Cu-65 70% of all copper is Cu-63, 30% is Cu-65 Average Mass = 63.55 amu Calculating Average Atomic Masses To find the average atomic mass you multiply the % abundance of each isotope by the mass of each isotope and take the sum of all masses • Cu-63 = 63 amu x .70 = 44.10 amu • Cu-65 = 65 amu x .30 = 19.45 amu + ------63.55 amu Atomic Mass & # of Particles • Moles (mol)- moles are a counting unit (dozen = 12) – a mole = 6.02 x 1023 particles • Avogadro's #: The number of particles in one mole – 6.02 x 1023 • Molar Mass- the mass of one mole of a substance (numerically equivalent to atomic mass) – Measured in grams per mole (g/mol) ***The Mole Road*** multiply (x) Mass(g) Molar Mass(g/mol) multiply (x) Moles divide (/) 6.02 x 1023 # of Atoms divide (/) • To convert from Moles Mass – Multiply the moles by the molar mass of the compound • To convert from Moles # of Atoms – Multiply the moles by 6.02 x 1023 Mole Road Sample Calculations • How many grams of He are in 2 moles He? Mole Road Sample Calculations • How many mol Ag in 3.01 x 1023 atoms Ag? END OF CHAPTER 3 NOTES!!!