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Atoms: The Building Blocks of Matter Chapter 3 • Nature’s basic particle is atom • Based on the Greek word for indivisible • Did not believe in atoms • Thought all matter was continuous • View held for almost 2000 years Basic Laws of Chemistry Law of Conservation of Mass – mass is neither created nor destroyed in an ordinary chemical or physical change Law of Definite Proportions – chemical compounds contain the same elements in the same proportions regardless of the sample Law of Multiple Proportions – elements may combine in small whole number ratios to form different compounds Dalton’s Atomic Theory - 1808 1. All matter is composed of tiny indivisible particles called atoms 2. Atoms of a given element are identical. Atoms of different elements differ in some fundamental way. 3. Atoms cannot be subdivided, created, or destroyed. 4. Atoms of different elements combine in small whole number ratios to form compounds Dalton’s Atomic Theory 4. Atoms of different elements combine in small whole number ratios to form compounds 5. In chemical reactions, atoms are combined, separated, or rearranged Structure of the Atom Discovery of the electron – 1897 Joseph John Thompson Discovery of the Nucleus Ernest Rutherford - 1911 Discovery of the Nucleus Ernest Rutherford - 1911 Discovery of the Nucleus Ernest Rutherford - 1911 The Nuclear Atom The Nuclear Atom Subatomic Particles Particle Electron Proton Neutron Symbol Relative Charge Mass Number Relative Mass (amu) Actual Mass (kg) 0 −1𝑒 1 1H 1 0𝑛 -1 0 0.0005486 9.109 x 10-31 +1 1 1.007276 1.673 x 10-27 0 1 1.008665 1.675 x 10-27 Atomic Number • • • • Atoms of different elements have different numbers of protons Atoms of the same element have the same number of protons Atomic # is the number of protons in the nucleus of an atom Defines the element Isotopes • • • Atoms with the same number of protons but different numbers of neutrons. Show almost identical chemical properties; chemistry of atom is due to its electrons. In nature most elements contain mixtures of isotopes. Isotopes Isotopes of Hydrogen Isotope Nuclear symbol Number of protons Number of electrons Number of neutrons Hydrogen-1 1 1H 2 1H 3 1H 1 1 0 1 1 1 1 1 2 Hydrogen-2 Hydrogen-3 Atomic Masses Mass Spectrometer Atomic Masses Isotope Atomic mass (amu) Hydrogen-1 1.007825 Hydrogen-2 2.014102 Hydrogen-3 3.016049 Average Atomic Mass (amu) 1.00794 Atomic Masses Isotope Percent abundance Atomic mass (amu) Hydrogen-1 99.985 1.007825 Hydrogen-2 0.015 2.014102 Hydrogen-3 negligible 3.016049 Average Atomic Mass (amu) 1.00794 Average Atomic Mass The average atomic mass depends on 1. Mass of the individual isotopes 2. Relative abundance of each of the element’s isotopes Weighted average Calculating Avg. Atomic Masses Isotope Percent abundance Atomic mass (amu) Hydrogen-1 99.985 1.007825 Hydrogen-2 0.015 2.014102 Hydrogen-3 negligible 3.016049 Average atomic mass (amu) 1.00794 Avg. Mass = (0.99985*1.007825) + (0.00015*2.014102) Avg. Mass = 1.0079 Calculating Avg. Atomic Masses Isotope Percent abundance Atomic mass (amu) C-12 98.90 12 C-13 1.10 13.003355 C-14 negligible 14.003242 Calculate the average atomic mass. Avg. Mass = (0.9890*12) + (0.0110*13.003355) Avg. Mass = 12.0111 Average atomic mass (amu) ? Calculating Avg. Atomic Masses Isotope Percent abundance Atomic mass (amu) Average atomic mass (amu) Cu-63 69.17 62.929599 ? Cu-65 30.83 64.927793 Calculate the average atomic mass. Avg. Mass = 63.546 Average Atomic Mass The average atomic mass reported on the periodic table can be thought of from two points of view • The very small – an average carbon atom weighs 12.01 amu • The very large – one mole of carbon atoms weighs 12.01 g Counting by Weighing • In most experiments, atoms are much too small to count individually • The relative atomic mass scale makes it possible to know how many atoms are present in a sample without counting them Counting by Weighing • Represents the mass of one mole of calcium atoms in grams • 40.08 g of Ca = 1.000 mol Ca atoms • 40.08 g of Ca = 6.02 x 1023 atoms • 20.04 g of Ca = 0.5000 mol Ca • 20.04 g of Ca = 3.01 x 1023 atoms Counting by Weighing • Use dimensional analysis for conversions between grams, moles, and numbers of atoms. Counting by Weighing Perform the following conversions. 1. 0.357 mol gold = ? g 2. 10.0 g silicon = ? mol 3. 1 carbon atom = ? g 4. 20.0 g tungsten = ? atoms Counting by Weighing Perform the following conversions. 1. 4.0 x 1022 atoms of boron = ? mol boron 2. 1.5 x 10-3 mol sodium = ? atoms sodium 3. 3.5 x 10-2 g lithium = ? mol lithium 4. 4.25 g nickel = ? atoms nickel