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Atoms: The Building Blocks of Matter Chapter 3 Democritus vs. Aristotle: Philosophical Ideas • Democrituscalled nature’s basic particle an atom, based on the Greek word meaning “indivisible.” • Aristotlecame after Democritus; did not believe in atoms, thought all matter was continuous. • Neither view supported by experimental evidence until the 18th Century. Foundations of Atomic Theory • Law of conservation of mass • Law of definite proportions • Law of multiple proportions Dalton’s Atomic Theory (1808) 1. 2. 3. 4. 5. All matter is composed of extremely small particles called atoms. Atoms of a given element are identical in size, mass, and other properties; Atoms cannot be subdivided, created, or destroyed. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. In chemical reactions, atoms are combined, separated, or rearranged. How did Dalton do? Modern Atomic Theory Two important concepts, (1) All matter is composed of atoms and (2) Atoms of any one element differ in properties from atoms of another element, remain unchanged. Atoms • Composed of: – Nucleus protons (+) & neutrons (no charge) – Electrons (-) – Collectively referred to as subatomic particles. Electrons • Joseph John Thompson and Robert A. Millikan • Discovered through experiments with cathode-ray tubes. • Negative charge • Very small mass 9.109 x 10-31 kg • Found in electron cloud around nucleus. • Discovery of the electron led to 2 other inferences about atomic structure: – Because atoms are electrically neutral, they must contain a positive charge to balance the negative electrons. – Because electrons have so much less mass than atoms, atoms must contain other particles that account for most of their mass. Atomic Nucleus • Ernest Rutherford, Hans Geiger, and Ernest Marsden • Discovered through experiments with alpha particles and gold foil. • The neucleus contains protons and neutrons. – Protons have a positive charge equal to the negative charge of the electron. • Atoms are electrically neutral because they contain equal #s of protons and electrons. – Neutrons are electrically neutral. • Proton mass = 1.673 x 10-27 kg – 1,836 times greater than the mass of the electron • Neutron mass = 1.675 x 10-27 kg Nuclear Forces • Nuclear forces are short-range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together. Atomic Number • The atomic number of an element is the number of protons in the nucleus of each atom of that element. • Determines the identity of the atom. • Look at the periodic table, what is the atomic number for hydrogen? Nitrogen? Sodium? Info on the Periodic Table Atomic # Symbol Mass # Isotopes • Isotopes are atoms of the same element that have different masses. – The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons. – Most of the elements consist of mixtures of isotopes. – Although isotopes have different masses, they do not differ significantly in their chemical behavior. – Nomenclature Isotopes of Hydrogen • The number of protons defines the element. • The number of neutrons varies. 1 1 H Protium Hydrogen-1 2 1 H 3 1 H Deuterium Tritium Hydrogen-2 Hydrogen-3 Mass Number • The total number of protons and neutrons in the nucleus of an atom is equal to the mass number. mass number atomic number 4 2 He Practice How many protons are in the nucleus of this helium atom? How many neutrons? 4 2 He 2 protons and 2 neutrons # of protons + # of neutrons = mass number Practice Problems • How many protons, electrons, and neutrons are there in an atom of chlorine-37? 17 protons 17 electrons 20 neutrons More Practice • How many protons, electrons, and neutrons are in each of the following atoms: – Bromine-90 – Carbon-13 • What element contains 15 electrons and 15 neutrons? Relative Atomic Masses • Expressing the mass of atoms in grams results in very small numbers that are not fun to work with. Therefore, scientists use relative atomic masses, based on carbon-12. • One atomic mass unit (amu) is exactly 1/12 the mass of a carbon-12 atom. Average Atomic Masses of Elements • Weighted average of the isotopes of an element • The sum of the masses of each isotope times their relative abundances Isotope U-238 U-235 U-234 Percent Abundance 99.23% 0.71% 0.0058% Relative Abundance Practice Problems • Calculate the average atomic mass of argon to 2 decimal places, given the following relative atomic masses and abundances of each of the isotopes: argon-3635.97 amu; 0.337% argon-3837.96 amu; 0.063% argon-4039.96 amu; 99.600% Answer: 39.95 amu Stop Relating Mass to #s of Atoms • The relative atomic mass scale makes it possible to know how many atoms of an element are present in a sample of the element with a measurable mass. – Mole – Avogadro’s number – Molar mass The Mole • A mole (mol) is the amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12. Avogadro’s Number • Avogadro’s number is the number of particles in exactly one mole of a pure substance. • 6.0221367 x 1023 • Typically rounded to 6.022 x 1023 Molar Mass • The mass of one mole of a pure substance is called the molar mass of that substance. • Usually written in units of g/mol. • Molar mass of an element is numerically equal to the atomic mass of the element in atomic mass units. • Can be used as a conversion factor in chemical calculations. How do they all relate? Practice Problems • What is the mass in grams of 3.50 mol of the element copper, Cu? Answer: 222 g Cu • A chemist produced 11.9 g of aluminum, Al. How many moles of aluminum were produced? Answer: 0.441 mol • How many moles of silver, Ag, are in 3.01 x 1023 atoms of silver? Answer: 0.500 mol • What is the mass in grams of 1.20 x 108 atoms of copper, Cu? Answer: 1.27 x 10-14 g/mol Now, you try some!!! • What is the mass in grams of 2.25 mol of the element iron, Fe? • What is the mass in grams of 0.0135 mol of the element sodium, Na? • How many moles of calcium, Ca, are in 5.00 g of calcium? • How many moles of gold, Au, are there in 3.60 x 10-10 g of gold? Yes, keep working!!! • How many moles of tin, Sn, are in 2500 atoms of tin? • How many moles of lead, Pb, are in 1.50 x 1012 atoms of lead? • What is the mass in grams of 7.5 x 1015 atoms of nickel, Ni? • What mass of gold, Au, contains the same number of atoms as 9.0 g of aluminum, Al? Just a few more!!! • Just kidding!!! • Any questions?