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Important Vocabulary for Ch. 3 1. Atomic mass: Round the average atomic mass on the periodic table to the nearest whole number unless the mass is given in name. 2. Anion: A negative ion (ex. O2-) 3. Cation: A positive ion (ex. Sr2+) 4. Isotopes: Atoms of the same element with different numbers of neutrons and therefore, different masses 5. Ions: Atoms of the same element with different numbers of electrons, denoted by a positive or negative superscript. Mullis 1 Protons = atomic number Neutrons = mass – protons Electrons = protons if no charge Electrons = protons + the charge if anion Electrons = protons – the charge if cation Example: 3 Li 6.941 lithium Li has 3 protons (atomic #) 4 neutrons (7-3) 3 electrons (no charge) Li+ has 3 protons (atomic #) 4 neutrons (7-3) 2 electrons (3 protons -1 (charge is +1)) Mullis 2 More examples: 35 Br 79.904 bromine 56 Ba 137.33 barium Br has 35 protons (atomic #) 45 neutrons (80-35) 35 electrons (no charge) Br- has 35 protons (atomic #) 45 neutrons 36 electrons (35 protons +1 (charge is -1)) Ba2+ has 56 protons (atomic #) 81 neutrons (137-56) 54 electrons (56 protons - 2 (charge is +2)) Mullis 3 56 Ba 137.33 barium 4 The Atom: Idea to Theory • Democritus (~ 400 BC) called nature’s basic particle an atom • Atom comes from Greek word meaning “indivisible” • 1808: Dalton proposed a theory with several statements which were later verified, but his “model” of an atom was that of a sphere. Mullis 5 Modern Atomic Theory • Democritus = Idea about an atom • Dalton = Scientific theory which could be tested • Dalton’s concepts which have “held true:” – All matter is composed of atoms – Atoms of any one element differ in properties from atoms of another element Mullis 6 John Dalton • Reinstated atomic theory into western science, composed 4 postulates and 3 laws to explain the atom as he found them to be, and explained that the atom is the smallest amount of an element to retain its basic properties. Dalton’s Atomic Theory 1. All matter is composed of extremely small particles called atoms. 2. Atoms of a given element are identical in size, mass and other properties. 3. Atoms cannot be divided, created or destroyed. 4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. 5. In chemical reactions, atoms are combined, separated or rearranged. Mullis 8 • Law of Conservation of Mass : Mass is neither created nor destroyed during ordinary physical or chemical changes. •Law of Definite Proportions: A chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample. •Law of Multiple Proportions: If two or more different compounds are made of the same two elements, then the ratio of the masses of the 2nd element combined with a certain mass of the 1st element is always a ratio of small whole numbers. Mullis 9 • Atomic Number – Number of protons or electrons in an element – Identifies the element • Atomic Mass – – – – – Nucleus contains most of the mass of an atom. Protons and neutrons are each ~ 1.67 x 10-24 g. Electrons are each ~ 9.11 x 10-28 g. Use atomic mass unit (amu) instead of gram. The mass of one proton is ~ 1 amu. • Mass Number – The sum of the number of protons and number of neutrons in the nucleus – Is approximately equal to the average atomic mass shown on periodic table. – Number of neutrons = mass number – atomic number Mullis 10 • Isotopes – Atoms of the same element with different numbers of neutrons – Have the same number of protons – Example: Carbon-12 and Carbon-14 • Radioactive Isotopes – Unstable in nature – Atoms with a neutron: proton ratio of > 1.5:1 are unstable. – Can be used to date fossils and rocks – The time it takes for half of the radioactive atoms in a piece of the fossil to change to another element is its half-life. Mullis 11