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Atomic Structure Chapter 4
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Dalton’s Atomic Theory 4.1 John Dalton – 18 th century All matter is composed of atoms All atoms of a given element are identical Atoms cannot be created, divided or destroyed Different atoms combine to form compounds In chemical reactions atoms are rearranged.
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Subatomic Particles 4-2 Discovering the Electron- Thomson identified the electron through use of a cathode ray tube. Electrons carry a negative charge Electrons have an extremely small mass Thomson created the plum pudding model (chocolate chip cookie dough model) Electrons --- charge = -1 e -
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Plum Pudding Model Negatively charged electrons were distributed throughout a uniform positive charge
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The Nuclear Atom 4.2 Rutherford’s Gold Foil Experiment Discovered the nucleus Atoms consist of mostly empty space through which the electrons move There is a tiny dense region (nucleus) which contains all of the atom’s positive charge and almost all its mass
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Gold Foil Experiment
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Plum Pudding Model Nuclear Model Because some of the alpha particles bounced back – they must have hit something with a positive charge
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What’s in the nucleus Protons – subatomic particle with a charge equal and opposite to an electron Proton --- charge = +1 p + Neutrons – subatomic particle with a mass almost equal to that of a proton but it has no charge – neutral Neutron --- charge = 0 n 0
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How atoms differ 4.3 Atomic Number: The number of protons in an atom is the elements atomic number The number of protons in an atom identifies it as an atom of a particular element Atoms are neutral so # protons = # electrons Example: Helium - He Atomic # = 2 so it has 2 p + and 2 e -
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How atoms differ 4.3 Isotopes and Mass Number Not all atoms of a particular element are identical. They all have the same atomic number (protons), but they can have a different number of neutrons Isotopes - same # of protons different # of neutrons
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How atoms differ 4.3 Example: 3 types of potassium - all have 19 protons (that’s what makes them K) - 20 neutrons; 21 neutrons; 22 neutrons Differ in mass, but they have the same chemical properties Mass # = protons + neutrons
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How atoms differ 4.3 Example cont. 3 isotopes of Potassium (same protons, different neutrons) Potassium – 39 (19 + 20) Potassium – 40 (19 + 21) Potassium – 41 (19 + 22)
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Mass of Individual Atoms 4.3 Atomic Mass Units (AMU) Table 4-2 1 amu is almost equal to the mass of one proton or one neutron The atomic mass of an element is the average mass of the isotopes of that element
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Mass of Individual Atoms 4.3 Example: Chlorine - two isotopes Chlorine – 35 and Chlorine – 37 How many protons and neutrons does each have? IsotopesMass% abundance Chlorine - 3535.00 amu75.0% Chlorine - 3737.00 amu25.0% Atomic Mass = 35.00 x 0.75 = 26.25 37.00 x 0.25 = 9.25 26.25 + 9.25 = 35.5 amu
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Mass of Individual Atoms 4.3 Practice Problem: IsotopesMass% abundance Boron – 1010.013 amu19.8% Boron – 1111.009 amu80.2% 10.013 x 0.198 = 1.982 11.009 x 0.802 = 8.829 1.982 + 8.829 = 10.811 amu
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Mass of Individual Atoms 4.3 IsotopesMass% abundance X – 66.015 amu7.50 % X - 77.016 amu92.5 % You Try: 6.015 x 0.0750 = 0.451 0.451 7.016 x 0.925 = 6.489 + 6.489 = 6.940 amu What element has a average atomic mass of 6.94 amu? Lithium (Li)
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Radioactive Decay 4.4 Nuclear Reactions – a rxn that involves a change in the atoms nucleus change in the # of protons atoms of one element turn into atoms of another element
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Radioactive Decay 4.4 Radioactivity – the process by which some substances spontaneously emit radiation They do this because they are unstable – they have a unstable ratio of protons to neutrons in their nucleus. They lose energy and undergo radioactive decay to become stable, non-radioactive atoms
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Radioactive Decay 4.4 3 types of radiation: Alpha, Beta, & Gamma Alpha Radiation- Alpha Particle - 2 p + and 2 n 0 - has a charge of 2+ - equivalent to a Helium -4 nucleus - symbol = α or 4 2 He Ex:
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Radioactive Decay 4.4 Beta Radiation- Beta Particle – fast moving electrons - negative charge ( -1) - symbol = 0 -1 β Ex:
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Radioactive Decay 4.4 Gamma Radiation Gamma Rays - 0 0 γ - high energy - have no mass, no charge - usually accompany alpha or beta - don’t make a new atom by themselves Ex:
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