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ATOMIC STRUCTURE, NUCLEAR CHEMISTRY & UNIT 6
What is an Atom? ATOMIC STRUCTURE, NUCLEAR CHEMISTRY & UNIT 6
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Democritus He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms.
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History of the Atom Aristotle – famous philosopher
Believed all substances made of 4 elements Blending these in different proportions to get all substances Fire – hot air – light Earth – cool, heavy Water – wet
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History of the Atom Late 1700s – John Dalton
Teacher who summarized results of his experiments and those of others Combined idea of elements with that of atoms
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Dalton’s Atomic Theory
All matter is made of tiny, indivisible particles call atoms Atoms of the same element are identical Atoms of different elements combine in whole number ratios to form compounds Chemical reactions involve the rearrangement of atoms. No atoms are created or destroyed.
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Atoms Smallest particle of an element that still retains the properties of that element
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JJ Thompson In 1897, Thompson discovered the first component part of the atom: the electron, a particle with a negative electric charge. In 1904, he proposed an initial model of an atom, since nicknamed "Thompsons pudding". He imagined the atom as a sphere full of an electrically positive substance mixed with negative electrons "like the raisins in a cake".
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Thomson’s Experiment Voltage source Vacuum tube Metal Disks
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Thomson’s Experiment Voltage source - + Vacuum tube Metal Disks
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Thomson’s Experiment Voltage source - +
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Thomson’s Experiment Voltage source - +
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Thomson’s Experiment Voltage source - +
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- + Thomson’s Experiment Voltage source
Passing an electric current makes a beam appear to move from the negative end to the positive end
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- + Thomson’s Experiment Voltage source
Passing an electric current makes a beam appear to move from the negative end to the positive end
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- + Thomson’s Experiment Voltage source
Passing an electric current makes a beam appear to move from the negative end to the positive end
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- + Thomson’s Experiment Voltage source
Passing an electric current makes a beam appear to move from the negative end to the positive end
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Thomson’s Experiment Voltage source - + By adding an electric field…
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- + Thomson’s Experiment Voltage source By adding an electric field… +
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- + Thomson’s Experiment Voltage source By adding an electric field… +
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- + Thomson’s Experiment Voltage source
By adding an electric field he found that the moving pieces were negative
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Millikan’s Experiment
Atomizer Oil droplets - + Oil Telescope
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Millikan’s Experiment
X-rays X-rays give some electrons a charge.
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Millikan’s Experiment
From the mass of the drop and the charge on the plates, he calculated the mass of an electron
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Other Particles Proton – positively charged pieces 1840 times heavier than electrons, discovered by E. Goldstein Neutron – no charge but the same mass as a proton, by J. Chadwick Where are these pieces?
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Rutherford’s Experiment
Ernest Rutherford – English physicist Believed in the plum pudding model of the atom
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Rutherford’s Experiment
Wanted to see how large the positive particles are Used radioactivity Shot alpha particles at gold foil Alpha particles – positively charged pieces,
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Rutherford’s Experiment
When an alpha particle hits a fluorescent screen it glows
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Fluorescent Screen Lead block Uranium Gold Foil
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Rutherford’s Experiment
He expected the alpha particles to pass through without changing direction very much Because… The positive charges were thought to be spread out evenly Not enough to stop the alpha particles
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What he expected
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Because
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He thought the mass was evenly distributed in the atom
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Since he thought the mass was evenly distributed in the atom
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What he got
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How he explained it Atom is mostly empty. Small dense, positive
piece at center. Alpha particles are deflected by it if they get close enough. +
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Density and the Atom Since most of the particles went through, it was mostly empty space Because the alpha particles turned so much, the nucleus was heavy Small volume, big mass, big density The small dense positive area is the nucleus
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The Nucleus
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Structure of the Nuclear Atom
Unit 6 – Part 2 Structure of the Nuclear Atom
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Subatomic Particles Name Symbol Charge Relative Mass Actual Mass (g)
Electron e- -1 1/1840 9.11 x 10-28 Proton p+ +1 1 1.67 x 10-24 Neutron n0
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Counting the Pieces Atomic Number – the number of protons in the nucleus All protons are alike so… # of protons determines kind of atom The same as the number of electrons in a neutral atom
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Counting the Pieces Mass number = # of protons + # of neutrons
Protons and neutrons account for most of the mass of an atom
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X Symbols Mass number Atomic number
Contain the element symbol, mass number, and atomic number Mass number X Atomic number
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F Symbols 19 9 Find: # of protons # of neutrons # of electrons
Atomic # Mass # F 19 9
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Br Symbols 80 35 Find: # of protons # of neutrons # of electrons
Atomic # Mass # Br 80 35
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Symbols If an element has 91 protons and 140 neutrons, what is the
Atomic # Mass # # of electrons Complete symbol
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Isotopes Dalton was wrong
Atoms of the same element can have different numbers of neutrons This means different mass numbers Called isotopes
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Naming Isotopes Can also put mass # after name of the element
Carbon-12 Carbon-14 Uranium-235
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Atomic Mass There are different kinds of oxygen atoms.
More concerned with the average atomic mass
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Atomic Mass Based on the abundance of each in nature
Do not use grams because numbers would be too small
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Measuring Atomic Mass Use the atomic mass unit (amu)
One twelfth of a carbon-12 atom Each isotope has its own atomic mass Determine average from % abundance
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Calculating Averages Multiply atomic mass by % abundance then add results
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Atomic Mass - Example Calculate the atomic mass of copper if copper has two isotopes % has a mass of amu and the rest has a mass of amu.
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Atomic Mass – Example 2 Magnesium has 3 isotopes % has a mass of % has a mass of The rest has a mass of amu. What is the atomic mass?
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Atomic Mass Is not a whole number because it is an average
The number on the periodic table
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