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Unit 3 The Structure of the Atom Early Theories of Matter Do you recognize the these names? Aristotle or Democritus Early Theories of Matter Atomic Theory Matter is composed of empty space through which atomos (indivisible particle) move Atoms – small indivisible particles whose shape determines the properties of the matter 460-370 B.C. 4.1 Early Theories of Matter Aristotle Elemental Theory everything in the world was made up of some combination of four elements: earth, fire, water, and air elements were acted upon by the two forces of gravity and levity gravity was the tendency for earth and water to sink levity the tendency for air and fire to rise 384-322 B.C. ARISTOTLE WINS! Because Aristotle was considered one of the greatest thinkers of his time, Aristotle sets back chemistry over 2,000 Years! Early Theories of Matter Atoms are small solid indivisible particles Different elements are made of different atoms, but atoms of the same element are identical Atoms combine in simple whole number ratios to make compounds In a reaction, atoms are rearranged 1766-1844 Early Theories of Matter How big are atoms? Reeeeeeeeeeeeeaaaaaaaaaaaaaaaaaaly small! In the year 2010, it was estimated that 7 billion people lived on Earth! The number of atoms of copper atoms in ONE solid copper penny is 50 billion times bigger! World Population 7 000 000 000 Atoms in a penny 29 000 000 000 000 000 000 000 Can you count to a million this semester? 1000000 dollars 2 s 1 dollar 1 min 1 hr 1 day 60 s 60 min 24 hr = 23 days At 4 hrs a day it would take you 138 days!!! 1 billion would take you 368 years! How long would it take everyone on Earth to count the atoms in a penny? 1,764,167 years Can We See Atoms? NO! While we can’t see them, we can sense them! Scanning Tunneling Electron Microscope (STM) Scanning Tunneling Electron Microscope (STM) Gold atoms Silicon atoms Not only can the probe sense the atoms electron cloud, it was discovered that it can also manipulate the atoms! Scanning Tunneling Electron Microscope (STM) “Corrals” “Round” Iron atoms on copper “Stadium” Iron atoms on copper Scanning Tunneling Electron Microscope (STM) “IBM Scientists Playing” “Atom” in Japanese Iron atoms on copper “IBM” Xenon on nickel “carbon monoxide man” Carbon monoxide on platinum Subatomic Particles and the nuclear atom Thomson (1897) – Cathode-Ray Experiments: discovered the electron . Established the charge to mass ratio for these particles 1856-1940 Cathode Ray Tube Subatomic Particles and the nuclear atom Millikan (1909) – Oil-Drop Experiment: determined the mass of the electron. All of the change in charges came in multiples of 1.602 10-19! This must be the actual charge of the electron! Subatomic Particles and the nuclear atom Combined with the charge/mass ratio from Thomson, he was able to accurately calculate the mass of a single electron to be 1/1840 the mass of a hydrogen nucleus (proton)! Even with his primitive equipment, his value was within 1% of the accepted value that we use today! Subatomic Particles and the nuclear atom Rutherford (1911) – Gold Foil Experiment: discovered the nucleus and basic structure of the atom. Electrons are outside the positively charged nucleus. Most of the atom is empty space. 1871-1937 Gold Foil Experiment Subatomic Particles and the nuclear atom Rutherford (1920) – Concluded there must be a particle in the nucleus carrying a charge equal but opposite of the electron and he called it a proton. James Chadwick (1932) discovered the existence of a neutrally charged particle in the nucleus called the neutron. Subatomic Particles and the nuclear atom Particle Symbol Relative Location Electrical Charge (amu) Actual Mass (g) Relative Mass Electron e- Outside Nucleus 1- 1/1840 9.11×10-28 Proton p+ Nucleus 1+ 1 1.673×10-24 Neutron n0 Nucleus 0 1 1.673×10-24 How Atoms Differ Moseley (1913) – discovered each element has a unique positive charge in their nuclei. Each element has a different number of protons. Atomic number - # protons Atoms are neutral # protons = # electrons How many protons does chlorine have? 17 17 How many electrons? 17 How Atoms Differ Thomson (1910) – It’s discovered that neon consists of atoms with 2 different masses. WHY? Isotope – atoms with the same number of protons, but different numbers of neutrons. Neon-20 Neon-22 p+ = 10 p+ = 10 n0 n0 = 12 = 10 How Atoms Differ If not all atoms of an element are identical, how can we tell them apart? Mass number – the total number of particles in the nucleus Mass number = # protons + # neutrons MASS NUMBER IS NOT ON THE PERIODIC TABLE!!! Mass number 7 Mass number 6 How Atoms Differ Hyphen Notation Element name – mass number Symbol Notation Mass # Atomic # Potassium-41 p+ = 19 41 19 symbol K e- = 19 p+ = 19 n0 = 22 e- = 19 n0 = 22 What about ions (atoms with a charge)? 37 17 Cl p+ = 17 - e- = 18 n0 = 20 46 20 Ca 2 p+ = 20 e- = 22 18 n0 = 26 How Atoms Differ Atomic mass – because the mass of atoms is so small (proton = 1.67×10-24g) we simplify atomic masses by measuring them in atomic mass units. Atomic mass unit (amu) – 1/12 the mass of carbon-12. Hydrogen-1 = 1.007825 amu Silicon-30 = 29.974 amu How Atoms Differ If elements have isotopes with different atomic masses, what is the atomic mass on the periodic table? Atomic Mass – weighted average mass of the isotopes of an element. This found by summing the mass contribution of each isotope of the element. % abundance ) 100 % abundance (mass second isotope) ( ) 100 average atomic mass (mass first isotope) ( There are 2 naturally occurring isotopes of copper – copper-63 and copper-65. If copper-63 has a mass of 62.930 amu and 69.17% abundance and copper-65 has a mass of 64.928 amu and 30.83% abundance, what is the average atomic mass of copper? 1. First, calculate the mass contribution of each isotope to the average atomic mass, being sure to convert each percent to a fractional abundance. For copper-63: Mass contribution = (62.930 amu)(.6917) = 43.52868 43.53 amu For copper-65: Mass contribution = (64.928 amu)(.3083) = 20.017302 20.02 amu 2. Finally, the average atomic mass of the element is the sum of the mass contributions of each isotope. 43.53 amu 20.02 amu 63.55 amu Unstable Nuclei and Radioactive Decay Can chemical reactions change the identity of an atom? NO! Why Not? You can’t mess with the nucleus! Does this mean the nucleus is not affected by any reactions? NO! Ch 23 - Unstable Nuclei and Radioactive Decay Why do they change? Stability! Unstable systems, like atoms with the wrong number of neutrons or a pencil sitting on its tip, gain stability by losing energy! The pencil loses energy when it falls, but gains stability sitting on the table How is carbon-14 formed? This can be used to find out how old something is in carbon-14 dating! Carbon-14 Dating How do we see radiation? Just look around you! You were thinking of radioactive particles! Remember Rutherford’s Alpha Particles 23.1-3 - Unstable Nuclei and Radioactive Decay Types of radioactive decay Alpha decay ( 42 He ) 1. Mass # drops by 4 and atomic number drops by 2 +2 charge 226 88 Ra Rn He 222 86 4 2 Beta Decay ( 01 e or -10 ) 2. Mass # stays same and atomic number increases by 1 -1 charge 14 6 C N e 14 7 0 -1 23.1 Unstable Nuclei and Radioactive Decay 3. Gamma decay ( γ ) 0 0 No charge No change in mass number or atomic number Usually accompanies alpha or beta decay Most of the energy lost in radioactive decay is from gamma decay 238 92 U Th He 2 234 90 4 2 0 0 Detecting Radiation 23.1 Unstable Nuclei and Radioactive Decay Balancing nuclear reactions Mass number and atomic number must be conserved. 238 92 U 234 90 Th 2 He 4