Chapter 4 Atomic Structure
Warm Up We have not discussed this material, what do you know already?? What is an atom? What are electron, neutrons, and protons? Draw a picture of an atom from what you know today.
History of the Atom 1. Democritus vs. Aristotle pg. 102-103 2. John Dalton and conservation of mass pg. 104-105 3. Cathode ray tube and Sir William Crookes pg. 107-108 4. Mass and charge of electron (J.J. Thompson) and oil drop experiment pg. 108-109 5. Plum pudding model vs. Rutherfords experiment pg. 110-112
Democritus Greek philosopher who asked questions about matter. Can you divide matter infinitely? Democritus says no! Tiny particles called atoms, indivisible! Matter is composed of atoms, which move through empty space. Atoms are solid homogeneous, indestructible, and indivisible. Atoms have different sizes and shapes. These properties, and movement determine properties of matter
John Dalton Matter is composed of small particles called atoms that are indivisible and indestructible! Atoms of a given element are identical in size, mass, and chemical properties, and are different from those of another element. Different atoms combine in simple whole number ratios to form compounds. In a chemical reaction, atoms are separated, combined or rearranged.
Dalton s Inaccuracies Atoms are not the smallest type of matter! Subatomic particles electrons, protons, and neutrons Atoms of the same element can have slightly different masses! - isotopes
The Atom EXTREMELY small particle of an element that retains the properties of that element is an atom. If atom is the size of an orange, an orange would be the size of the EARTH
Subatomic Particles - Electron Cathode Ray Tube Thin beam of electrons travels from cathode to anode! Cathode rays are a Cathode Vacuum Anode stream of charged particles. Particles carry a negative charge now called electrons!
J.J. Thompson Determined that the mass of the charged particle (electron) was much less than that of the hydrogen atom. Dalton was WRONG about the atom being the smallest particle!
Millikan Oil-Drop Experiment Determined the charge of an electron. Charge up the oil particles with electrons. Change the electric field changes the rate of oil droplets! Charge of electron 1.602 x 10-19 coulombs Mass of electron = 9.1 x 10-28 grams
Plum Pudding Model Matter isn t all negatively charged, so how do we have negatively charged subatomic particles without positively charged ones?? J.J. Thompson thought an atom was a positively charged sphere with electrons hanging out within.
Rutherford and the Nucleus Experiment proved that plum pudding model was incorrect! Atom is mostly empty space through which e - can move. Almost all of the positive charge and atomic mass resides in the center NUCLEUS! Nucleus is positively charged to deflect alpha particles and to balance electron charge.
Subatomic Particles Electron VERY tiny, negatively charged Proton located in the nucleus, charge opposite of an electron (positive!) Neutron located in the nucleus, same mass as a proton, NO charge!
Atomic Theory Today Quantum Mechanical Model All atoms are made up of electrons, protons, and neutrons. Electrons are located outside of the nucleus, protons and neutrons are located inside the nucleus. Electrons exist in a cloud surrounding the nucleus. Attracted to the nucleus so they hang around! Nucleus accounts for 99.97% of the atomic mass, and occupies a VERY small volume. A neutral atom has the same number of electrons and protons!
Atomic Model Protons = Electrons
Warm Up! What experiment determined the mass and charge of an electron? Dalton concluded that the atom was the smallest particle of matter. Was he correct? What did the gold foil experiment prove?
Warm Up! Element Atomic # Mass # Calcium 20 46 Oxygen 8 17 Mercury 80 204 What is the number of protons, electrons, and neutrons for each? What is the isotope symbol for each?
Isotopes and Ions Isotope Atoms with the same number of protons but different number of neutrons. Things to remember The # of protons of an element NEVER changes, and is ALWAYS the same as the Atomic #. If the # neutrons is different = ISOTOPE If the # electrons is different = ION + = cation Less electrons - = anion More electrons
Mass of Atoms Mass of electron = 1/1840 th of a proton Mass of proton mass of neutron 1 atomic mass unit (amu) mass of proton Carbon 12 atom = 12 amu Why aren t the masses of elements in whole numbers?
Atomic Mass = Average of Isotopes Weighted average mass mass of each isotope contributes to total mass according to how much of that isotope exists. Three isotopes = 39 K 40 K 41 K K Potassium 19 19 19 Percent Composition: 93.26% 0.01% 6.73%
Calculate the Atomic Mass of K 1. Use % composition and convert to relative abundance (divide by 100) 93.26% composition =.9326 relative abundance 2. Amu = ((Mass of Isotope 1 )x(relative Abundance 1 )) + ((Mass of Isotope 2 )x(relative Abundance 2 )) ((0.9326)x(39)) + ((0.0001)x(40)) + ((0.0673)x(41)) = 39.1347 amu
Amu = (R.A.)x(Mass) + ((R.A.)x(Mass)) What element is this? Isotope Mass of Isotope Percent abundance 6 X 6.015 amu 7.59% 7 X 7.016 amu 92.41% Find the atomic mass What element is this? (Use the Periodic Table) Boron has two isotopes: Boron-10 (% abundance 19.8%, mass = 10.013 amu) and Boron-11 (% abundance 80.2%, mass 11.009 amu). Calculate the atomic mass of Boron.
Bromine has two isotopes with the first having a mass of 78.918336 amu and occupying 50.69% and the second isotope having a mass of 80.916289 amu and occupying 49.31%. What is the average atomic mass of bromine? Verify the atomic mass of Magnesium: 24 Mg = 23.985042 amu and percent abundance of 78.99%, 25 Mg = 24.985837 amu and percent abundance of 10.00%, 26 Mg = 25.982593 amu and percent abundance of 11.01%.
Warm Up Isotope Percent Abundance Mass (amu) X-50 4.35 49.946 X-52 83.79 51.941 X-53 9.50 52.941 X-54 2.36 53.939 Find the atomic mass and identify the element. How many electrons, and protons does this element have? How many electrons protons and neutrons does 52 X 2+ have? Is it in cationic or anionic form?
Warm Up! What is an isotope? What is a radioactive isotope? What does it mean for something to be radioactive?
Warm Up! What are the three types of radioactive particles? What are the charges on those particles? Complete the following nuclear reaction (Remember that = e - ) 137 137 55Cs + 56Ba
Warm Up!! What happens to the atomic mass number and the atomic number of a radioisotope when it undergoes alpha emission? High speed electrons emitted by an unstable nucleus are particles. What isotope of what element is produced if krypton-81 undergoes beta decay? Write out the nuclear reaction.
Radioactivity emit radiation Nuclear reactions change an element into a new element!! Lots of energy involved! Unlike a chemical reaction because we are doing more than rearranging we CHANGE the identity. Change in the atom s nucleus. UNSTABLE nuclei are unhappy and lose energy by emitting radiation radioactive decay. They form STABLE atoms of a different element.
Radioisotopes Isotopes of atoms with unstable nuclei. Undergo radioactive decay to attain stability. Emit 3 types of radiation alpha, beta, gamma,
What are the charges on radioactive particles?
Types of Radiation Alpha radiation (remember the gold foil experiment?!?!) made up of POSITIVE alpha particles. 2 protons and two neutrons (no electrons!) 4 He 2+ or 2
Alpha decay 238 4 234 92U He + 2 90Th 226 4 222 88Ra He + 2 86Rn 247 4 243 96Cm He + 2 94Pu
Types of Radiation Beta radiation negatively charged beta particles Unstable neutron turns into a proton and ejects 1 electron e - or
Types of Radiation Gamma radiation emits gamma rays, high energy photon that has no mass nor charge. Gamma rays almost always accompany alpha and beta radiation and account for the energy lost in the nucleus. Usually omitted from nuclear equations. 238 4 234 92U 2He 90Th + + 2
Penetrating Power of Radiation
Penetrating Power Least Alpha particles most mass and charge. Isotopic mass 4 2He 2+ Beta particles less mass (only the mass of an electron) and a neg charge. Most Gamma rays have no mass and no charge.
In the Nucleus Radioactive decay transmutation Atomic # is altered = identity of element changed Nucleons + + + Strong nuclear force between all nucleons. Repulsive force between 2 protons (electrostatic). Neutron attraction have to overcome the repulsive forces as atomic # increases we need more neutrons to stabilize the nucleus!!!
Low atomic # s have a 1:1 neutron to proton ratio 4 He 2 High atomic # s are stabilized by a 1.5:1 ratio 200 Hg 80 If atom is not in band (belt) of stability it undergoes radioactive decay to get there!
Decay Practice decay 238 Pu 94 4 He 2 + 234 U 92 Thorium-229 is used to increase the lifetime of fluorescent bulbs. What type of decay occurs when thorium-229 decays to form radium-225? Write out the nuclear equation. 229 Th 90 Write a balanced nuclear equation for the decay shown on the right. Identify A and B 212 Bi 83 208 Tl 81 4 He 2 + + 208 Tl A 81 208 Pb B 82 4 He 2 Bismuth -212 + 225 Ra 88 A B
Warm Up!! What is the band of stability and how does it relate to the proton to neutron ratio? How does the neutron to proton ratio change when polonium-210 decays into lead-206? What type of decay does polonium-210 undergo? (Low atomic # elements are happy with a 1:1 ratio of neutrons to protons. Heavier elements need a 1.5:1 ratio and all elements above 82 are radioactive.)
Half Life Time required for one half of the nuclei to decay into its products. Strontium-90 half life is 29 years. If you had 10 g now, in 29 years you would have 5g.
Half Life Calculations N = N 0 (½) n N remaining amount of element N 0 initial amount of element n number of half lives that have passed Kr-85 has a half life of 11 years. Kr is used in indicator lights of appliances. If a refrigerator light contains 2.0 mg of Kr-85, after 33 years, how much is left? N =? N 0 = 2 mg n = 33 years/11 years (years that have passed/half life)
Kr-85 has a half life of 11 years. Kr is used in indicator lights of appliances. If a refrigerator light contains 2.0 mg of Kr-85, after 33 years, how much is left? N = 2.0 mg (½) (33/11) N = 2.0 mg (½) 3 N = 2.0 mg (⅛) N = 0.25 mg left after 33 years
Half Life Practice The half life of Ra-222 is 3.8 days. How much is left of a 10 mg sample after 15.2 days? N = N 0 (½) n N = 10mg (½) (15.2/3.8) N = 10mg (½) 4 N = 10mg (1/16) N = 0.625mg
Half Life Practice Bandages can be sterilized by exposure to gamma radiation from cobalt-60, which has a half life of 5.27 years. How much of a 10 mg sample of cobalt-60 is left over after 10.54 years? After four half lives? N = N 0 (½) n N = 10 mg (½) 10.54/5.27 N = 10 mg (½) 4
Half Life Calculations Do the problem intuitively Think about how many half lives have passed and just do the division Two half lives (10 mg/2)/2 = 2.5 mg Four half lives 10 mg/2/2/2/2 = 0.625 mg
Warm - Up! Americium-241 has a half life of 430 years. How much of a 15 mg sample is left after 2150 years? A radioisotope has a half life of 197 years. How much remains of a 2.0 g sample after 10 years? Strontium has a half life of 29 years. How long will it take for a 56 g sample to decay to 1.75 g?
Nuclear Reactions Induced Transmutation FORCE an element to change its identity by bombarding it with radioactive particles! 4 He 2 + 14 17 7 N O + 8 1 H 1 Particle accelerators move particles at extremely high speeds to overcome repulsive forces.
Nuclear Fission Splitting of a nucleus into fragments - LARGE release of energy! Large atoms want to be smaller for stability (Atomic # larger than 60 = large atoms) Kr - 92 neutron U-235 U-236 Ba - 141
Neutrons Perpetuate Fission
Critical Mass Fissionable material must have sufficient mass to split Not massive enough subcritical no chain reaction Extremely massive violent nuclear reaction
Violent Nuclear Reactions Subcritical masses that get together to form supercritical mass. Equal to 20,000 tons of TNT
Nuclear Reactors Reactor core controlled by Cd or B to absorb neutrons Assignment : Look up Chernobyl Nuclear Power Plant!
Nuclear Fusion Bind low atomic mass (less than 60 because 60 is ideal) to form more stable atom. Combination of nuclei called fusion. How the sun works 4 1 1 H + 2 energy + 4 He 2 Need very high energy to initiate and sustain.
Nuclear waste What happens to all the radioactive waste accumulated at a power plant? Treated with advanced technologies so the material doesn t deteriorate Sealed and buried underground
Describe the process that occurs during a nuclear chain reaction and explain how to monitor a chain reaction in a nuclear reactor. True/False Great amounts of energy can be liberated from small amounts of matter in a nuclear reaction. The amount of U-235 in a nuclear reactor should always be kept subcritical. Nuclear power plants do not contribute to air pollution. Nuclear power use is dangerous because plants are commonly are out of control.