UNIT 2 – THE STRUCTURE OF THE ATOM

2 Democritus, 400 BC --Matter is made of small, indivisible particles – “atomos” In the 1700’s, scientists making accurate measurements discovered several new laws Law of Conservation of Matter --matter is not created or destroyed during chemical or physical changes 2. Law of Definite Composition -- compounds have an unvarying composition 3. Law of Multiple Proportions -- elements combine in simple ratios

3 John Dalton, Atomic Theory Dalton thought that atoms were hard, unbreakable spheres 1. Each element is composed of one kind of atom 2. Atoms of different elements have different masses and properties 3. Atoms can combine and recombine with each other; but they don’t change from one kind to another; they aren’t created or destroyed 4. In compounds, atoms combine in simple, constant ratios

4 J.J. Thomson, experimented with cathode ray tubes: vacuum tubes with high voltage passing through --found that cathode rays could be: 1. deflected by magnets 2. bent toward positive electric fields 1. cathode rays were streams of negatively charged particles  electrons CONCLUSIONS: 2. atoms are made of sub-atomic particles with opposite electrical charge

5

6 Ernest Rutherford, 1910: -- took alpha particles (2 protons + 2 neutrons) and shot them toward a thin gold foil Gold Foil Experiment

7 Most of the particles went straight through; a few were deflected Rutherford’s Results:

8 Interpretation of Rutherford’s Experiment- His observations and conclusions: 1. Since most of the particles went straight through, the atom is mostly empty space. 2. Some of the alpha particles were deflected so the atom contains a nucleus that is: a. very small b. dense (a large mass in a small volume) c. positively charged

9 The nucleus contains most of the atom’s mass and all its positive charge mass charge proton neutron 1 u +1 1 u 0 Atomic number = number of protons determines the element Atomic mass = number of protons + neutrons (mass number) B atomic mass atomic number

10 Not all atoms of the same element are identical p p n n n p Atomic number: Atomic mass: H-1 H-2 H-3 These are called isotopes of hydrogen Isotopes: atoms of the same element with different masses -- same p, different n

11 Isotopes of hydrogen have the same chemical properties All are: -- flammable gases --able to react with oxygen to form water They differ in mass and nuclear properties All elements exist as mixtures of isotopes -- the atomic mass of an element is the mass of an average atom A carbon-12 atom has a mass of yet the atomic mass of the element carbon is because a few carbon-13 and carbon-14 atoms are mixed in

12 Sample Problem Element X consists of 35% X-21 and 65% X-23 What is the average atomic mass of X? Solution Assume you have 100 atoms: 35 of them would be X of them would be X-23 Find the total mass of all 100 atoms: 35x 21 = x 23 = u = mass of 100 atoms 2230 / 100 = u The average is always between the isotope masses and always closer to the one that is most abundant

13 Charged Atoms + charge in nucleus, – charge outside Add + and – to find the total charge If the charge = 0 ( p = e ), it is a neutral atom If the charge = 0 ( p = e ), it is an ion

14 ELECTRONS Niels Bohr, said that electrons orbit the nucleus in set paths, like planets around the sun: or These orbits were designated: K L M N O P Q closest to nucleus farthest from nucleus lowest potential energy highest potential energy

15 Bohr said that e - could be in any of these 7 orbits, but NOT in between. Add energye - move to higher energy levels, absorbing the energy Later, the e - falls back to a lower energy level, and the energy is released e e e energy absorbed energy released in the form of electromagnetic radiation

16 Electromagnetic Radiation -- energy that travels as waves -- can travel through a vacuum -- moves at 3.0 x 10 8 m/s, or “c” (speed of light) λ -- wavelength -- the number of waves that move past a fixed point in one second is called the frequency, “f” -- frequency is measured in Hertz (Hz), or waves per second

17 -- As λ decreases, f increases -- c = λ f

18

19 In 1900, Max Planck discovered: (high frequency means high energy) energy = (planck’s constant)(frequency) --so, as λ, f, E E = h f the energy carried by electromagnetic energy is directly proportional to its frequency

20 Back to atoms.... When all the e - in an atom are in the lowest possible energy orbits, the atom is in the ground state – its normal condition Add energy, and the e - move to higher energy levels, and the atom is in the excited state – an unstable condition Energy is released when the e - return to the ground state K L M N L K jump--- short, light emitted has low f, M K jump--- medium energy released light emitted has med f, N K jump--- long, light emitted has high f, red green violet little energy released high λ med λ lots of energy released low λ

21 Each electron jump releases one photon of a certain λ This produces the spectrum of the element Since each element starts with a different electron arrangement, each element has a different spectrum

22 Bohr’s model worked well for hydrogen, but not for more complex elements. Other scientists in the 1920’s came up with several modifications to his theory. The Current Understanding of Electron Arrangement : The Wave-Mechanical model We can’t know an electron’s distance from the nucleus, as Bohr thought, KL distance from nucleus probability but we can calculate how likely it is to be found at certain distances

23 The wave-mechanical model says that electrons are in an electron cloud – Instead, we have Principal Energy Levels or Shells: K L M N O P Q Each level can hold a given number of electrons: PEL No. of e or higher 32 If the PEL = n, the number of electrons = 2n 2 there are NO orbits

24 The Principal Energy Levels are divided into orbitals (NOT ORBITS) of different energy An orbital is the region where the electron is most likely to be found

25 Rules for electrons in atoms: 1. No 2 electrons in an atom have exactly the same energy 2. Octet Rule – the last shell can never hold more than 8 e - So, a shell may not be filled with electrons before the next shell begins to be filled Atomic No. 18 Ar: 2 – 8 – 83 rd shell can hold 10 more e - Atomic No. 19 K: 2 – 8 – 8 – 14 th shell begins to fill Atomic No. 19 K: 2 – 8 – 9 violates octet rule Example:

26 Electron configurations on the periodic table are for: Neutral atoms-- when its charge is –2, the electrons are… In the ground state -- Ex. oxygen is 2–6 2–8 oxygen is 2—5—1 in the excited state O 8 2 – 6 Ex. oxygen is 2–6

27 Atoms are most stable when their outermost energy level has as many e - as possible (8 e - ; or 2 e - for the 1 st shell) This situation is called a stable octet Elements that have a stable octet when neutral are called the noble gases (group 18) These elements undergo few, if any, chemical reactions Other elements change during chemical reactions to obtain a stable octet

28 Electrons in the outermost energy level are called valence electrons Ex. Atomic No. 13 Al 2 – 8 – 3 Lewis electron-dot structures: -- use one dot for each valence electron Al NeLi Ex.

29 Nuclear Chemistry Natural Radioactivity -- the changing, or decay, of an atom’s nucleus -- accompanied by the release of particles and energy -- changes one element into another (transmutation) -- occurs in some isotope of every element -- occurs in all atoms of elements number 84 and above

30 Types of Emissions 1. Alpha particle: ( α ) 4 He 2 mass = 4 u charge = +2 Example: 238 U  92 4 He alpha radiation can be stopped by a few sheets of paper 2. Beta particle: ( β) 0 e mass = near 0 charge = A high speed electron Example: 14 C  6 0 e + -- beta radiation results from a neutron disintegration 1 n  1 p + 0 e beta radiation can be stopped by wood or metal 234 Th N 7

31 3. Gamma ray: ( γ ) mass = 0 charge = 0 -- high energy electromagnetic radiation -- no change to nucleus -- stopped by lead, thick steel, or high-density concrete 4. Positron: ( β + ) mass = near 0 charge = +1 Example: 11 C  0 e B 5 -- a positron is the antimatter particle to an electron The proton : neutron ratio in the nucleus determines the decay mode too many neutrons beta too many protons positron very large nuclei alpha See ref. table N 0e0e +1

32 Emissions can be separated by: 1. Testing their penetrating power 2. Seeing how they respond to an electric field

33 Every radioisotope follows a decay series, until it becomes a stable (non-radioactive) atom Example: U-238 to Pb-206

34 Half - Life the amount of time it takes for half the atoms of a radioisotope to decay U-238  4.5 billion years Po-214  seconds See Reference Table N Example: if you have 100 grams of Cs-137: after 30.2 yr, 50 grams remain unchanged after 60.4 yr, 25 grams remain after 90.6 yr, 12.5 grams remain

35 Example: A company has 96 grams of Sr-90, and wants to dump it into the river. Legally they can only dump 3 grams. How long must they wait until only 3 grams are left? Solution: Half-life Sr-90 = 29.1 yrs Time Amount Left g 29.1 yr g 58.2 yr g 87.3 yr g yr g yr g

36 The amount of C-14 is constant (formation = decay) The C-14 enters living things When an organism dies, it no longer takes in C-14, and the existing C-14 decays... half-life = 5715 years So, after 5715 years, bones have half as much C-14 as living things After about 10 half-lives, too little C-14 is left to measure The changing of U-238 to Pb-206 (half-life 4.5 billion years) can be used to find the age of rocks ,860

37 Biological Effects of Radiation In low doses, radiation primarily affects the DNA of living cells -- it can change them into cancer cells The effect is greatest on dividing cells: 1. Blood cells -- leukemia; anemia 2. Skin cells -- skin cancer 3. Sex cells Unborn babies -- birth defects } 5. Cancer cells -- radiation therapy attempts to destroy cancer cells without harming healthy cells Co-60 is used in cancer treatment I-131 is used to treat thyroid disorders

38 Radioisotopes can also be used: -- as tracers to study chemical or biological reactions -- to kill bacteria and sterilize food and other items -- to measure the thickness of various materials

39 Radioisotopes can be made artificially in an accelerator Accelerators use magnetic or electric fields to speed up a charged particle, and steer it into a collision with another atom -- 4 He + 27 Al  30 P + 1 n alpha ordinary radioisotope neutron particle aluminum of phosphorus atom -- causing an artificial transmutation Example: A scientist uses a particle accelerator to collide an alpha particle with a nitrogen-14 atom. A proton is produced, along with one other atom. What’s the other atom?

40 Nuclear Fission -- is the breaking of a large nucleus into pieces -- accompanied by the release of lots of energy -- occurs when a fissionable nucleus, usually U-235 or Pu-239 is hit by a particle such as a neutron

41 Protons: 92 = Mass: =

42 The neutrons released may be absorbed by another material, or they may strike another fissionable nucleus If enough other fissionable nuclei are present (a critical mass), the process continues -- a chain reaction If the mass of fissionable nuclei is less (subcritical), the reaction stops If the mass is very large (supercritical), a nuclear explosion results

43 mass of fuel is supercritical --95% U-235 mass of fuel is critical --3% U-235 Natural uranium is 0.7% U-235; not enough for a chain reaction It must be enriched to reach a critical mass

44 Electricity from Nuclear Fission -- Nuclear Power A fission reaction generates heat, --which turns water to steam --which generates electricity as it spins a turbine reactor core

45 The Reactor Core consists of: --Fuel rods (Uranium, enriched for U-235; or Plutonium) --Control rods (Boron or Cadmium) absorb neutrons to stop or slow the chain reaction Advantages 1.No air pollution problems (acid rain, global warming, etc) 2.Does not use up fossil fuels— less dependence on foreign oil sources 3. Safer (?) 4. Cost (?) Disadvantages 1.Chance of radiation release accident 2.Used fuel (nuclear waste) remains dangerous for years 3.May allow spread of nuclear weapons 4. Cost (?)

46 Nuclear Fusion --the combination of small nuclei to form larger atoms -- releases even more energy than fission ( E = mc 2 ) 2 H + 3 H  4 He + 1 n Example: Fusion could potentially serve as a power source: -- uses hydrogen for fuel ( from water) -- produces almost no radioactive wastes -- provides the energy for the sun and other stars

47 BUT -- Fusion requires very high temperatures and pressures to overcome the repulsion between nuclei ( + and + ) -- about 10,000,000 ºC Producing these temperatures and containing the hot gases are problems yet to be solved