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Atomic Theory

Atomic Theory. Matter (400 BC) - The Greeks were the first to study matter. Greeks believed that matter was composed of indivisible particles called atoms. 2 original theories on matter :

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Atomic Theory

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  1. Atomic Theory Matter (400 BC) - The Greeks were the first to study matter. Greeks believed that matter was composed of indivisible particles called atoms. 2 original theories on matter: 1) Matter is continuous - can be chopped up and never reach a base particle (disliked due to crystals).

  2. 2) Matter is discontinuous - if you chop matter up you would eventually hit a base particle = atom. Atom - the smallest part of an element that still retains the properties of that element = base particle.

  3. With the invention of the balance matter was studied quantitatively. From this several laws were formulated: 1) Law of Conservation of Mass - Lavoisier. 2) Law of Definite Composition - Proust. 3) Atomic Theory - Dalton.

  4. Dalton’s Atomic Theory 1) All matter is composed of tiny indivisible particles called atoms. Incorrect - due to fission - a nuclear process that breaks down the atom into 3 subatomic particles: 1) neutron (neutral) in the nucleus (center of the atom). 2) proton (positive) in the nucleus. 3) electron (negative) in the electron cloud (the space surrounding the nucleus.

  5. 2) All atoms of the same element have the same mass. Incorrect due to isotopes - atoms of the same element with different numbers of neutrons. A = atomic mass = #n + #p Atomic mass is the number of protons (for the same element this number is the same) and the number of neutrons,but if the number of neutrons changes then the mass changes.

  6. 3) Atoms of different elements have different masses. True - different elements have different masses because they have a different number of protons and the atomic mass is the sum of the protons and the neutrons. 4) Atoms unite and disunite in definite proportions of atoms. True! (ex) 2H2 + O2 ------> 2H2O

  7. Law of Multiple Proportions - The ratio of masses of one element that combines with a constant mass of another element (in different compounds) can be expressed as a small whole number ratio. (ex) Mass of SnMass of O SnO 119 g 16 g SnO2 119g 32g Divide both by the smallest for the ratio of atoms. 16g/16g = 1 32g/16g = 2 Therefore the ratio of O atoms is 1:2

  8. History of the Atom 1) J. J. Thomson - cathode (discharge) ray tube (put diagram on the board) Results: could get the negative particles to pop off the negative plate and migrate to the positive plate (opposites attract), but could not get positives to come off and migrate to the negative plate. Got a discharge of electricity which seemed to be made of very tiny negative particles - discovered the electron!

  9. 2) Robert Millikan - oil drop experiment. (put diagram on the board) Results: He calculated the charge on the oil droplet. He always got some multiple of a base number. Concluded that this base number must be the charge of an electron = -1.6 X 10-19 C SI unit for electrical charge is the Coulomb = C. Using the data of both Thompson and Millikan it was possible to calculate the mass of an electron = 9.1 X 10-31 kg.

  10. Protons were later discovered in a device similar to Thomson’s discharge tube. Found particles traveling in the opposite direction of the electrons which had a positive charge. Protons have the same amount of electrical charge as the electron, but opposite in sign = 1.6 X 10-19 C. A proton’s mass is approximately 1836 times more massive than an electron. The mass of a proton and a neutron are approximately the same.

  11. At this point in time 2 theories tried to explain and predict the atom’s construction: 1) Dalton - solid, indivisible - WRONG! Not indivisible! 2) Thomson (plum pudding model) - solid blob of positive charge (evenly distributed throughout) with negative electrons embedded on the surface. This is what was accepted at this time.

  12. 3) Radioactivity (late 1800, early 1900’s) - Becquerel and the Curies Realized that some atoms fall apart on their own. When they did so they emitted one of three types of particles: 1) gamma rays (neutral in charge) - also called x -rays - have very high energy. 2) alpha rays (positive in charge). 3) beta rays (negative in charge) - similar to Thomson’s electron, but higher in energy.

  13. ALPHA PARTICLES • The alpha particle is the heaviest. It is produced when the heaviest elements decay. Alphas are rays not waves. The alpha particle is an helium atom and contains two neutrons and two protons. The alpha particles is relatively large and heavy. A sheet of paper or a 3-cm layer of air is sufficient to stop them. The alpha particle emitter will not penetrate the

  14. outer layer of our skin, but is dangerous if inhaled or swallowed. The delicate internal workings of the living cell forming the lining of the lungs or internal organs, most certainly will be changed (mutated) or killed outright by the energetic alpha particle. The number of lung cancer cases among uranium miners from inhaled and ingested alpha sources is much higher than those of the public at large.

  15. BETA PARTICLES • Beta rays are much lighter energy particles. The beta particle is an energetic electron given off by the nucleus of unstable isotopes to restore an energy balance. They can be stopped, for instance, by an aluminium sheet a few millimetres thick or by 3 meters of air. Although the beta particle is around 8000 times smaller than the alpha particle, it is capable of penetrating much deeper into living matter. Each encounter with a living cell, and there may be many before the beta energy is dissipated, is likely to dam age some of the chemical links between the living molecules of the cell or cause some permanent genetic change in the cell nucleus

  16. GAMMA RAYS • The next "particle" is the very high energy "X-ray" called the gamma ray. It is capable of damaging living cells as it slows down by transferring its energy to surrounding cell components. Lead shields are required to stop it from penetrating the skin. The gamma has the highest penetrating ability and is most harmful to us.

  17. 4.) Rutherford’s Experiment - Used alpha particles. - Tested Thomson’s model of the atom. - Put diagram on the board. Rutherford’s Results: 1) Most alphas passes straight through the foil and hit the detectors straight out in front. 2) A few alphas were deflected at slight angles. 3) 1 out of 20,000 hit the foil and bounced straight back toward the box.

  18. Rutherford believed that the atom consists of a smalldensecentral core where all of the positive charge is concentrated. This core he called the nucleus. He believed that the rest of the atom was mainly empty space with negative electrons spread throughout. He called this part of the atom the electron cloud. Rutherfordexplains his results using the above model for his atom: 1) Most alphas passed straight through because they passed through the vast empty space of the electron cloud. Most passes straight through because the empty space was so big.

  19. 2) A few alphas (+) were defected at slight angles because they got close to the (+) nucleus. Since like charges repel, they were deflected at slight angels. It happened only occasionally because both the nucleus and the alphas are so small that the chance of them coming close to one another was so small. 3) 1/20,000 alphas (+) directly hit the (+) nucleus head on and since like charges repel they were deflected back toward the box. So few were deflected back because both the alphas and the nucleus were so small that the chances of a direct hit were so small.

  20. Thomson tries to explain Rutherford’s results: 1) Most alphas passed straight through because there is not enough positive charge at the point of impact. 2) Could not explain why some alphas were deflected at slight angles. 3) Could not explain why 1/20,000 bounced back toward the box. Rutherford’s results disproved Thomson’s theory of the atom!

  21. 5) Neils Bohr Planetary model of the atom - he compared the movement of the electrons around the nucleus to the movement of the planets around the sun. 6) Chadwick (1932) - showed the existence of the neutron (neutral). Put diagram on the board.

  22. Things to know: The electron cloud gives the atom most of its volume and the least of its mass. The electron cloud keeps two atoms from occupying the same space (like charges repel). Electrons have a negative charge and a mass of 9.11 X 10-28 g. The mass of a proton and a neutron are about the same and are both much more massive than an electron. The electron has so little mass that it is not considered when determining the mass of an atom.

  23. All electrons are identical except for the amount of energy they possess. Nucleus contains n + p. Atom = neutral #p(+) = #e-(-) Cation (+) - lost e(-), therefore more p than e- Anion (-) - gained e-, therefore more e- than p.

  24. If like charges repel, how can so many protons (+) exist in the small dense central core of the nucleus? Wouldn’t they repel? When a proton and a neutron are very close together a strong attraction occurs between them. Proton - proton and neutron - neutron attractive forces exist when such pairs are very close together. These short range p-n, n-n, and p-p forces hold the nucleus together = nuclear forces.

  25. Isotopes and Atomic Number Atomic number (Z) = whole number on P. Table = tells us the # protons. Atomic mass (A) = decimal # on P. Table = tells us the # of protons and neutrons. Mass number (A) = # of protons and neutrons = rounded off atomic mass. Nuclear notation (Z = p) ZXA (A = p + n)

  26. # n = A -Z (ex) 20Ca40 (ex) (82Pb207)4+ (ex) (53I 127)1-

  27. Nucleons - particles that make up the nucleus (p + n). Isotope - atoms of the same element with different # of n. Since the # of p for an element remains the same and the atomic mass is the # of n + p, when the number of n changes so does the atomic mass of that element.

  28. Isotopes of hydrogen 1) protium - the most common for hydrogen nucleus contains 1p surrounding by 1 e-. Nuclear notation= 1H1 Z = 1p N = A - Z = 1 - 1 = 0n Atom = neutral 1p = 1e-

  29. 2) deuterium (heavy H) 1H2 Z = 1p #n = A -Z = 2 -1 = 1n Atom = neutral 1p = 1e-

  30. 3) tritium (radioactive) - long 1/2 life 1H3 Z = 1 p #n = A - Z = 3 - 1 = 2n Atom = neutral 1 p = 1 e-

  31. Series - elements in the same row 1st series - H, He 2nd series - Li - Ne 3rd series - Na - Ar Nuclear Reactions Nuclear change = nucleus changes Nuclear Reactions Decay, makes isotopes, fission, fusion 1) alpha decay - He nucleus (2He4)

  32. Alpha decay (ex) 84Po210 -----> 84Po210 -----> 2He4 84Po210 -----> 2He4 + 82Pb206 Alpha decay (ex) 92U238 -----> 92U238 -----> 2He4 92U238 -----> 2He4 + 90Th234

  33. Alpha decay (ex) 84Po212 -----> 84Po210 -----> 2He4 84Po210 -----> 2He4 + 82Pb208

  34. (ex) 0n1 -----> 0n1 -----> -1B0 0n1 -----> -1B0 + 1H1 2) Beta decay = -1B0 (high energy e-) (ex) 90Th234 -----> 90Th234 -----> -1B0 90Th234 -----> -1B0 + 91Pa234

  35. (ex) 91Pa234 -----> 91Pa234 -----> -1B0 91Pa234 -----> -1B0 + 92U234 Half life - the time it takes for half the original pile to decay. (ex) Es99 - 320 days No102 - 3 seconds

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