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Chapter 4

Chapter 4. Structure of the Atom. History. In the 1800’s, early philosophers believed all matter consisted of either air, earth, water, or fire. History Democritus. Coined the term atom Believed atoms consisted mostly of empty space

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Chapter 4

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  1. Chapter 4 Structure of the Atom

  2. History • In the 1800’s, early philosophers believed all matter consisted of either air, earth, water, or fire.

  3. HistoryDemocritus • Coined the term atom • Believed atoms consisted mostly of empty space • Different properties of matter are due to the size shape and movement of the atoms

  4. HistoryDalton • Proposed first Atomic Theory • 1. Atoms All matter is composed of very small particles called atoms • 2. Identical All atoms of a given element are identical • 3. Created Atoms cannot be created, divided into smaller particles, or destroyed

  5. HistoryDalton • 4. Compounds Atoms combine in simple whole number ratios to form compounds • Dalton was a school teacher • Lead to advancement in chemistry • Part of his theory was proven incorrect upon later experimentation

  6. Law of Definite Proportionsby Joseph Proust • Same as 4th postulate of Dalton’s Atomic Theory • Regardless of the amount, a compound of the same elements is in the same proportion by mass

  7. Subatomic Particles and the Nuclear Atom • The electron was discovered by J.J. Thomson in the 1890’s. • 1. used a cathode ray tube determine the electron • 2. was able to determine charge to mass ratio but was unable to calculate exact charge

  8. Subatomic Particles and the Nuclear Atom • Millikan determine the charge of an electron using an oil drop and a charged plate • Millikan’s results were within one percnet of today’s accepted value

  9. Subatomic Particles and the Nuclear Atom • Thomson proposed the plum pudding model of the atom • Atom model was spherically shaped with uniformly distributed positive charge with individual negatively charged electrons attached

  10. Rutherford Proposes a New Atom • Rutherford experimented with alpha particle reacting with matter • Other scientists tried to see if alpha particles would pass through a gold foil sheet. • almost all of the alpha particles went through the gold foil as if it were not even there.

  11. Rutherford Proposes a New Atom • some of the alpha particles were deflected only slightly, usually 2° or less. • a very, very few (1 in 8000 for platinum foil) alpha particles were turned through an angle of 90° or more. (Rutherford cites 1 in 20,000 for gold in his 1911 paper.)

  12. Rutherford Proposes a New Atom • From these results he found he concluded: • Atoms consisted mostly of empty space through which electrons move • There is a tiny dense region called the nucleus which has a positive charge • Concludes most of the atom’s mass is contained in the nucleus

  13. Nucleus of the Atom • 1920 - Rutherford coined the term proton for the positively charged part of the nucleus • 1932 - James Chadwick showed that the nucleus also contained a neutrally charged part of the nucleus called the neutron

  14. Subatomic Particles

  15. How Atoms Differ • Atomic Number- The number of protons in an atom is the atomic number. In an element, the number of protons equals the number of electrons • A = P = E • All atoms of same element have same number of protons

  16. How Atoms differ • Isotopes – Atoms of the same element with different number of neutrons • Mass Number – The sum of number of protons and neutrons

  17. How to find the number of neutrons. • Mass # = # of n0 + # of p+ • Hence, n0 = mass # - # of p+ • Example: How many neutrons are in Mercury-204? • First find the atomic # to find the number of protons • Next use the equation n0 = mass # - # of p+ • n0 = 204 – 80 • n0 = 124

  18. Example Problem • How many protons, electrons, and neutrons are in a Carbon isotope with a mass number of 12? • If a Zinc isotope has 30 protons and 34 neutrons, what is the atomic number and mass number?

  19. Nuclide number • is a symbol that notes the number of protons and neutrons of a specific atom 14 Mass # C Element 6 Atomic #

  20. Atomic Mass • The atomic mass is the bottom number below the element on the periodic table. • The atomic mass is the weighted average mass of the isotopes of that element

  21. Atomic Mass • Example: Chlorine’s atomic mass is 35.453 amu • Chlorine exists naturally as a mixture of 75% chlorine-35 and 25% chlorine-37 • To calculate atomic mass multiply the mass of each isotope by it’s relative abundance and add them together. • 0.75(35amu) + 0.25(37amu)= atomic mass • 26.25amu + 9.25amu = atomic mass • 35.5amu = atomic mass

  22. Ions • Ion : atom which have gained or lost electrons • Cation : contains fewer electrons than a neutral atom, therefore, it has a positive charge. • Anion: contains more electrons than a neutral atom, therefore, it has a positive charge.

  23. Nuclear Forces • You know protons and neutrons make up the nucleus, • You also know protons are positively charged, and like charges repel each other. • Therefore, some force must hold the nucleus together.

  24. Nuclear Forces • Strong Force – Force which holds the protons and neutrons in the nucleus together. • Relatively strong force which acts over a very short range • 100 times stronger than electromagnetic force • Electromagnetic force has no limit on distance, unlike strong force, therefore, protons which are far apart repel each other.

  25. Nuclear Forces • The larger the atom the weaker the strong force. • Therefore, larger atoms tend to have more nuclear decay • Also, atoms with certain numbers of protons tend to decay. If they have the same number of protons and neutrons they tend to be more stable.

  26. Unstable Nuclei and Radioactive Decay • Nuclear Reactions : Reactions which involve a change in an atom’s nucleus • Radiation : Rays and particles emitted by the radioactive elements • Radioactivity: Substances spontaneously emitted radiation

  27. Types of Radiation • 1. Alpha Radiation : an alpha particle emitted from radioactive nuclei, consists of 2 protons and 2 neutrons, but no electrons • Not very harmful • Large atoms are not very stable and need to decrease mass • What is the charge and mass of an alpha particle? • +2 and 4amu • Example • 22688Ra → 22286Rn + 42He • Radium Radon Alpha Particle

  28. Types of Radiation • Beta Radiation: fast moving electron emitted from a radioactive element called a Beta Particle. • Can cause serious health problems especially in bones • Atoms want to have a 1 : 1 neutron to proton ratio • Beta emission is used to decrease the neutron to proton ratio. • What is the charge and mass on a beta particle? • -1 and 0 • 146C → 147N + 0-1β

  29. Types of Radiation • Gamma Radiation: Gamma rays are released from radioactive nuclei. • Gamma rays have no mass or charge. • Gamma rays are very harmful and have a very high energy

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