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Chapter 4: Atoms. The Building Blocks of Matter. An atom is the smallest particle of an element that retains the chemical properties of that element. Section 1. The Atom: From Philosophical Idea to Scientific Theory Page 67. The Early Atom.
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Chapter 4: Atoms The Building Blocks of Matter An atom is the smallest particle of an element that retains the chemical properties of that element.
Section 1 The Atom: From Philosophical Idea to Scientific Theory • Page 67
The Early Atom • As early as 400 B.C., Democritus called nature’s basic particle the “atomon” based on the Greek word meaning “indivisible”. • Aristotle succeeded Democritus and did not believe in atoms. Instead, he thought that all matter was continuous. It was his theory that was accepted for the next 2000 years. (Read page 43 of your textbook.)
Three Basic Laws of Matter: • Law of Conservation of Mass • Law of Definite Proportions • Law of Multiple Proportions
Basic Laws of Matter • Law of Conservation of Mass- mass is neither created nor destroyed during ordinary chemical reactions or physical changes. CH4 + 2O2 → 2H2O + CO2 16g + 64g → 36g + 44g Antoine Lavoisier stated this about 1785
Antoine Lavoisier and his wife, Marie-Anne "It took them only an instant to cut off that head, and a hundred years may not produce another like it." Joseph-Louis Lagrange
Alka Seltzer in Water • Ziploc bag • Alka seltzer tablet • Water Using the reaction between the tablet and the water, prove that the Law of Conservation of matter is true.
HOMEWORK • Read Section 1 • Complete Questions 1-3 of the Section 1 Review (page 71) on a separate sheet of paper to be collected.
Basic Laws of Matter • Law of Definite Proportions – no matter how much salt you have, it is always 39.34% Na and 60.66% Cl by mass. Example: Sodium chloride always contains 39.34% Na and 60.66% Cl by mass. 2NaCl → 2Na + Cl2 100g→ 39.34g + 60.66g 116.88g → ? + ? Joseph Louis Proust stated this in 1794.
Basic Laws of Matter • Law of Multiple Proportions-Two or more elements can combine to form different compounds in whole-number ratios. Example John Dalton proposed this in 1803.
Dalton’s Atomic Theory • In 1808, Dalton proposed a theory to summarize and explain the laws of conservation of mass, definite proportions, & multiple proportions. I was a school teacher at the age of 12!
Dalton’s Atomic Theory John Dalton - 1808 • All matter is composed of extremely small particles called atoms. 2. Atoms of a given element are identical in size, mass, and other properties.** 3. Atoms cannot be subdivided, created, or destroyed.** 4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. 5. In chemical reactions, atoms are combined, separated, or rearranged. **Today, we know these parts to have flaws.
Flaws of Dalton’s Theory… 2. Atoms of a given element are identical in size, mass, and other properties. 3. Atoms cannot be subdivided, created, or destroyed. Isotopes – atoms with the same number of protons but a different number of neutrons Subatomic particles – electrons, protons, neutrons, and more
Section 2 • The Structure of the Atom • Page 72
The Atom • Atom - the smallest particle of an element that retains the chemical properties of that element. CARBON
The Structure of the Atom • The atom is composed of two main regions, the nucleus & the electron cloud.
Nucleus of an Atom • Nucleus- very small region located at the center of the atom. The nucleus accounts for most of an atoms mass but very little volume, making it a very dense region. • The nucleus contains protons, neutrons, and more. proton = p+ neutron = no others – neutral, too
Electron Cloud of an Atom • The electron cloud is the negatively charged region of the atom that accounts for most of the atom’s volume but very little of the atom’s mass. The electron cloud is composed of a number of electrons, of which depends the element. electron = e-
Checking for Understanding • What are the two main regions of the atom? • Does an electron from gold, act like gold? NO, an electron is like any other electron, no matter the source. The nucleus and the electron cloud are the two main regions. • What is the charge on the nucleus? The nucleus is positive since it holds protons (+), neutrons (0) and other neutral particles.
Subatomic Particles • Protons- positively charged particles found in the nucleus of an atom. • Neutrons- neutral particles found in the nucleus of an atom. • Electrons- negatively charged particles found in the electron cloud. • Others – photon, boson, gluon, lepton, muon, quark, tau, neutrino, meson, …
Properties of Subatomic Particles 1 amu (atomic mass unit) = 1.660540 x 10-27 kg or exactly 1/12 the mass of a carbon-12 atom
Discovery of the Subatomic Particles • The discovery of the subatomic particles came about from the study of electricity & matter. • Benjamin Franklin’s kite experiment in 1752 demonstrated that lightning was electrical.
Charged Particles • In 1832, Michael Faraday proposed that objects are made of positive and negative charges.
Discovery of the Electron • In the late 1870’s many experiments were performed in which electric current was passed through gases at low pressures due to the fact that gases at atmospheric pressure don’t conduct electricity well. • These experiments were carried out in glass tubes called cathode-ray tubes or Crookes tubes. • Sir William Crookes developed these tubes.
Discovery of the Electron • When current was passed through the cathode ray tube, the surface of the tube, directly opposite the cathode, glowed. • It was thought that this glow was caused by a stream of particles called cathode rays. • The rays traveled from cathode (negative) to anode (positive).
Discovery of the Electron • Negatively charged objects deflected the rays away. • Therefore, it was determined that the particles making up the cathode rays were negatively charged.
Joseph John Thomson • In 1897 the English physicist Joseph John Thomson was able to measure the ratio of charge of the cathode ray particles to their mass. • He found that the ratio was always the same regardless of the metal used to make the cathode or the nature of the gas inside the cathode ray tube. • Thomson concluded that cathode rays were composed of identical, negatively charged particles called electrons.
Joseph John Thomson • Thomson’s experiments revealed that the electron has a very large charge-to-mass ratio. • Thomson determined that electrons were present in all elements because he noted that cathode rays had identical properties regardless of the element used to produce them.
Cathode Ray Tube Experiment Accomplishments • Proved that the atom was divisible and that all atoms contain electrons. • This contradicted Dalton’s Atomic Theory. • This allowed a new model of the atom.
Checking for UnderstandingCathode Ray Tube • Why were the cathode rays deflected? They were negatively charged, so they were repelled from the negative plate and attracted to the positive plate. • Why did they assume there was a positive portion to the atom? They knew the atom was neutral, so by default, there must be a positive portion if there are negative particles. • How did this contradict Dalton’s model of the atom? Dalton stated that atoms cannot be subdivided. Electrons are subatomic particles.
Robert A. Millikan • In 1909, Robert Millikan performed an Oil Drop Experiment & calculated the charge of the electron.
Oil Drop Experiment • Millikan dropped negatively charged microscopic oil particles into a chamber containing metallic plates and viewed them with a microscope. • By applying voltage to the metallic plates, Millikan created an electric field. • He was able to suspend the oil droplets by adjusting the electric field to the appropriate strength and direction to overcome gravity.
Oil Drop Experiment • Knowing the mass of the droplets and the strength of the electric field necessary to suspend them, he was able to calculate the charge of the electron. • He noticed that the charge was always a whole-number multiple of 1.602 X10-19 Coulombs. • He determined that the charge of the electron to be 1.602 X 10-19 C.
Checking for UnderstandingOil Drop Experiment • What year did Millikan perform this experiment? 1909 • How did he view the oil droplets? He viewed them with a microscope. • He did NOT measure the charge on the electron; he calculated it. What did he measure? He knew the mass of the droplets and the strength of the electric field.
Discovery of X-Rays • In 1895 William Conrad Roentgen discovered X-rays, a form of radiation.
Radioactivity • In 1896, the French scientist Henri Becquerel was studying a Uranium mineral. He discovered it was spontaneously emitting high-energy radiation. • In 1898, Marie and Pierre Curie attempted to isolate radioactive components of the mineral.
Radioactivity • In 1899, Ernest Rutherford, a British scientist, began to classify radiation: alpha (a), beta (b), and gamma (g).
Radiation • Look closely at the paths of radiation. Do you notice something about the amount of deflection of each type of particles?
Discovery of the Nucleus • In 1911, Ernest Rutherford performed a Gold Foil Experiment. • He and his colleagues bombarded a thin piece of gold foil with fast moving, positively charged alpha particles.
Alpha Particles • Alpha (a) particles are Helium-4 nuclei. • This means they are two protons and two neutrons (with no electrons). • Thus, they are positive.
Gold Foil Experiment • As expected, most of the alpha particles passed straight through with little or no deflection. • However, 1/8000 of the positively charged alpha particles were deflected, some back at the source.
