KD McMahon Reseda High School Atoms and The Periodic Table Aristotle and Democritus In the fourth century BC two models were presented by Greek philosophers to explain the nature of matter. Aristotle, perhaps the most important philosopher and naturalists that ever lived, explained that all matter was composed of four earth elements: earth, air, fire, and water. Combinations of these four elements made up everything in the universe. Rearrangement of these elements would result, according to Aristotle, in the formation of different or new substances. Alchemy, an odd combination of science and superstition, arose out of this idea. Alchemists attempted to turn base earth elements, such as lead, into gold or the "philosopher's stone." Democritus believed that matter was made up of tiny particles which he called atoms. Individuals that agreed with this "school" of thought were known as "atomists." According to Democritus, atoms were the smallest thing that exist. Atoms could not be subdivided into smaller particles, that is, they couldn't be cut in two. That is why he called them "a tome" (atom) which means "not cut." Democritus further speculated that atoms came in many sizes, shapes, colors, and weights. Some were smooth while others, he believed, were bumpy. Democritus said that atoms had existed forever, would continue to do so, and would never change. Many of Democritus' ideas were fairly close to the truth. Unfortunately, Aristotle's model of the atom was more popular. For centuries, alchemists pursued futile dreams of manipulating the "four elements" to achieve riches and immortal life. John Dalton and the Atomic Theory John Dalton (1766- 1844) was a teacher (he started teaching at the age of 12) and a prolific scientist. His principle interests were meteorology and chemistry. In 1808, Dalton presented his Atomic Theory. Dalton's atomic theory included the following ideas: * All elements are composed of submicroscopic indivisible particles called atoms. * Atoms of the same element are identical. The atoms of any one element are different from those of any other element. * Atoms of different elements can physically mix together or can chemically combine with one another in simple whole-number ratios to form compounds. * Chemical reactions occur when atoms are separated, joined, or rearranged. However, atoms of one element are never changed into atoms of another element (transmutation) as a result of a chemical reaction. Dalton developed a system of symbols to depict atoms and the compounds they formed. He envisioned that compounds were held together by the interlocking of spines that extended outward from each atom; sort of a modern day Velcro. Question #1 Why did Democritus call his fundamental particle of matter the “atom?” Question #2: Why was Democritus’ theory rejected for so many centuries? Question #3: Based on Dalton’s atomic theory what would be wrong with the following formula for water: HO1/2 Question #4: According to Dalton’s atomic theory would it have been possible for the alchemists to have turned the element of lead into the element gold? Explain. The Experiments of J.J. Thomson The English chemist, Sir J.J. Thomson (1856-1940) was able to demonstrate that the atom was not indivisible. He showed that it was composed of smaller particles. From his experimental results he proposed one of the first models of the atom. During the 1850's Geissler sealed a positive (anode) and negative (cathode) electrode in an evacuated tube. When a high voltage was applied to the electrodes a glowing beam traveled from the cathode to the anode. It was assumed that this beam was a form of light, hence it was called a "cathode ray." The tubes were known as "cathode ray tubes." Thomson was not convinced that these "rays" were a form of energy. He performed a variety of experiments using special cathode ray tubes that he designed himself. Thomson devised a cathode ray tube with a paddle wheel built inside. When the high voltage electricity was turned on the paddle wheel began to rotate and move away from the cathode and towards the anode. Thomson concluded that the cathode rays were not a form of light energy, but were actually tiny particles with mass. When these particles collided with the paddle wheel they transfered their momentum to the wheel causing it to move. Thomson devised another cathode ray tube with positive and negatively charged plates. Thomson observed that the particles were being repelled by the negatively charged plate and attracted to the positive plate. Thomson concluded from this evidence and from his previous experiments that tiny particles were being emitted from the atoms of the cathode. These tiny particles were negatively charged. He called these particles "electrons." In 1886, E. Goldstein, using a cathode ray tube in which the cathode had holes, observed rays traveling in the opposite direction to the cathode ray. These rays emitted from the anode were called anode or canal rays. Thomson recognized that these rays must be composed of positive material. He reasoned that this material must be present along with the negative electrons. This would explain why matter is not electrically charged. Given these experimental results, Thomson proposed in 1897 what has since been referred to as the "plum pudding" model of the atom. This model depicts the atom as a diffuse cloud of positive charge with the negative electrons embedded randomly in it, like plums in pudding. Question #5: What three facts about the atom did Thomson learn from studying cathode and anode rays? Question #6: Suppose Thomson obtained results like that shown below. What would he have concluded about the particle he discovered? Millikan and the Mass of the Electron In addition to theorizing his Plum Pudding Model of the atom, Thomson was able to calculate the charge-to-mass ratio of the electron: e/m = -1.76 X 108 C/g where e represents the charge on the electron in coulombs and m represents the mass in grams. In 1909, Robert Millikan (1868-1953), working at the University of Chicago, used the charge-to-mass ratio to determine the mass of the electron. Millikan designed a clever experiment using oil drops. He produced very tiny oil droplets using an atomizer. As these droplets fell through a hole in the chamber they were given a charge by bombarding them with X-rays. By adjusting the charge on the plates he could halt the fall of the droplets. The voltage and the mass of an oil drop could then be used to calculate the charge on the oil drop. Millikan's experiments showed that the charge on an oil drop is always a wholenumber multiple of the electron charge. Using this information he calculated the mass of the electron as 9.11 X 10-28 grams. Question #7: Thomson determined that the charge to mass ration of the electron (e/m) = -1.76 X 108 C/g (C = Coulombs). Millikan was able to use this information and the results he obtained from his oil drop experiment to determine mass of the electron. He determined the mass of the electron to be 9.11 X 10-28 grams. Use this information to calculate the charge of the electron in coulombs. Rutherford and the Planetary Atom In the first few years of the 20th century, Ernest Rutherford, a student of J.J. Thomson, began some experiments on the nature of radiation that would eventually lead him to propose his own model of the atom. In 1896, the French scientist, Henri Becquerel, found that a piece of a mineral containing uranium could produce its image on a photographic plate in the absence of light. He attributed this phenomenon to a spontaneous emission of something which he called "radiation" which originated from the uranium. Rutherford performed experiments on uranium, and other radioactive elements, to determine the nature of radiation. Rutherford placed a piece of uranium in a lead box. A beam of radioactivity was allowed to pass through an opening in the box. This beam passed through two charged plates. A fluorescent screen would detect radiation by flashing whenever it was struck by the radiation. Rutherford observed that the radiation was diffracted into three beams by the charged plates. He concluded that the beam attracted to the negative plate consisted of positively charged particles. He called these particles "alpha particles" Likewise, he concluded that those particles attracted to the positively charged plate had to be negatively charged. He called these particles "beta particles." He recognized that the radiation that was not diffracted by the charged plates had to be a form of electromagnetic radiation. He called this "gamma radiation." Rutherford used his newly discovered knowledge of the nature of radiation to test Thomson's model of the atom. Rutherford set up the apparatus which would bombard thin gold foil with alpha particles; these particles would then be detected by the fluorescent screen. He anticipated that all of the alpha particle detection would occur on the screen directly behind the foil. This would be consistent with Thomson's model in which the positive charge is diffusely distributed throughout the atom. The positively charged alpha particles would easily pass through the foil and its diffusely charged gold atoms and strike the screen behind it. Rutherford got surprisingly different results. He observed alpha particle detection even in front of the foil. This meant that the particles were being deflected by the gold foil atoms. Rutherford observed, "It was about as credible as if you had fired a 15inch shell at a piece of tissue paper and it came back and hit you!" Rutherford suggested that the atom was mostly empty space with a highly charged center. Most of the particles pass through the atom undisturbed, but a few get too close to the center and are deflected. To account for these results Rutherford proposed a new model of the atom in 1913. This model had the following characteristics: The atom is mostly empty space with the majority of its mass concentrated in the center of the atom which he called the "nucleus." This nucleus was composed of charged particles called "protons." Protons have an equal amount of charge as an electron, but they are positive, not negative. Protons have a mass nearly 2000 times greater than an electron. The electrons rotate around the nucleus like planets around the sun. This is why Rutherford's model is often referred to as "Planetary Model." Question #8: Discuss the role of serendipity in Becquerel’s discovery of radiation. Question #9: Suppose Rutherford had obtained the results illustrated here. Draw a model of an atom that would be consistent with these results. Explain your model and why it is consistent with the experimental results. Question #10: Discuss the results Rutherford actually obtained . Explain how this evidence led him to propose the Planetary model of the atom. Draw and label his atomic model. Moseley and Atomic Number During this same time, Henry Moseley (18871915) was a lecturer in physics at Ernest Rutherford's laboratory at the University of Manchester. He did important research on the role of the proton in the atom which led to a better understanding of the elements. Just before the turn of the century, Roentgen discovered X-rays. Shortly after this discovery a British scientist, Charles Barkla, found that each element, on being struck by cathode rays, emitted their own characteristic X-rays. This led Moseley to make a systematic study of the elements using X-rays. Moseley observed that as he went across a row of elements the frequency of the X-rays emitted became progressively higher as the atomic mass increased. This suggested to Moseley that there might be another "number" in the atom in which there is a truely linear relationship with the X-rays emitted. He decided that the frequency emitted reflected the size of the electron's orbit around the nucleus. He reasoned that the closer the electrons were to the nucleus, the smaller would be their orbits, and the higher the frequency of the Xrays they emitted. Since the frequency increased with the weight of the atom, then in heavier atoms the electrons must be drawn closer to the nucleus. What was the force that drew them closer? Moseley suggested that what drew the electrons closer in was the increase in the nucleus' positive charge which attracted the negative electrons. He further reasoned the nuclear charge must increase from element to element right through the periodic table. The most reasonable way to account for this is to suppose that each element has one more unit of positive charge, one more proton, than the one before. This number of protons in an atom is known as that element's "atomic number." When Moseley plotted the frequency of X-rays emitted versus the element's atomic number he obtained the true linear relationship he was seeking. Using this graph he was able to predict the existence of unknown elements, and the frequency of X-rays they would emit. Chemists now know that it is the number of protons in an atom's nucleus which makes one element different from another. In addition, the number of protons and electrons are equal in neutral atoms. Other scientists had to complete Moseley's work when his life was tragically cut short during World War I. Question #11: When Moseley plotted the square root of the frequency of X-rays emitted by elements against their atomic mass he obtained nearly a straight line. What did this suggest to Moseley? This eventually led to the discovery of another “number” that could be used to describe the atom. What is this number and what does it tell us about the atom? Aston and Isotopes While studying can rays, Aston, an English scientist working in Thomson's lab, made an important discovery that would eventually lead to the discovery of another subatomic particle. Aston accelerated canal rays consisting of Neon nuclei through a specialized anode ray tube. As the neon nuclei passed by charged plates and a magnetic field they were deflected. The extent of their deflection was detected on a fluorescent screen at the end of the tube. To Aston's surprise three different types of neon atoms were detected on the screen. The neon atoms differed in mass. The more massive neon was deflected least because of its greater momentum. The least massive neon was deflected the most. Analysis of these three types of neon atoms demonstrated that they had masses of 20, 21, and 22 amu (atomic mass units). These values are known as "mass numbers." Aston called these atoms that were the same element, but that had different masses, "isotopes." Isotope means "same place;" indicating that isotopes are atoms of the same element and would all be in the "same place" on the Periodic Table. Continued research by Aston and others showed that all of the elements have isotopes, ie: atoms that have the same number of protons (and therefore atomic number), but have different masses. Question #12 Identify the isotopes of Neon in the diagram as either 20, 21, or 22. How do you know that this is the correct pattern? Question #13 Why did Aston’s discovery suggest that there was another fundamental particle that made up the atom? Chadwick and the Neutron Aston and his colleagues knew that the proton and electrons could not account for the existence of isotopes. In 1920, Rutherford suggested that there must be an undiscovered particle in the atom. In 1932, his student, John Chadwick, discovered the particle. Chadwick bombarded beryllium atoms with alpha particles. The resulting collisions broke the beryllium nuclei apart sending subatomic particles in all directions. The subatomic particles passed by charged plates. Some of the particles where deflected, as expected, towards the negative electrode. These particles were protons that formerly made up the beryllium nuclei. Some of the particles however, were not deflected by the charged plates. Chadwich concluded that these heretofore unknown particles were uncharged and with further analysis he determined that they had a mass equal to that of the proton. He called these particles "neutrons." Mass Number and Atomic Mass By the early 1930's the major subatomic particles had been discovered and their physical properties had been described. Particle Proton Electron Neutron Mass (grams) 1.67262 X 10-27 9.10939 X 10-31 1.67493 X 10-27 Relative Mass (g) 1.007 5.486 X 10-4 (~ 0) 1.009 Relative Charge +1 -1 0 The mass numbers that Aston had observed could now be explained. Neon has an atomic number of 10; each neon atom has 10 protons in the nucleus. This accounts for 10 amu of mass. The isotope of neon with a mass of 20 must have 10 protons and 10 neutrons. Neon-21 must have 10 protons and 11 neutrons. Neon-22 has 10 protons and 12 neutrons. As can be seen from the table below the mass numbers of an element's isotopes and the element's atomic mass are close in value, but different. Isotope 32 S 35 S 34 S 35 S Mass Number 32 33 34 36 Atomic Number 16 16 16 16 Mass (amu) 31.972 32.971 33.967 35.967 % Abundance 95 0.76 4.22 0.014 The atomic mass of sulfur is 32.064 amu. How is the atomic mass of an element determined from its isotopes? An element's atomic mass is a "weighted average" of the mass numbers of all of the isotopes of that element. Weighting takes into account the varying abundances of each isotope. Consider the data for sulfur: (.95)(32) + (.007)(33) + (.042)(34) + (.00014)(36) = 30.4 + 0.23 + 1.43 + 0.005 = 32.065 The data to calculate the atomic mass for sulfur and the other elements is obtained by "mass spectroscopy." In a mass spectrometer, atoms are vaporized and passed into a beam of high-speed electrons. The high-speed electrons knock off electrons of the atoms being analyzed and change them into positive ions. An applied electron filed then accelerates these ions into a magnetic field. Because an accelerating ion produces its own magnetic field, an interaction with the applied field occurs, which tends to change the path of the ion. The amount of path deflection for each ion depends on its mass. The most massive ions are deflected the smallest amount; this causes the ions to separate. A comparison of the position where the ions hit the detector plate gives very accurate values of the relative masses. When C-12 and C-13 are analyzed in a mass spectrometer, the ratio of their masses is found to be: mass of C-13 = 1.0836129 mass of C-12 Since the atomic mass unit is defined such that the mass of C-12 is exactly 12 amu, then on this same scale: mass C-13 = (1.0836129)(12amu) = 13.0034amu The masses of other atoms can be determined in a similar fashion. The mass spectrometer is also used to determine the isotopic composition of an element. For example, when a sample of natural neon is injected into a mass spectrometer, the resultant graphs are obtained: The areas of the "peaks" or heights of the bars indicated the relative numbers of Ne-20, Ne-21, and Ne-22. The average atomic mass = (0.9092)(20) + (0.00257)(21) + (0.0882)(22) = 20.18 amu Question #14: Complete the chart on the following page for the hypothetical elemental samples. Assume all atoms are neutral. Element A B C D Atomic # 12 Mass # 10 23 30 # of protons # of neutrons 10 # of electrons 5 11 20 Mendeleev and the Periodic Table Dmitri Ivanovich Mendeleev (1834-1907) knew that certain elements shared similar properties. He believed that a careful analysis of these properties would someday yield a unity and predictability to chemistry that was still absent in the field. One of the properties he studied was an element's "combining power." The hydrogen atom, for example, could not take on more than one atom at a time. Oxygen, on the other hand, could combine with two other atoms. In 1852, the English chemist, Edward Frankland, coined the term "valence" (from the Latin word meaning "power") to denote combining power. Hydrogen, therefore, has a valence of one and oxygen has a valence of two. Mendeleev then proceeded to make a card for each of the known elements. The card had the element's name, symbol, valence, and atomic weight (atomic number was unknown at the time). Mendeleev arranged the cards in various ways looking for any kind of pattern he might find to the elements. He discovered that when he arranged the elements be increasing atomic weight the properties of the elements (as indicated by their valence) would repeat periodically. He constructed a table consisting of eight columns and nine rows. Elements which belonged to the same column had similar properties. Mendeleev's table revealed that the properties of the elements are repeated in an orderly way when they are arranged by increasing atomic weight. Thus, the properties of the elements are a periodic function of their atomic weights. This statement is called "Periodic Law." Mendeleev's table became known as the "Periodic Table of the Elements." Mendeleev observed that there were several blanks in his table. he predicted that these blanks represented elements which had not yet been discovered. Mendeleev accurately foretold these missing element's atomic weights and chemical properties. In 1874, three years after Mendeleev published his Periodic Table, the first of these predicted elements was discovered. The Modern Periodic Table Mendeleev's Periodic Table was not without its flaws. There were infact several places in the table in which elements seemed to be in the wrong place. Henry Moseley's discovery of atomic numbers would eventually lead to a modification of the Periodic Table into the form that we are familiar with today. When chemists arranged the elements in the Periodic Table according to increasing atomic number the problems that were inherent with Mendeleev's table were eliminated. Question #15: Mendeleev arranged his table by increasing…. He discovered that the chemical properties of the elements would repeat themselves every ______ elements. This periodic function of the elements became know as …. Question #16: After Moseley made his discovery, the Periodic Table was rearranged so that the elements increased by …. Question #17: Label the alkali metals, alkali-earth metals, transition elements, halogens, noble gases, the lanthanide, and actinides.