3The earliest models of the atom were developed by the ancient Greek philosophers. Leucippus of Miletus (490-??? B.C.).First to introduce the idea of the atom, anindivisible unit of matter. This idea was laterextended by his student, Democritus.Democritus (about B.C.) thought that all forms of matter were made of tiny particles called “atoms” from the Greek “atomos” indivisible.
4According to Democritus atoms are: Unchangeable and indivisible.Identical except for their size and shape.Always in motion.
5Democritus imagined that atoms of iron were shaped like coils, making iron rigid, strong, and malleable. Atoms of fire were sharp, lightweight, and yellow.
6Aristotle ( B.C.) rejected the theory of Democritus and endorsed that of Empedocles that stated that matter was made of 4 elements: air, earth, fire , and water.
7Empedocles ( B.C.) believed that these elements have always existed in fixed amounts, and that there two major forces which act upon these elements to both create and destroy: Love and Strife. According to legend, he died by falling into a volcano's crater after failing to become a god as he predicted.
8Aristotle’s influence dominated the thinking of scientists and philosophers until the beginning of the 17th century
16Dalton’s Atomic Theory Elements are composed of minute indivisible particles called atoms.Atoms under special circumstances can be decomposed.Modern research has demonstrated that atoms are composed of subatomic particles.Atoms of the same element are alike in mass and size.Atoms of different elements have different masses and sizes.Chemical compounds are formed by the union of two or more atoms of different elements.
17Dalton’s Atomic Theory Atoms combine to form compounds in simple numerical ratios, such as one to one, two to two, two to three, and so on.Atoms of two elements may combine in different ratios to form more than one compound.
18Dalton’s atoms were individual particles. Atoms of each element are alike in mass and size.
19Dalton’s atoms were individual particles. Atoms of different elements are not alike in mass and size.
20Daltons atoms combine in specific ratios to form compounds.
22The Law of Definite Composition A compound always contains two or more elements combined in a definite proportion by mass.
23Composition of WaterWater always contains the same two elements: hydrogen and oxygen.The percent by mass of hydrogen in water is 11.2%.The percent by mass of oxygen in water is 88.8%.Water always has these percentages. If the percentages were different the compound would not be water.
24Composition of Hydrogen Peroxide Hydrogen peroxide always contains the same two elements: hydrogen and oxygen.The percent by mass of hydrogen in hydrogen peroxide is 5.9%.The percent by mass of oxygen in hydrogen peroxide is 94.1%.Hydrogen peroxide always has these percentages. If the percentages were different the compound would not be hydrogen peroxide.
25The Law of Multiple Proportions Atoms of two or more elements may combine in different ratios to produce more than one compound.
26Combining Masses of Hydrogen and Oxygen Mass Hydrogen(g)Mass Oxygen(g)Water1.08.0Hydrogen Peroxide16.0Hydrogen peroxide has twice as much oxygen (by mass) as does water.
27Combining Ratios of Hydrogen and Oxygen Hydrogen peroxide has twice as many oxygens per hydrogen atom as does water.The formula for water is H2O.The formula for hydrogen peroxide is H2O2.
29Surrounding the atomic nucleus are electrons Surrounding the atomic nucleus are electrons. The name electron comes from the Greek word for amber, a brownish-yellow fossil resin studied by the early Greeks. They found that when amber was rubbed by a piece of cloth, it attracted such things as bits of straw. This phenomenon, known as the amber effect, remained a mystery for almost 2000 years. In the late 1500s other materials that behaved like amber were called “electrics”. The concept of electric charge awaited experiments by Benjamin Franklin nearly two centuries later. Franklin experimented with electricity and postulated the existence of an electric fluid that could flow from place to place. An object with an excess of this fluid he called electrically positive, and one with a deficiency of the fluid he called electrically negative. The fluid was thought to attract ordinary matter but to repel itself. We still follow Franklin’s lead in how we define positive and negative electricity. Franklin’s 1752 experiment with the kite in the lightning storm showed that lightning is an electrical discharge between clouds and the ground. This discovery told him that electricity is not restricted to solid or liquid objects and that it can travel through a gas.
30Ben Franklin ( ) flew kites to demonstrate that lightning is a form of static electricity (ESD). He would run a wire to the kite and produce sparks at the ground, or charge a Leyden jar. This led Franklin to invent the lightning rod. Franklin also made several electrostatic generators with rotating glass balls to experiment with. These experiments led him to formulate the single fluid (imponderable fluid) theory of electricity. Previous theories had held there were two electrical fluids and two magnetic fluids. Franklin theorized just one imponderable electrical fluid (a fluid under conservation) in the universe. The difference in electrical charges was explained by an excess ( + ) or defect ( - ) of the single electrical fluid. This is where the positive ( + ) and negative ( - ) symbols come from in electrical science.
32Michael Faraday discovered that certain substances, when dissolved in water, conducted an electric current.He found that atoms of some elements moved to the cathode (negative electrode) and some moved to the anode (positive electrode).He concluded they were electrically charged and called them ions (Greek wanderer).
34Svante Arrhenius reasoned that an ion is an atom (or a group of atoms) carrying a positive or negative electric charge.Arrhenius accounted for the electrical conduction of molten sodium chloride (NaCl) by proposing that melted NaCl dissociated into the charged ions Na+ and Cl-.NaCl → Na+ + Cl-Δ
35NaCl → Na+ + Cl-In the melt the positive Na+ ions moved to the cathode (negative electrode). Thus positive ions are called cations.In the melt the negative Cl- ions moved to the anode (positive electrode). Thus negative ions are called anions.
39The diameter of an atom is 0.1 to 0.5 nm. This is 1 to 5 ten billionths of a meter.Even smaller particles than atoms exist. These are called subatomic particles.If the diameter of this dot is 1 mm then 10 million hydrogen atoms would form a line across the dot.
42In 1875 Sir William Crookes invented the Crookes tube.
43Crookes tubes experiments led the way to an understanding of the subatomic structure of the atom.
44Crookes tube emissions are called cathode rays. Below are Crookes cathode-ray tubes. The cathode-rays (streams of electrons) can be clearly seen.
45"Maltese Cross" Crookes Tube Demonstrates that radiant matter is blocked by metal objects
46Other Interesting Crookes Tubes May Be Found At the Sites Below:
47In 1897 Sir Joseph Thomson demonstrated that cathode rays: travel in straight lines.are negative in charge.are deflected by electric and magnetic fields.produce sharp shadowsare capable of moving a small paddle wheel.
51J.J. Thomson determined and is given credit for finding: The charge to mass (e/m) ratio of the cathode ray.The cathode ray was re-named the “electron”.Thomson “discovered” the electron.
52Can atoms be split apart. Does each atom have inner workings Can atoms be split apart? Does each atom have inner workings? Parts which can be separated? Parts which can perhaps be put to some use? These questions had already come to mind in 1898, when J. J. Thomson isolated the electron. That was the first solid proof that atoms are indeed built of much tinier pieces. Thomson speaks of the electron in this recorded passage...Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen, which itself is so small that a crowd of these atoms equal in number to the population of the whole world would be too small to have been detected by any means then known to science.
53Robert Millikan Determined the charge of the electron. Experiment called the Oil Drop Experiment.
58Eugen Goldstein, a German physicist, first observed protons in 1886: Thomson determined the protons’ characteristics.Thomson showed that atoms contained both positive and negative charges.This disproved the Dalton model of the atom which held that atoms were indivisible.
68X-rays were discovered by Wilhelm Röentgen in 1895
69Röentgen observed that a vacuum discharge tube enclosed in a thin, black cardboard box had caused a nearby piece of paper coated with the salt barium platinocyanide to phosphorescence.From this and other experiments he concluded that certain rays, which he called X-rays, were emitted from the discharge tube, penetrated the box, and caused the salt to glow.
70Radioactivity was discovered by Henri Becquerel in 1896.
71Shortly after Röentgen’s discovery, Antoine Henri Becquerel attempted to show a relationship between X-rays and the phosphorescence of uranium salts.Becquerel wrapped a photographic plate in black paper, sprinkled a sample of a uranium salt on it, and exposed it to sunlight.
72When Becquerel attempted to repeat the experiment the sunlight was intermittent. He took the photographic plate wrapped in black paper with the uranium sample on it, and placed the whole setup in a drawer.
73Several days later he developed the film and was amazed to find an intense image of the uranium salt on the plate.He repeated the experiment in total darkness with the same result.This proved that the uranium salt emitted rays that affected the photographic plate, and that these rays were not a result of phosphorescence due to exposure to sunlight.
74Two years later, in 1898, Marie Curie coined the name radioactivity. Radioactivity is the spontaneous emission of particles and/or rays from the nucleus of an atom.
75Marie Curie, in a classic experiment, proved that alpha and beta particles are oppositely charged. three types of radiation are detected by a photographic plateradiation passes between the poles of an electromagnetBeta rays are strongly deflected to the positive pole.Gamma rays are not deflected by the magnet.Alpha rays are less strongly deflected to the negative pole.a radioactive source was placed inside a lead block
78In 1899 Rutherford began to investigate the nature of the rays emitted by uranium. He found two particles in the rays. He called them alpha and beta particles.
79Rutherford in 1911 performed experiments that shot a stream of alpha particles at a gold foil. Most of the alpha particles passed through the foil with little or no deflection.He found that a few were deflected at large angles and some alpha particles even bounced back.
81An electron with a mass of 1/1837 amu could not have deflected an alpha particle with a mass of 4 amu.Rutherford knew that like charges repel.Rutherford concluded that each gold atom contained a positively charged mass that occupied a tiny volume. He called this mass the nucleus.
82If a positive alpha particle approached close enough to the positive mass it was deflected. Most of the alpha particles passed through the gold foil. This led Rutherford to conclude that a gold atom was mostly empty space.
83Because alpha particles have relatively high masses, the extent of the reflections led Rutherford to conclude that the nucleus was very heavy and dense.
84Deflection Scattering Deflection and scattering of alpha particles by positive gold nuclei.
85Ideas about the atom were refined by one of Thomson's students, Ernest Rutherford. He showed that the mass in an atom is not smeared out uniformly throughout the atom, but is concentrated in a tiny, inner kernel: the nucleus. Rutherford wanted to understand the nucleus, not for any practical purpose, but because he was attracted to the beauty of its simplicity. Fundamental things should be simple not complex. Here is how he explains himself inThe bother is that a nucleus, as you know, is a very small thing, and we know very little about it. Now, I had the opinion for a long time, that's a personal conviction, that if we knew more about the nucleus, we'd find it was a much simpler thing than we suppose, that these fundamental things I think have got to be fairly simple. But it's the non-fundamental things that are very complex usually. I am always a believer in simplicity being a simple person myself.
86The gamma ray, a third type of emission from radioactive material, was discovered by Paul Villard in 1900.
87Alpha, Beta, and Gamma Radiation NameNuclide SymbolParticle SymbolMass (amu)ChargeAlpha4+2Beta–1Gamma Ray
89Rutherford’s experiment showed that an atom had a dense, positively charged nucleus. Chadwick’s work in 1932 demonstrated the atom contains neutrons.Rutherford also noted that light, negatively charged electrons were present in an atom and offset the positive nuclear charge.
90Rutherford put forward a model of the atom in which a dense, positively charged nucleus is located at the atom’s center.The negative electrons surround the nucleus.The nucleus contains protons and neutrons
100Atoms of the same element can have different masses. They always have the same number of protons, but they can have different numbers of neutrons in their nuclei.The difference in the number of neutrons accounts for the difference in mass.These are isotopes of the same element.
101Greek roots isos (equal) and topos (place) Greek roots isos (equal) and topos (place). Hence: "the same place," meaning that different isotopes of a single element occupy the same position on the periodic table.
102Isotopes of the Same Element Have Equal numbers of protonsDifferent numbers of neutrons
103Isotopic Notation Mass number is also the number of nucleons in the nucleus.Nucleons = protons and/or neutrons
104Relationship Between Mass Number and Atomic Number
105The mass number minus the atomic number equals the number of neutrons in the nucleus. -=109-47=62
112The mass of a single atom is too small to measure on a balance. Using a mass spectrometer, the mass of the hydrogen atom was determined.
113A Modern Mass Spectrometer Positive ions formed from sample.Electrical field at slits accelerates positive ions.Deflection of positive ions occurs at magnetic field.From the intensity and positions of the lines on the mass spectrogram, the different isotopes and their relative amounts can be determined.A mass spectrogram is recorded.
114A typical reading from a mass spectrometer A typical reading from a mass spectrometer. The two principal isotopes of copper are shown with the abundance (%) given.
115Using a mass spectrometer, the mass of one hydrogen atom was determined to be 1.673 x 10-24 g
117The mass of a hydrogen atom is very small. To overcome this problem a system of relative atomic weights using “atomic mass units” was devised to express the masses of elements using simple numbers.Numbers of this size are too small for practical use.1.673 x g
118The standard to which the masses of all other atoms are compared to was chosen to be the most abundant isotope of carbon.
119A mass of exactly 12 atomic mass units (amu) was assigned to
1201 amu is defined as exactly equal to the mass of a carbon-12 atom 1 amu = x g
125Most elements occur as mixtures of isotopes. Isotopes of the same element have different masses.The listed atomic mass of an element is the average relative mass of the isotopes of that element compared to the mass of carbon-12 (exactly …amu)
126Average atomic mass (amu) To calculate the atomic mass multiply the atomic mass of each isotope by its percent abundance and add the results.IsotopeIsotopic mass (amu)Abundance (%)Average atomic mass (amu)69.0930.9163.55( amu)=43.48 amu( amu)=20.07 amu63.55 amu
127Isotope Practice (Fill-in the Blanks) symbolatomic nomass no# e# n# p81610Pt11730P–353744836344540Ca+288781957878153018151512753533684363479342040182020