Presentation on theme: "Development of the Modern Atomic Theory"— Presentation transcript:
1Development of the Modern Atomic Theory Atomic Structure: Basic ConceptsTopic2Development of the Modern Atomic TheoryIn 1782, a French chemist, Antoine Lavoisier ( ), made measurements of chemical change in a sealed container.He observed that the mass of reactants in the container before a chemical reaction was equal to the mass of the products after the reaction.
2Development of the Modern Atomic Theory Atomic Structure: Basic ConceptsTopic2Development of the Modern Atomic TheoryLavoisier concluded that when a chemical reaction occurs, mass is neither created nor destroyed but only changed.Lavoisier’s conclusion became known as the law of conservation of mass.
3Development of the Modern Atomic Theory Atomic Structure: Basic ConceptsTopic2Development of the Modern Atomic TheoryIn 1799, another French chemist, Joseph Proust, observed that the composition of water is always 11 percent hydrogen and 89 percent oxygen by mass.Regardless of the source of the water, it always contains these same percentages of hydrogen and oxygen.
4Development of the Modern Atomic Theory Atomic Structure: Basic ConceptsTopic2Development of the Modern Atomic TheoryProust studied many other compounds and observed that the elements that composed the compounds were always in a certain proportion by mass. This principle is now referred to as the law of definite proportions.
5Dalton’s Atomic Theory Atomic Structure: Basic ConceptsTopic2Dalton’s Atomic TheoryJohn Dalton ( ), an English schoolteacher and chemist, studied the results of experiments by Lavoisier, Proust, and many other scientists.
6Dalton’s Atomic Theory Atomic Structure: Basic ConceptsTopic2Dalton’s Atomic TheoryDalton proposed his atomic theory of matter in 1803.Although his theory has been modified slightly to accommodate new discoveries, Dalton’s theory was so insightful that the essence of his theory remains intact up to the present time.
7Dalton’s Atomic Theory Atomic Structure: Basic ConceptsTopic2Dalton’s Atomic TheoryThe following statements are the main points of Dalton’s atomic theory.1. All matter is made up of atoms.2. Atoms are indestructible and cannot bedivided into smaller particles. (Atoms areindivisible.)3. All atoms of one element are exactlyalike, but are different from atoms ofother elements.
8Dalton’s Atomic Theory 4. Atoms of different elements combine to form compounds.5. Compounds contain atoms in small whole number ratios6. Atoms can combine in more than one ratio to form different compounds
9Thomson’s experiments used a vacuum tube. Atomic Structure: Basic ConceptsTopic2The ElectronBecause of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts.In 1897, a British physicist, J.J. Thomson, discovered that this solid-ball model was not accurate. Atom was divisibleThomson’s experiments used a vacuum tube.
10A vacuum tube has had all gases pumped out of it. Atomic Structure: Basic ConceptsTopic2The ElectronA vacuum tube has had all gases pumped out of it.At each end of the tube is a metal piece called an electrode, which is connected through the glass to a metal terminal outside the tube.These electrodes become electrically charged when they are connected to a high-voltage electrical source.
11Atomic Structure: Basic Concepts Topic2Cathode-Ray TubeWhen the electrodes are charged, rays travel in the tube from the negative electrode, which is the cathode, to the positive electrode, the anode.Because these rays originate at the cathode, they are called cathode rays.
12Atomic Structure: Basic Concepts Topic2Cathode-Ray TubeThomson found that the rays bent toward a positively charged plate and away from a negatively charged plate.He knew that objects with like charges repel each other, and objects with unlike charges attract each other.
13Atomic Structure: Basic Concepts Topic2Cathode-Ray TubeThomson concluded that cathode rays are made up of invisible, negatively charged particles referred to as electrons.These electrons had to come from the matter (atoms) of the negative electrode.
14Atomic Structure: Basic Concepts Topic2Cathode-Ray TubeFrom Thomson’s experiments, scientists had to conclude that atoms were not just neutral spheres, but somehow were composed of electrically charged particles.Reason should tell you that there must be a lot more to the atom than electrons.Matter is not negatively charged, so atoms can’t be negatively charged either.
15Atomic Structure: Basic Concepts Topic2Cathode-Ray TubeIf atoms contained extremely light, negatively charged particles, then they must also contain positively charged particles—probably with a much greater mass than electrons.
16These rays travel in a direction opposite to that of cathode rays. Atomic Structure: Basic ConceptsTopic2ProtonsIn 1886, scientists discovered that a cathode-ray tube emitted rays not only from the cathode but also from the positively charged anode.These rays travel in a direction opposite to that of cathode rays.
17Atomic Structure: Basic Concepts Topic2ProtonsLike cathode rays, they are deflected by electrical and magnetic fields, but in directions opposite to the way cathode rays are deflected.Thomson was able to show that these rays had a positive electrical charge.Years later, scientists determined that the rays were composed of positively charged subatomic particles called protons.
18However, in 1910, Thomson discovered that neon consisted Atomic Structure: Basic ConceptsTopic2ProtonsAt this point, it seemed that atoms were made up of equal numbers of electrons and protons.However, in 1910, Thomson discovered that neon consistedof atoms of two different masses.
19The third was too scarce for Thomson to detect. Atomic Structure: Basic ConceptsTopic2ProtonsAtoms of an element that are chemically alike but differ in mass are called isotopes of the element.Today, chemists know that neon consists of three naturally occurring isotopes.The third was too scarce for Thomson to detect.
20Atomic Structure: Basic Concepts Topic2NeutronsBecause of the discovery of isotopes, scientists hypothesized that atoms contained still a third type of particle that explained these differences in mass.Calculations showed that such a particle should have a mass equal to that of a proton but no electrical charge.The existence of this neutral particle, called a neutron, was confirmed in the early 1930s.
21Rutherford’s Gold Foil Experiment Atomic Structure: Basic ConceptsTopic2Rutherford’s Gold Foil ExperimentIn 1909, a team of scientists led by Ernest Rutherford in England carried out the first of several important experiments that revealed an arrangement far different from the cookie-dough model of the atom.
22Rutherford’s Gold Foil Experiment Atomic Structure: Basic ConceptsTopic2Rutherford’s Gold Foil ExperimentThe experimenters set up a lead-shielded box containing radioactive polonium, which emitted a beam of positively charged subatomic particles through a small hole.
23Rutherford’s Gold Foil Experiment Atomic Structure: Basic ConceptsTopic2Rutherford’s Gold Foil ExperimentToday, we know that the particles of the beam consisted of clusters containing two protons and two neutrons and are called alpha particles.The sheet of gold foil was surrounded by a screen coated with zinc sulfide, which glows when struck by the positively charged particles of the beam.
25The Nuclear Model of the Atom Atomic Structure: Basic ConceptsTopic2The Nuclear Model of the AtomTo explain the results of the experiment, Rutherford’s team proposed a new model of the atom.Because most of the particles passed through the foil, they concluded that the atom is nearly all empty space.
26The Nuclear Model of the Atom Atomic Structure: Basic ConceptsTopic2The Nuclear Model of the AtomBecause so few particles were deflected, they proposed that the atom has a small, dense, positively charged central core, called a nucleus.
27The Nuclear Model of the Atom Atomic Structure: Basic ConceptsTopic2The Nuclear Model of the AtomThe new model of the atom as pictured by Rutherford’s group in 1911 is shown below.
28Atomic Structure: Basic Concepts Topic2Atomic NumbersThe atomic number of an element is the number of protons in the nucleus of an atom of that element.It is the number of protons that determines the identity of an element, as well as many of its chemical and physical properties.
29Atomic Structure: Basic Concepts Topic2Atomic NumbersBecause atoms have no overall electrical charge, an atom must have as many electrons as there are protons in its nucleus.Therefore, the atomic number of an element also tells the number of electrons in a neutral atom of that element.
30The mass of a neutron is almost the same as the mass of a proton. Atomic Structure: Basic ConceptsTopic2MassesThe mass of a neutron is almost the same as the mass of a proton.The sum of the protons and neutrons in the nucleus is the mass number of that particular atom.
31Atomic Structure: Basic Concepts Topic2MassesIsotopes of an element have different mass numbers because they have different numbers of neutrons, but they all have the same atomic number.
32Atomic Structure: Basic Concepts Topic2Atomic MassIn order to have a simpler way of comparing the masses of individual atoms, chemists have devised a different unit of mass called an atomic mass unit, which is given the symbol u.An atom of the carbon-12 isotope contains six protons and six neutrons and has a mass number of 12.
33Therefore, 1 u = 1/12 the mass of a carbon-12 atom. Atomic Structure: Basic ConceptsTopic2Atomic MassChemists have defined the carbon-12 atom as having a mass of 12 atomic mass units.Therefore, 1 u = 1/12 the mass of a carbon-12 atom.1 u is approximately the mass of a single proton or neutron.
34Information in the Periodic Table Atomic Structure: Basic ConceptsTopic2Information in the Periodic TableThe number at the bottom of each box is the average atomic mass of that element.This number is the weighted average mass of all the naturally occurring isotopes of that element.