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• In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic Theory Atomic Structure: Basic Concepts Atomic Structure: Basic Concepts • 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. Topic 2

In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

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Page 1: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container.

Development of the Modern Atomic Theory

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts

• 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.

Topic 2Topic 2

Page 2: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• Lavoisier’s conclusion became known as the law of conservation of mass.

• Lavoisier concluded that when a chemical reaction occurs, mass is neither created nor destroyed but only changed.

Development of the Modern Atomic Theory

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 3: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• In 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.

Development of the Modern Atomic Theory

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 4: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• Proust 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.

Development of the Modern Atomic Theory

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 5: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• John Dalton (1766-1844), an English schoolteacher and chemist, studied the results of experiments by Lavoisier, Proust, and many other scientists.

Dalton’s Atomic Theory

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 6: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• Dalton 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.

Dalton’s Atomic Theory

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 7: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• The following statements are the main points of Dalton’s atomic theory.

Dalton’s Atomic Theory

1. All matter is made up of atoms.

2. Atoms are indestructible and cannot be divided into smaller particles. (Atoms are indivisible.)

3. All atoms of one element are exactly alike, but are different from atoms of other elements.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 8: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

Dalton’s Atomic Theory

4. Atoms of different elements combine to form compounds.

5. Compounds contain atoms in small whole number ratios

6. Atoms can combine in more than one ratio to form different compounds

Page 9: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• Because 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.

The Electron

• In 1897, a British physicist, J.J. Thomson, discovered that this solid-ball model was not accurate. Atom was divisible

• Thomson’s experiments used a vacuum tube.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 10: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• A vacuum tube has had all gases pumped out of it.

The Electron

• 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.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 11: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• When the electrodes are charged, rays travel in the tube from the negative electrode, which is the cathode, to the positive electrode, the anode.

Cathode-Ray Tube

• Because these rays originate at the cathode, they are called cathode rays.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 12: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• Thomson found that the rays bent toward a positively charged plate and away from a negatively charged plate.

Cathode-Ray Tube

• He knew that objects with like charges repel each other, and objects with unlike charges attract each other.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 13: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• These electrons had to come from the matter (atoms) of the negative electrode.

Cathode-Ray Tube

• Thomson concluded that cathode rays are made up of invisible, negatively charged particles referred to as electrons.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 14: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• From Thomson’s experiments, scientists had to conclude that atoms were not just neutral spheres, but somehow were composed of electrically charged particles.

Cathode-Ray Tube

• 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.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 15: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• If atoms contained extremely light, negatively charged particles, then they must also contain positively charged particles—probably with a much greater mass than electrons.

Cathode-Ray Tube

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 16: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• In 1886, scientists discovered that a cathode-ray tube emitted rays not only from the cathode but also from the positively charged anode.

Protons

• These rays travel in a direction opposite to that of cathode rays.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 17: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• Like cathode rays, they are deflected by electrical and magnetic fields, but in directions opposite to the way cathode rays are deflected.

Protons

• 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.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 18: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• At this point, it seemed that atoms were made up of equal numbers of electrons and protons.

Protons

• However, in 1910, Thomson discovered that neon consistedof atoms of two different masses.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 19: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

Protons

• Today, chemists know that neon consists of three naturally occurring isotopes.

• Atoms of an element that are chemically alike but differ in mass are called isotopes of the element.

• The third was too scarce for Thomson to detect.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 20: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

Neutrons

• Calculations showed that such a particle should have a mass equal to that of a proton but no electrical charge.

• Because of the discovery of isotopes, scientists hypothesized that atoms contained still a third type of particle that explained these differences in mass.

• The existence of this neutral particle, called a neutron, was confirmed in the early 1930s.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 21: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

Rutherford’s Gold Foil Experiment

• In 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.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 22: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

Rutherford’s Gold Foil Experiment

• The experimenters set up a lead-shielded box containing radioactive polonium, which emitted a beam of positively charged subatomic particles through a small hole.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 23: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• 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.

• Today, we know that the particles of the beam consisted of clusters containing two protons and two neutrons and are called alpha particles.

Rutherford’s Gold Foil Experiment

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 24: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

The Gold Foil Experiment

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 25: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

The Nuclear Model of the Atom

• Because most of the particles passed through the foil, they concluded that the atom is nearly all empty space.

• To explain the results of the experiment, Rutherford’s team proposed a new model of the atom.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 26: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

The Nuclear Model of the Atom

• Because so few particles were deflected, they proposed that the atom has a small, dense, positively charged central core, called a nucleus.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 27: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

The Nuclear Model of the Atom• The new model of the atom as pictured by

Rutherford’s group in 1911 is shown below.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 28: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

Atomic Numbers

• It is the number of protons that determines the identity of an element, as well as many of its chemical and physical properties.

• The atomic number of an element is the number of protons in the nucleus of an atom of that element.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 29: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

Atomic Numbers

• Therefore, the atomic number of an element also tells the number of electrons in a neutral atom of that element.

• Because atoms have no overall electrical charge, an atom must have as many electrons as there are protons in its nucleus.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 30: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

• The sum of the protons and neutrons in the nucleus is the mass number of that particular atom.

Masses

• The mass of a neutron is almost the same as the mass of a proton.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 31: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

Masses

• Isotopes of an element have different mass numbers because they have different numbers of neutrons, but they all have the same atomic number.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 32: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

Atomic Mass

• In 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.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 33: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

Atomic Mass

• Chemists 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.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2

Page 34: In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. Development of the Modern Atomic

Information in the Periodic Table

• The 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.

Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Atomic Structure: Basic ConceptsAtomic Structure: Basic Concepts Topic 2Topic 2