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In 1782, a French chemist, Antoine Lavoisier ( ), made measurements of chemical change in a sealed container. Development of the Modern Atomic Theory 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 Topic 2

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 Concepts Topic 2 Topic 2 Click box to view movie clip.

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

John Dalton ( ), an English schoolteacher and chemist, studied the results of experiments by Lavoisier, Proust, and many other scientists. Dalton’s Atomic Theory Atomic Structure: Basic Concepts Topic 2 Topic 2

Dalton proposed his atomic theory of matter in Although his theory has been modified slightly to accommodate new discoveries, Dalton’s theory was so insightful that it has remained essentially intact up to the present time. Dalton’s Atomic Theory Atomic Structure: Basic Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

The first step to solving a problem, such as what makes up matter, is observation. Hypotheses, Theories, and Laws Scientists use their senses to observe the behavior of matter at the macroscopic level. Atomic Structure: Basic Concepts Topic 2 Topic 2

Hypotheses, Theories, and Laws They then come up with a hypothesis, which is a testable prediction to explain their observations. Atomic Structure: Basic Concepts Topic 2 Topic 2 To find out whether a hypothesis is correct, it must be tested by repeated experiments.

Hypotheses, Theories, and Laws Scientists accept hypotheses that are verified by experiments and reject hypotheses that can’t stand up to experimental testing. Atomic Structure: Basic Concepts Topic 2 Topic 2

Hypotheses, Theories, and Laws A theory is an explanation based on many observations and supported by the results of many experiments. Atomic Structure: Basic Concepts Topic 2 Topic 2

Hypotheses, Theories, and Laws Atomic Structure: Basic Concepts Topic 2 Topic 2 As scientists gather more information, a theory may have to be revised or replaced with another theory.

Hypotheses, Theories, and Laws A scientific law is simply a fact of nature that is observed so often that it becomes accepted as truth. A law can generally be used to make predictions but does not explain why something happens. In fact, theories explain laws. Atomic Structure: Basic Concepts Topic 2 Topic 2

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. Thomson’s experiments used a vacuum tube. Atomic Structure: Basic Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2 Click box to view movie clip.

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 consisted of atoms of two different masses. Atomic Structure: Basic Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2 Click box to view movie clip.

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 Concepts Topic 2 Topic 2

The Gold Foil Experiment Atomic Structure: Basic Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2 Click box to view movie clip.

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2

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 Concepts Topic 2 Topic 2