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. CH 3

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 CH 3

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 CH 3

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 CH 3

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 CH 3

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

The following statements are the main points of Dalton’s atomic theory. ATOMS acronym Dalton’s Atomic Theory A -- All matter is made up of atoms. T – Total atoms of an element are identical. O – Only an element can have that specific type of atoms and are different from atoms of other elements. Atomic Structure: Basic Concepts

Dalton’s Atomic Theory M – make compounds in whole-number ratios. S – Smallest particle and cannot be divided (indivisible). Atomic Structure: Basic Concepts Remember ATOMS!

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

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

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

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

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

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

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

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

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

Millikan’s Oil Drop Exp. In 1909, Robert Millikan performed a famous oil drop experiment and determined the size of the charge of an electron and also it’s mass! What Millikan did was to put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling

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

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

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 by James Chadwick who bombarded a nucleus with alpha particles (video).video Atomic Structure: Basic Concepts

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

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

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

The Gold Foil Experiment Atomic Structure: Basic Concepts

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

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

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