Section 4.2 Defining the Atom

Objectives Define atom Distinguish between the subatomic particles in terms of relative charge and mass. Describe the structure of the atom, including locations of the subatomic particles.

What is an atom?? The smallest particle of an element that still retains the properties of the element is called an atom. The scanning tunneling micro- scope (STM) allows scientists to see individual atoms. A cobalt atom

The Electron 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. In 1897, a British physicist, J.J. Thomson, discovered that this solid-ball model was not accurate.

The Electron Thomson’s experiments used a vacuum tube (a tube that has had all the gases pumped out of it) called the cathode ray tube.

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.

The Electron When 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.

The Electron Thomson found that the rays bent toward a positively charged plate (placed next to the glass tube) and away from a negatively charged plate. He also knew that objects with like charges repel each other, and objects with unlike charges attract each other.

The Electron Video clip Video Clip II

The Electron Thomson concluded that cathode rays are made up of invisible, negatively charged particles referred to as electrons. He also concluded that these electrons had to come from the matter of the negative electrode. That is, they had to have come from the atoms of the metal and so were subatomic.

The Electron In Thomson’s experiments, he determined the mass of the charged particle to be MUCH LESS than a hydrogen atom. Scientists had to conclude that atoms were not just solid spheres, but somehow were composed of smaller, negatively charged particles. (Dalton was wrong!)

The Electron In addition, since atoms themselves are not negatively charged but neutral, there had to be other particles in the atom, especially positively charged ones. And, because atoms have a measurable mass and the negatively charged particles that atoms were found to contain were extremely light, scientists had to conclude that atoms must contain positively charged particles with a much greater mass than electrons.

The Plum Pudding Model Thomson proposed a model of the atom called the plum pudding model. The atom contained uniformly distributed positive charge containing electrons.

The Electron (cont.) Robert Millikan (1868-1953) determined the charge of the electron with an experiment called the “oil drop” experiment. The value of the charge (1.6 x 10-19 C) has since been equated to a charge of -1. He also calculated the mass of an electron at 9.1 x 10-28 g (= 1/1840 of a hydrogen atom).

The Nuclear Atom In 1911, 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 plum pudding model of the atom.

The Nuclear Atom The experimenters set up a lead-shielded box containing radioactive polonium, which emitted a beam of positively charged subatomic particles through a small hole. Today, we know that the particles of the beam consisted of clusters containing two protons and two neutrons and are called alpha particles.

The Nuclear Atom This beam of particles was then “shot” at a thin sheet of gold foil. The gold foil was surrounded by a screen coated with zinc sulfide (which glows when struck by the positively charged particles of the beam).

The Nuclear Atom This experiment was called the “gold foil” experiment

The Nuclear Atom Based on the plum pudding model, Rutherford predicted that the paths of the fast-moving alpha particles would only be slightly altered by a collision with an electron. They should pass straight through the foil.

The Nuclear Atom He was amazed to discover that some of the alpha particles were deflected at wide angles. Video

The Nuclear Atom To explain the results of the experiment, Rutherford’s team proposed a new model of the atom called the nuclear model Because most of the particles passed through the foil, they concluded that the atom is nearly all empty space.

The Nuclear Atom The Nuclear Model (cont.) 2. Because the deflections had to be caused by a repulsive force between the alpha particles and another structure, they proposed that the atom has a small, dense, positively charged central core, called a nucleus. The nucleus was centrally located within the atom and contained all the mass and positive charge of the atom.

The Nuclear Atom The Nuclear Model (cont.) 3. Electrons move rapidly through the space surrounding the nucleus. They are held there by the positively-charged nucleus.

Completing the Atom By 1920, Rutherford refined his concept of the nucleus. He concluded that the nucleus contained positively charged particles called protons. A proton is a subatomic particle carrying a charge equal to but opposite of that of the electron; a charge of +1.

Completing the Atom In 1935, James Chadwick showed that the nucleus contained another particle. Since it was neutral, it was called a neutron. A neutron is a subatomic particle that has the same mass of a proton but no electrical charge.

The properties of the three (3) subatomic particles that are found in all atoms are summarized below. ** Scientists have determined that protons and neutrons are composed of subatomic particles called quarks.