听力原文： What is an atom? The modern view of an atom has come from many fields of chemistry and physics. The idea of an atom came from ancient Greek science and philosophy and from the results of 18th and 19th century chemistry. It includes three aspects, concept of the atom, measurements of atomic mass and repeating or periodic relationship between the elements. From the ancient Greeks through today, we have pondered what ordinary matter is made of. To understand the problem, here is a simple demonstration from this book, The Extraordinary Chemistry of Ordinary Things by Carl H. Snyder. At first, take a pile of paper clips which are all of the same size and color; second, divide the pile into two equal piles; third, divide each of the smaller piles into two equal piles; then repeat step 3 until you are down to a pile containing only one paper clip. Now one paper clip still does the job of a paper clip such as hold loose papers together. Now, take a pair of scissors and cut that one paper clip in half. Can half of the paper clip do the same job as the single paper clip? Certainly not. If you do the same thing with any element, you will reach an indivisible part that has the same properties of the element, like the single paper clip. This indivisible part is called an atom. The idea of the atom was first devised by Democritus in 530 BC. In 1808, an English school teacher and scientist named John Dalton proposed the modern atomic theory. Modern atomic theory simply states the following. Every element is made of atoms—piles of paper clips. All atoms of any element are the same—all the paper clips in the pile are the same size and color. Atoms of different elements are different in size and properties—like different sizes and colors of paper clips. Atoms of different elements can combine to form. compounds—you can link different sizes and colors of paper clips together to make new structures. In chemical reactions, atoms are not made, destroyed, or changed—no new paper clips appear, no paper clips get lost and no paper clips change from one size/color to another. In any compound, the numbers and kinds of atoms remain the same—the total number and types of paper clips that you start with are the same as when you finish. Daltons atomic theory formed the groundwork of chemistry at that time. Dalton envisioned atoms as tiny spheres with hooks on them. With these hooks, one atom could combine with another in definite proportions. But some elements could combine to make different compounds for example; hydrogen + oxygen could make water or hydrogen peroxide. So, he could not say anything about the numbers of each atom in the molecules of specific substances. Did water have one oxygen with one hydrogen or one oxygen with two hydrogens? This point was resolved when chemists figured out how to weigh atoms. The ability to weigh atoms came about by an observation from an Italian chemist named Amadeo Avogadro. Avogadro was working with gases such as oxygen, hydrogen and noticed that when temperature and pressure was the same, these gases combined in definite volume ratios. Avogadro said that at the same temperature and pressure, equal volumes of the gases had the same number of molecules. So, by weighing the volumes of gases, he could determine the ratios of atomic masses. For example, a liter of oxygen weighed 16 times more than a liter of hydrogen, so an atom of oxygen must be 16 times the mass of an atom of hydrogen. Work of this type resulted in a relative mass scale for elements in which all of the elements related to carbon. Once the relative mass scale was made, later experiments were able to relate the mass in grams of a substance to the number of atoms and an atomic mass unit—amu was found; 1 amu or Dalton is equal to 1.66×10-24 grams. At this time, chemists knew the atomic masses of elements and their chemical properties, and an astonishing phenomenon jumped out at them! Later we will have a look at the following part. The Properties of Elements Showed a Repeating Pattern.Narrator Listen to part of a lecture in the chemistry class. Now get ready to answer the questions. You may use your notes to help you answer. What is the talk mainly about?