The idea that matter is made up of atoms dates back to the ancient Greeks. According to the Greek philosopher Democritus, if a pure substance—say, a piece of iron—were cut into smaller and smaller bits, eventually a smallest piece of that substance would be obtained which could not be divided further. This smallest piece was called an atom, which in Greek means “indivisible.” Today an atom is still the smallest piece of a substance, but we do not consider it indivisible. Rather it is viewed as consisting of a central nucleus (containing protons and neutrons) surrounded by electrons, Chapter 27.Today the atomic theory is universally accepted. The experimental evidence in its favor, however, came mainly in the eighteenth, nineteenth, and twentieth centuries, and much of it was obtained from the analysis of chemical reactions. We will often speak of the relative masses of individual atoms and molecules— what we call the atomic mass or molecular mass, respectively. (The terms atomic weight and molecular weight are sometimes used.) These masses are based on arbitrarily assigning the most abundant form of carbon atom, the atomic mass of exactly 12.0000 unified atomic mass units (u). In terms of kilograms, The average atomic mass of hydrogen is 1.0079 u, and the values for other atoms are as listed in the Periodic Table inside the back cover of this book, and also in Appendix B.† The molecular mass of a compound is the sum of atomic masses of the atoms making up the molecules of that compound. [An element is a substance, such as neon, gold, iron, or copper, that cannot be broken down into simpler substances by chemical means. Compounds are substances made up of elements, and can be broken down into them; examples are carbon dioxide and water. The smallest piece of an element is an atom; the smallest piece of a compound is a molecule. Molecules are made up of atoms; a molecule of water, for example, is made up of two atoms of hydrogen and one of oxygen; its chemical formula is ] An important piece of evidence for the atomic theory is called Brownian motion, named after the biologist Robert Brown, who is credited with its discovery in 1827. While he was observing tiny pollen grains suspended in water under his microscope, Brown noticed that the tiny grains moved about in erratic paths (Fig. 13–1), even though the water appeared to be perfectly still. The atomic theory easily explains Brownian motion if we assume that the atoms of any substance are continually in motion. Then Brown’s tiny pollen grains are jostled about by the vigorous barrage of rapidly moving molecules of water. In 1905, Albert Einstein examined Brownian motion from a theoretical point of view and was able to calculate from the experimental data the approximate size and mass of atoms and molecules. His calculations showed that the diameter of a typical atom is about At the start of Chapter 10, we distinguished the three common phases (or states) of matter—solid, liquid, gas—based on macroscopic, or “large-scale,” properties. Now let us see how these three phases of matter differ, from the atomic or microscopic point of view. First of all, atoms and molecules must exert attractive forces on each other, because only this explains why a brick or a block of aluminum holds together in one piece. The attractive forces between molecules are of an electrical nature (more on this in later Chapters). When molecules come too close together, the force between them must become repulsive (electric repulsion between their outer electrons). We need this assumption to explain that matter takes up space. Thus molecules maintain a minimum distance from each other. In a solid material, the attractive forces are strong enough that the atoms or molecules move only slightly (oscillate) about relatively fixed positions, often in an array known as a crystal lattice, as shown in Fig. 13–2a. In a liquid, the atoms or molecules are moving more rapidly, or the forces between them are weaker, so that they are sufficiently free to pass around one another, as in Fig. 13–2b. In a gas, the forces are so weak, or the speeds so high, that the molecules do not even stay close together. They move rapidly every which way, Fig. 13–2c, filling any container and occasionally colliding with one another. On average, the speeds are sufficiently high in a gas that when two molecules collide, the force of attraction is not strong enough to keep them close together and they fly off in new directions.给我讲讲这个