PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000 Chapter 2 The structure of matter and radiation Everything in the universe – matter and radiation -- is made of a few fundamental ingredients we call particles. In this Chapter we take a grand tour of these elementary particles and the composite objects that build up to atoms, molecules, and solids – all the states of matter – and radiation. We will learn about big accelerators that probe the depths of the fundamental particles and create exotic forms of matter, including antimatter. We will also learn about re- cent inventions such as the Scanning Tunneling Microscope that images atoms directly. A world made of atoms Demokritos in ancient Greece was the first to propose that all matter is fun- damentally made of indivisible units he called atoms, literally meaning uncuttable. The rest is empty space. Aristotle, however, ridiculed the idea, believing that mat- ter is continuous. Aristotelian physics prevailed for about two thousand years until the 17th century when alchemists and the early chemists demonstrated the existence of atoms by experimentation and measurement. Compelling evidence in favor of atoms accumulated rapidly and today we know that 92 such distinct species of atoms, called “elements”, are found in our physical world. Most of them have familiar names, hydrogen, oxygen, iron, sulfur, and so on, but others have names you probably never heard before, such as hafnium, scandium, and praseodemium. They are denoted by shorthand abbre- viations consisting of one or two letters, usually related to their Latin name that often coincides with the English name. For example, H is used for hydrogen, O for oxygen, I for iodine, Ca for calcium. However, Fe is used for iron (from the Latin ferrum), Na for sodium (from the Latin natrium). Most substances are made up of more than one species of One of the key experimental re- atoms -- they are compound sub- sults obtained by early chemists stances. was that substances combine in definite proportions to make up The elements are usually arranged other substances, e.g. two parts hy- in a Periodic Table that reflects the fact drogen and one part of oxygen to that atoms in the same column have make water. Such discoveries ulti- similar chemical properties, i.e., they mately led to convincing evidence combine with other elements in similar about the existence of atoms as the ways to form similar compound sub- building blocks of all matter. stances. 9 PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000 For example, NaCl, KCl, KBr and so on are very similar “salts”. NaCl is the famil- iar table salt whereas KCl is used on streets in the winter to melt ice. Similarly you have NH3 (ammonia) and also PH3 (phosphine) AsH3 (arsine) and so on. The designation KCl simply means that the substance is made of equal numbers of K atoms and Cl atoms. Similarly, NH3 means that the substance is made up of three H atoms for every one N atom. Note that these “chemical formulas” don’t tell you anything else about the substance, e.g. if it is gaseous or liquid at room temperature or how the atoms are arranged relative to each other. We’ll have a good deal to say about such issues later in the chapter. The Periodic Table of the elements shown on the previous page contains 113 elements. From number 93 and above, the elements are not found in nature. They are only made in the laboratory. Making the next element -- and getting to name it -- has been a battlefield of national pride between Americans and Rus- sians. They have gone all the way to atomic number 118 by now. The early chemists thought the elements were Demokritos’s “uncuttable” at- oms. They thought of them as solid-like objects that somehow hooked up to- gether in different ways, but did not have any good ideas how they did that. It turned out in the end that the elements are not the ultimate “atoms”. For this reason, before we look at how atoms combine to make up everything around us, it is useful to first take a quick peak inside atoms. We’ll return later and take a closer look. For the time being we just need some bare essentials. Inside the atom The Periodic Table of the elements was an early telltale sign that atoms must have some internal structure that gives rise to the similarities and differences in the ways various elements combine to form compound substances. In the 19th century several other experimental observations pointed toward internal struc- ture. Around the turn of the century, it all became clear. Each species of atom is made up of a nucleus (the word means pit in Latin, as in olive pit, one of those 10 PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000 erudite Latin choices of yore) surrounded by a “cloud” of electrons. The idea of “cloud” is that the electron is very tiny and is moving ex- tremely fast so it is literally everywhere at the same time. Atoms, therefore, are not solid-like objects but mostly empty space. They are very tiny, generally spherical and only about 10-8 cm across. There are about 1018 (a trillion trillion) of them in the dot of ink at the end of this sen- tence. If you were to blow up an atom to the The electron cloud around the size of a football stadium, the nucleus would nucleus of a hydrogen atom be the size of a grain of sand! Different species of atoms simply have a different number of electrons. In fact, the serial number of each element in the Periodic Table, called the atomic number, is simply the number of electrons in that species of atom. Now you are probably wondering what sets apart the col- It was just over 100 years ago, in 1997 that J. J. Thompson in England first pried electrons out of atoms and made a beam with them in a glass tube from which air had been removed completely. He used a relatively new tech- nology for creating such a “vacuum” in a glass tube. At each end of the tube was a metal plate and the two plates were hooked to a battery (Batteries were invented in the 18th century by the Italian Volta; we will learn how they work later in the course). The beam glowed and Thompson did some clever experiments with which he established that the glowing beam was made up of tiny particles that are about 2000 times lighter than hydrogen atoms, the lightest atoms of them all! He coined the name electron because he sus- pected (correctly) that the critter is responsible for electricity (electron is the Greek word for amber; which was the first material known to produce electri- cal phenomena; see next chapter). Fourteen years later, Ernest Rutherford, an Australian working in Cam- bridge, England, did some other clever experiments and established that an atom is made of a hard tiny nucleus at the center and electrons buzzing around it. He and his students made the discovery by bombarding very thin gold foils with beams of particles (by that time scientists learned how to make such beams) and charting the directions in which they scattered. Most went through the foil virtually undeflected and very few bounced directly back as if they had struck something very hard. Indeed they had! Since Rutherford, accelerating beams of particles to high energies and smashing them into targets has been a big business, known as high-energy physics. The big machines that produce and accelerate the particles are popularly known as atom smashers. Fancy detectors controlled by computers now record the products of collisions from which physicists extract the deep- est secrets of nature about the structure of matter. 11 PHYSICS FOR EVERYONE by S. T. Pantelides -- ? 2000 umns of the periodic table, i.e., what makes the elements in the same column exhibit similar chemical behavior. It’s quite simple: the electron cloud is actually made up of shells, like the shells of an onion. There are rules about the maxi- mum number of electrons that can go in each consecutive shell, very much like there is a maximum number of people that can sit in each row of seats in an am- phitheater (the rules pop out of an equation that governs the behavior of elec- trons in an atom). It turns out that for elements of the first column of the periodic table, the outermost shell has only one electron, for elements of the second col- umn the outermost shell has two electrons, and, you guessed it right, so it goes all the way to the eighth column. Neat! The outermost shell, called the valence shell, never has more than eight electrons. Mendeleev would have been very proud! Molecules – Gases and Liquids Atoms make up molecules. A molecule is simply a bunch of atoms hooked up together. When atoms come close to each other, their electron clouds pene- trate and overlap. Since all electrons are the same, indistinguishable from each other, it is not possible to label which electrons belong to which atom. The elec- trons are in effect shared by the atoms. It is this overlap of electron clouds or sharing of electrons that is the effective “glue” that holds atoms together so they can form macroscopic matter as we know it. All the action of penetrating and overlapping in fact takes place among the valence electrons, namely those occu- pying the outermost or valence shells around the nucleus. That’s why the number of electrons in this shell determines the overall chemical behavior of elements.