20 Atoms
Atoms are tiny. A hydrogen atom has a 10-10 meter radius, which is the radius of the orbit of the electron around a proton. Mass of proton is 1836 times the mass of the electron, which is 9.11x10-31 kg.
The radius of the electron is smaller than has been so far possible to measure. The proton radius is 10-15 meter.
If proton is 0.1 meter in radius then the electron orbits 105 = 100,000 meter = 100 km = 60 miles away.
Atoms are mostly empty space. Just like the solar system. Nucleus of biggest natural atom is Uranium. Its nucleus is just three times larger than that of hydrogen, which is one proton.
Tell me how to figure how many Earths would fit between the sun and the Earth.
Heat a material and it melts, vaporizes, ionizes, nucleus separates, elementary particles become light, then just energy. Each of these temperatures has been measured. Reverse the process to visualize the cooling Big Bang material. Hydrogen and some helium atoms formed as after Big Bang cooled. See figure. Almost no heavier elements. they form from solar fusion up to Iron = element number 26. Supernova forms atoms heavier than iron. We are made of the material of stars. Uranium is 92. Higher element numbers made in lab but don't last long.
Atom element number determined by number of protons. Number of neutrons can vary. Atom just heavier. Neutron activation analysis adds uniform energy to nucleus then watches specific emissions that result because each nucleus has uniquely identifying emissions. Police measure gun shot residue on palms by swiping hand with towel and using NAA to analyze. They'll know how many bullets.
Mass spectrometry curves charged mass trajectories where electrical force supplies the centripetal force. Larger masses move in larger radius od curvature. dentify unknown molecules by vaporizing and passsing through a mass spectrometer. Mass and light spectrometers used in rising baloons and planetary landers. Try spectrum lights.
Remember magnetic compass for navigation. First used by Chinese to navigate on ground. About 1000 a.d. in boats. A few centuries later reached Europe.
Atom and the nucleus each act like magnets because of the spinning charges. MRI works by measuring alignment and unalignment times in externmal magnetic field.
Alpha decay = helium nucleus = 2 proton and 2 neutrons. reduces Z by 2.
Everything made of atoms. Why can we see through glass but not wood? Colors of light emmited or absorbed by orbiting electrons as they change orbital size. Material is clear if it absorbs no visible light. Each material absorbs, reflects, transmits different wavelengths. The spectrum of thousands of materials were measured and posted. We look at star light to know ehich atoms are present in that star. Color of objects usually due to outer electron energy levels but many geometrical, diffraction, and scattering causes, too.
properties of crystal material = solid state physics. QM equations (F=ma for atoms) tell us how to make lasers, diodes, transistors,
Can't take a photo of atoms, or use visible light microscope. Scanning tunneling microscope "feels" location of atoms using quantum tunneling of electrical current effect as a pointed tip is passed by the atoms.
Brownian motion. Albert in 1905.
An atom typically stays in your body for just a few years. You are being replaced daily.
Breathing Caesar's oxygen atom breath. How many atoms do you breathe per day? In a lifetime? How many are in the Earth's atmosphere? See problem6 and 7 for 1022 oxygen atoms per breath and 1044 oxygen atoms in the atmosphere makes one oxygen atom per breath from ceaser or whomever. Same for one thimble of water holding 1023 atoms of water while the ocean holds 1046 atoms of water. So any thimbleful contains one atom from a thimble spilled in the past at any location of the Earth.
Molecules form through electrical bonding sort of like gravity or two ice skaters holding hands while spinning around each other. Differing regions of a large molecule are either electrically positive or negative. Molecules of life built this way. see fig of large protein structure.
Antimatter. Each particle has an anti-particle of same mass but opposite electrical charge. Proton and anti-proton.
physical strength, stretch, thermal, light reflection, heat expansion, heat conductance properties of solids--and liquids--due to single-atom orbiting electrons and interatomic electrical forces.
From us humans:
About the year 1900, scientists began to be able to measure nature on an atomic scale. Examples of atoms include the basic and familiar chemicals hydrogen, helium, carbon, and oxygen. It has been found that atoms consist of a massive, central core or nucleus composed of positively-charged protons and a number of uncharged neutrons. This nucleus is orbited by negatively-charged electrons, see www.lbl.gov/abc/Basic.html. The electrons are held in orbit around the protons by their mutual electrical force in a manner analogous to the way in which the Earth and Moon are held together by their mutual gravitational force. Visit VisionLearning at http://web.visionlearning.com/custom/chemistry/animations/CHE1.3-an-animations.shtml for animations of various atoms) It takes some force to pull an electron away from the nucleus to which it is electrically attached. Protons and neutrons are about 2,000 times more massive than electrons.
This picture of electrons orbiting a massive clump of protons and neutrons was determined in the year 1911 by Ernest Rutherford. He passed a beam of low-mass helium nuclei through a thin sheet of high-mass gold atoms and measured the resulting trajectories of the helium nuclei. Gold can be hammered into very thin sheets; Rutherford's sheet was so thin that it contained only a few layers of gold atoms. Usually the beamed helium nuclei passed right through the thin gold sheet but sometimes they struck a gold nucleus and bounced straight back toward the incoming beam. This revealed that a nucleus consists of lumps of mass and that there is mostly empty space between the adjacent nuclei of the sheet. The mass and charge of these lumps can be measured in many ways. The lumps were later named protons and neutrons. For an animation of this scattering process visit http://galileo.phys.virginia.edu/classes/109N/more_stuff/Applets/rutherford/rutherford.html
Inside the nucleus, the positively charged protons are repelled from each other by the electric force; this is a tremendously large repulsive force. Despite this large repulsive force they are held together by the even-stronger but short-ranged and attractive "strong nuclear force." That is, the protons are subject to both the electrical and the strong nuclear forces. This nuclear force is about a million times stronger than the electrical force with which the orbiting electrons are held to the nucleus (this is also the reason nuclear explosions are a million times stronger than chemical explosions). When a nucleus is split into two pieces, say by the collisional force of an incoming projectile particle, the two separated pieces are no longer close enough to each other to be held together by the short-ranged and mutually attractive strong-nuclear force. The separated, positively charged pieces will then fly apart due to their mutual electrical repulsion.
Normally, atoms are electrically neutral because they have equal numbers of protons and electrons. When an atom is heated, its electrons gain speed. When sufficiently heated they will gain enough speed to break free from their electrical bond with the nucleus and move away, leaving behind a positively charged atom. The positively charged atom is called an ionized atom, or ion. This means that the atom is now electrically charged instead of being neutral because it is missing some negatively charged electrons. The temperature at which each chemical element ionizes has already been measured.
The nucleus of each type of atom always contains a fixed number of protons but it can have a bit of a range in its number of neutrons. Each variation will then have a different mass and is called an isotope of that particular type of atom. For example, a carbon nucleus always contains six protons but has either six, seven, or eight neutrons. Uranium has ninety-two protons, ninety-two electrons and 140 to 150 neutrons. Isotopes and their variations in mass make possible the techniques of radioactive dating and such that are used to study our own history..
Scientists have found that only ninety-two different atoms or chemical elements–from hydrogen to uranium–occur in nature, see http://education.jlab.org/itselemental/index.html. Each different chemical element contains from one to ninety-two protons inside its central nucleus and an equal number of orbiting electrons–unless sufficient heat is added that the electrons begin escaping. The number of neutrons varies for each particular chemical atom but the number of protons does not. That is, hydrogen always has one proton, helium always has two, and uranium always has ninety-two.
Hydrogen atoms are the smallest of all and consist of a single proton orbited by a single, electrically-bound electron. Hydrogen atoms are so small that 100 million of them fit across the width of your finger. Most often, hydrogen nuclei have no neutrons, but about 0.8% of them are found to contain both a proton and one neutron. Since the mass of protons and neutrons are about the same, this variety of hydrogen is twice as massive as the form that contains no neutron.
Helium is the next largest atom and contains two protons, two electrons, and usually two neutrons. It has been found that about 0.3% of helium atoms have three neutrons instead of two. (Helium is the gas inside a child's balloon; it also makes your voice sound funny when you breathe it.) If you press two hydrogen atoms together with a megaton force then they will merge into a single helium atom and release a lot of previously-stored nuclear energy.
Atoms can become electrically attached to other atoms. We use the term molecule to refer to any collection of two or more atoms. Large molecules can be built by combining many smaller ones. The molecule's collection of atoms remains electrically stuck together unless it is sufficiently heated or comes into contact with another molecule that has the right charge distribution to disrupt it.
Each atom within a solid piece of material, such as a block of iron, behaves as if it is attached to each of its neighbors with little springs, see www.physics.brocku.ca/courses/1p21_reedyk/images/F09001.jpg; the springs represent the electrical force. As this material is heated, the atoms are seen to jiggle more rapidly, see http://vis.lbl.gov/Vignettes/vanhove-98.ijs/output.mpg. At a certain temperature the atoms are no longer able to hold each other in place and the material melts. The melting-point has been measured for thousands of types of atoms and molecules.
Those one hundred or so varieties of atoms combine into endless varieties of molecules. A molecule consists of two or more atoms that are electrically bound together. For example, we all remember that water, H-2-O, consists of two hydrogen atoms and one oxygen atom. The largest molecules within our bodies consist of billions of atoms and are electrically neutral because they contain equal mixtures of positive and negative charge. Chemists study the ways atoms interact with each other. They have found many rules that help them figure out which final chemicals will result after several chemicals have been mixed together. (During each day of an eight-year college curriculum, a chemistry student will study many different chemical reactions; by graduation, the properties of thousands of chemicals have become familiar.) We use many chemicals in our daily lives. The most familiar everyday uses include medicine, soap, glue, oil, paint, and cleaning products. Our bodies consist of millions of interrelated chemical reactions.
What's the difference between chemistry and physics? Physicists are interested in the nature of the electrical force between atoms. Some chemists might think this is an interesting aspect of interacting atoms but they are usually more interested in the properties of the millions of chemicals that result from these electrically bound combinations. Chemists want to know how atoms interact and how more-complex molecules can be formed. They study the properties of known chemicals and look for uses for them. They also look for new chemicals to serve specific purposes. Physicists want to know why atoms and molecules occur. They look for the fundamental building blocks of atoms instead of new combinations of atoms.
The electrical bonding between the atoms of a molecule occurs in a few ways. In one situation, the electrical binding-force occurs when an electron simultaneously orbits two nearby atoms. In another situation, two neutral atoms can become electrically bound when their component charges become displaced. This happens, for example, when a charge is placed near an electrically neutral block. When that neutrally charged block is undisturbed, it consists of little packets–its atoms–of equal amounts of positive and negative charge. When an exterior positive charge is held near the block, the block's positive charges are slightly repelled while its negative charges are slightly attracted toward that exterior charge. This makes a slight separation between the positive and negative charges within the block. The near side of the block is then slightly negative, while the far side is slightly positive. The result is that the neutrally charged block is slightly attracted toward the exterior charge. The same thing can occur in nearby molecules. The electrical charge of a large molecule is unevenly distributed about its three-dimensional shape. When two multi-atom molecules face each other, a slightly positive charge of one side of the first molecule and a slightly negative charge of one side of the second molecule can result in an electrical force that holds the two molecules together. We will see that, inside our bodies, this process builds very large molecules containing thousands to billions of atoms.
The difference in the mass of different chemicals also allows us to identify substances. To do this, a small amount of an unknown substance is heated until it is vaporized into a gas of ionized molecules. The gas molecules have varying speeds and bounce around within a container. The container has a tiny hole that allows those molecules moving in the right direction to escape into an adjacent tube. There are both magnetic and electric fields within the tube and each exerts a force on the moving molecules. The magnetic field does not exert an equal force on every molecule; instead, its force is larger on faster-moving molecules. The force of the electric field does not vary with molecular speed. Those molecules moving with a certain speed will experience a force from the magnetic field that is exactly counterbalanced by the force of the electric field. These molecules then move straight down the tube while all others are pushed into the walls of the tube. The combination of these two fields has the property that it allows only those molecules with a certain preselected speed to pass along the tube. The researcher selects the desired speed by altering the ratio of electric and magnetic fields. Those molecules having non-selected speeds will not traverse the entire length of the tube. This means that each molecule emerges from the end of the tube with the same speed. Those emerging molecules next pass into an area containing a second magnetic field forcing them into a circular orbit. For each different molecule, the size of its circular orbit depends on its mass: more-massive molecules move in a larger circle than do less-massive molecules. Measuring the radius of motion allows the mass of the molecule to be measured. This so-called mass spectrometer is used to identify a chemical by measuring its mass and then comparing it with previously known molecular masses. The mass of tens of thousands of different molecules have been measured and tabulated.
The mass spectrometer has many uses. It can be placed on a rocket and used to identify the chemicals of the upper atmosphere or of the atmospheres and surfaces of other planets and moons. Also, differences in the chemical contents of different geological sources of obsidian and such can be measured and tabulated. When archaeological excavations find an obsidian artifact, the artifact’s chemical composition can be compared with those tabulated sources to determine its geographical source. This tells much about the people who used the artifact, including their trading patterns. Criminal investigators use mass spectrometers to identify the tiny chemical remnants found in clothes, cars, rugs, hair, poisons, pencil marks, and arson materials gathered at the scene of a crime. The mass spectrometer is also used in the radioactive dating procedure.
Our knowledge of the nucleus has given us lots of useful machines. The medical imaging process called Magnetic Resonance Imaging (MRI) is an important example. To understand the operation of these imaging devices recall that the magnetic needle of a compass always points toward the north pole of the Earth. It has been found that the moving, positively charged protons inside each nucleus act as a pointing magnet and will line up with an external magnetic field. If that external magnetic field is suddenly turned off, each different nucleus–that is, each different chemical–will take a different amount of time to become unaligned from that previously existing field. MRI machines makes use of the alignment and un-alignment times of different nuclei in order to determine the types of material found inside your body. Some of these materials include bone, tissue, normal cells, and cancerous cells. This allows a picture to be made of your interior without sawing you in half.
If energy is added to a nucleus, it will soon re-emit that energy to return to its less-energetic state. Since the nucleus of each chemical emits a different spectrum of energy, a chemical can be identified by measuring the energy emissions of its nucleus. This is called neutron activation analysis. For example, after you fire a gun there is an invisibly tiny amount of gunpowder left on your palms. The personnel in a crime lab can collect a remnant from the palms of a suspect and then examine the spectrum of that heated remnant to determine if that person had recently fired a gun.
Quantum Mechanics is the description of nature on the atomic level. It was developed in the 1920s (quantum mechanics is what happens when nobody in the U.S. is allowed to drink for ten years) as a necessary refinement of Newtonian mechanics. For large-scale objects, Newtonian and quantum mechanics agree to about thirty decimal places. In 1926, Erwin Schroedinger found the version of Newton's equation that describes atomic-sized particles. Schroedinger's equation describes the behavior of the atoms inside all materials. After a few decades, people learned to use the equation to make lasers and transistors and such–like those used inside computers and cd-players. Nobody in the 1920s could imagine the machines that would result from the application of Schroedinger's equation.
Electric charges were found to come in two varieties, positive and negative, but the carriers of the strong-nuclear force have been found to come in three varieties. During this century, we will begin to make machines that use the strong nuclear force. Nobody today can imagine the future machines that will be made whose operation will be governed by the strong nuclear force. The strong nuclear force is not yet fully understood, in fact, there is much left to be learned–you might like to join in the research.
The age of many objects can be measured by the technique of radiocarbon dating. This physical process will be described here so that we can refer to it throughout the remaining chapters. As mentioned above, the nuclei of a particular chemical element can contain a small range in its number of neutrons. The number of neutrons in a given nucleus can change when an external neutron collides with it and either knocks away others or becomes absorbed. That colliding neutron may have been emitted by the chemicals of the Earth, the Sun, or even another star. Certain numbers of neutrons result in an unstable nucleus, like having an overcrowded house. Stability is reestablished when the nucleus emits "radiation." This radiation has been found to occur in three forms: an electron (named beta radiation); a light wave, like an x-ray (named gamma radiation), or a helium nucleus (named alpha radiation) that consists of two protons and two neutrons. The proper mispronunciation of "helium nucleus" is "helius nucleum."
X-rays, electrons, and helium nuclei are often useful, and like everything else, they can also be harmful. One hundred years ago, Madam Curie was one of the first scientists to study radiation. From the studies of these early scientists, we began learning the hard way about the dangers of radiation. During the last hundred years, physicists have measured millions of facts concerning radiation. Today, the most familiar use of radiation is to create monster movies. Radiation can also be used to fight cancer. It can be used to kill microscopic germs in newly harvested potatoes, fruit, and meat and such. This process is called pasteurization and gives food a longer shelf life. Radiation is used to provide power in spacecrafts and in heart pacemakers. Radioactive tracers are used in medical tests. For a list of additional uses you might like to read The Ubiquitous Atom by Grace and Larry Spruch. Radiation is emitted from the dirt below us, the bricks around us, and from the stars above us. Every day our bodies absorb a small amount of radiation.
Next, we define the term "half-life" so that we can then go on to understand the radioactive dating technique. Imagine that you shake a bag of one thousand coins and then pour the coins onto the ground. Next, pick up the coins that show "heads" and place them back into the bag while taking away those showing "tails." Pretty close to half the coins will be heads and half will be tails so that the bag will now contain about five hundred coins–that is, the bag has lost half its original contents. (By the way, five years after finishing a course at school, I have forgotten about half of the material covered. After five more years, I have forgotten half of what remained, leaving one-quarter of the original amount.) Shake the bag again and then dump the coins onto the ground once more. Again, half the remaining coins will be heads, and half will be tails. If you pick up the heads, you will find about 250. We see that about half the contents of the bag are lost after each shake. Initially the bag had 1,000 coins. After shaking and removing "tails" the bag contained 500, then 250, then 125, 63, 32, 16, 8, 4, 2, 1, and finally zero coins. (See Student Study Guide for Energy for a Technological Society by Joseph Priest, page 89.)
In the same way it has been measured that half the radioactive atoms of an unstable material will emit radiation during each half life. This changes the unstable atoms into a stable non-radioactive form. For example, if you have 10 kilograms (22 pounds) of a radioactive material then half of it will change into another chemical during a period of time equal to its “half life.” The half life of each type of radioactive atom has been measured. The half life of an atom never changes. Radiation is a natural process. Just as a hammer will always fall whenever it is pushed off a table, during each half-life, half the nuclei in a radioactive material will become more-stable by radiating either light waves, electrons, or helium nuclei.
Different radioactive chemicals have different half lives; this makes them suitable for determining different sizes of time spans. For example, one can measure how much time has elapsed since a rock had last been in a molten state. This shows that the molten Earth solidified about 4.5 billion years ago. This technique can also be used to measure the age of a given volcanic eruption.
It was mentioned above there can be a small range of numbers of neutrons within a nucleus. For example, we saw that carbon atoms always have six protons but six, seven, or eight neutrons for a total of twelve, thirteen, or fourteen neutrons and protons. The percentages of each type (each type is called an isotope) of carbon have been measured. If you start with 1,000 atoms of radioactive carbon-14, you will find that after 5700 years have elapsed , which is its half-life, only 500 of these carbon-14 atoms will be remaining; the other half of them will have emitted radiation and turned into a more-stable atom. After another 5700 years elapse, only one-fourth of the original carbon-14 atoms remain.
When the Earth formed it contained a certain amount of carbon; a small percentage was the radioactive carbon-14 variety. The carbon-14 essentially disappears within about ten half lives, which would be 57,000 years. The Earth is much older than 57,000 years. (We have measured the age of the Earth by counting radiation tracks and by measuring the relative amounts of radioactive uranium and thorium, as described in Chapter 4.) Since the Earth is much older than 57,000 years it means that carbon-14 is continually being produced, otherwise it would all be gone.
Scientists have found that energetic neutrons from the Sun and the other stars collide with nitrogen nuclei in the Earth's atmosphere and create carbon-14. This collision rate will certainly be variable but not drastically so. Little variation is likely to have occurred in the last 50,000 years because the Sun's lifetime is much longer than that. Some carbon-14 combines with oxygen to form a carbon-dioxide molecule. Living things absorb this as they breathe. Every living creature contains carbon, and all living creatures have identical relative mixtures of the non-radioactive carbon-12 and the radioactive carbon-14 varieties. After death, the relative proportion of carbon-14 steadily decreases in time. Every 5700 years after death, half the remaining carbon-14 will have undergone radioactive decay. A measurement of the relative proportions of carbon-12 and carbon-14 atoms makes it possible to determine how many carbon-14 half-lives have occurred since the object died. This is the radioactive dating technique. The greater the number of years since the death of the creature, the longer its carbon-14 has been radiating away.
Return to Lecture Table of Contents
