Have you ever seen an atom? Why do we think atoms exist? Do you believe in atoms?
Although atoms can’t be seen directly, atomic phenomena are evident in our everyday world... Televisions Diagnostic X rays Chemical changes in our bodies
The Existence of Atoms: Evidence from Chemistry Why believe in the existence of something we have never seen? Observations can provide convincing evidence. Much early evidence of atoms came from chemistry, the study of the differences in substances and how they can be combined to form other substances. Early Greek philosophers believed in four elements: fire, earth, water, and air. Since certain substances were retrievable, early scientists were also tempted to believe that certain elements were made up of tiny indivisible atoms.
The Birth of Modern Chemistry Lavoisier discovered that the total mass of chemical reactants and products is conserved in chemical reactions. Combustion combines oxygen from air with carbon in coal or wood to form carbon dioxide and water vapor. The role of these gases is easily overlooked.
Dalton established that the reactants combine in the same proportions by mass. 8 grams of oxygen always combine with 1 gram of hydrogen to form water. Dalton’s law of definite proportions Each element’s atoms have the same mass. In chemical reactions, atoms of one element combine with atoms of another element in definite proportions to form molecules. By studying several reactions involving the same elements, the relative atomic masses can be determined.
If water is formed by a ratio of 8 grams of oxygen to 1 gram of hydrogen, and carbon dioxide is formed by a ratio of 8 grams of carbon to 3 grams of oxygen, what are the relative masses of oxygen, carbon, and hydrogen atoms? a) 8:3:2 b) 16:6:2 c) 16:12:1 d) 32:12:2 c) If two oxygen atoms combine with one carbon atom in a ratio 8:3 (or 32:12) and one oxygen atom combines with two hydrogen atoms in a ratio 8:1 (or 16:2) then the relative masses are Two oxygen atoms : one carbon atom : four hydrogen atoms = 32:12:4 One oxygen atom : one carbon atom : one hydrogen atom = 16:12:1
Fuel Cells and the Hydrogen Economy Almost all energy comes from fossil fuels: oil, natural gas, and coal These produce significant amounts of pollution. They also produce carbon dioxide, a greenhouse gas that contributes to global warming. They are also nonrenewable. One alternative to fossil fuels is the hydrogen fuel cell. Converts hydrogen and oxygen into water, generating electricity. Similar to a battery, except the hydrogen and oxygen are stored externally and so can be replenished.
Fuel Cells and the Hydrogen Economy The proton exchange membrane (PEM) fuel cell is most promising for use in cars and light trucks. Hydrogen gas is fed to the anode. H 2 molecules react chemically with the platinum catalyst and splits into two protons and two electrons. The PEM allows the protons through to the cathode. The electrons must travel through a circuit through an electric motor to get to the cathode. The protons and electrons arriving at the cathode react with oxygen molecules to form oxygen. 2H 2 + O 2 => 2H 2 O
Fuel Cells and the Hydrogen Economy Uses oxygen from the air Pollution-free, producing only electricity and water NASA has been using fuel cells for years Practical challenges: Developing reasonably priced, long-lasting fuel cells Developing effective hydrogen delivery and storage systems Developing environmentally sound methods for producing the hydrogen
By listing the elements in order of increasing atomic mass, Mendeleev organized the elements into a table with elements of similar properties aligned into columns. This is called the periodic table.
Cathode rays, Electrons, and X-rays By the end of the nineteenth century, chemists were using the concept of atoms to explain their properties. Physicists were less convinced. The discovery of cathode rays was the beginning of atomic physics. Two electrodes are sealed in a glass tube. As the tube is evacuated, a glow discharge appears in the gas between the electrodes. With further evacuation, the discharge disappears, and a glow appears on the end of the tube opposite the cathode.
An invisible radiation seemed to emanate from the cathode to produce the glow on the opposite wall of the tube. The invisible radiation was called cathode rays. If the north pole of a magnet is brought down toward the top of a cathode-ray tube, the spot of light is deflected to the left across the face of the tube. This indicates the cathode rays are negatively charged particles. Two electrodes are sealed in a glass tube. As the tube is evacuated, a glow discharge appears in the gas between the electrodes. With further evacuation, the discharge disappears, and a glow appears on the end of the tube opposite the cathode.
J. J. Thomson used both electric fields and magnetic fields to deflect the beam. The combined effect allowed him to estimate the velocity of the particles. With the deflection produced by the magnetic field alone, this allowed him to estimate the mass of the particles. We now call these particles electrons.
An electron beam in a cathode-ray tube passes between two parallel plates that have a voltage difference of 300 V across them and are separated by a distance of 2 cm. In what direction will the electron beam deflect? a) upward b) downward c) out of the page d) into the page a) The electron beam is deflected upward because the force on a negative charge is opposite to the direction of the electric field.
An electron beam in a cathode-ray tube passes between two parallel plates that have a voltage difference of 300 V across them and are separated by a distance of 2 cm. What is the value of the electric field between the plates? a) 6 N/C b) 150 N/C c) 600 N/C d) 15,000 N/C d) From V = Ed: E = V / d = (300 V) / (0.02 m) =15,000 N/C
An electron beam in a cathode-ray tube passes between two parallel plates that have a voltage difference of 300 V across them and are separated by a distance of 2 cm. What is the magnitude of the force exerted on the electron? a) 2.4 x N b) 24,000 N c) 1.5 x N d) 15,000 N a) From F = qE, q = 1.6 x C: F = (1.6 x C)(15,000 N/C) = 2.4 x N
An electron beam in a cathode-ray tube passes between two parallel plates that have a voltage difference of 300 V across them and are separated by a distance of 2 cm. What is the acceleration of the electron? a) 2.64 x m/s 2 b) 2,640 m/s 2 c) 26,400 m/s 2 d) 2.64 x m/s 2 d) From F = ma, m = 9.1 x kg: a = F / m = (2.4 x N) / (9.1 x kg) = 2.64 x m/s 2
An electron beam in a cathode-ray tube passes between two parallel plates that have a voltage difference of 300 V across them and are separated by a distance of 2 cm. What path will the electron follow? a) straight line, up and to the right b) straight line, down and to the right 26,400 m/s 2 c) parabolic curve, up and to the right d) parabolic curve, down and to the right c) It will move in a parabolic path toward the positive charged plate, similar to a body moving in a constant gravitational field.
You probably use cathode rays almost every day. The heart of most television sets is the cathode ray tube, or CRT. Do you know how a TV works?
The electrodes that produce and focus the electron beam are called the electron gun. An electric current passes through the filament to heat the cathode to emit electrons. Electrons are accelerated from the cathode to the anode by the high voltage. Electrons passing through the hole in the anode make up the electron beam. After leaving the electron gun, the beam of electrons travel across the tube, producing a bright spot of light when it strikes the glass face of the tube. Magnets deflect the beam so that it strikes different points on the face of the tube at different times. The beam scans across the entire face of the tube in a fraction of a second, to form the picture.
Thomson’s discovery provided the first known subatomic particle. The mass of an electron is 9.1 x kg. The charge of an electron is 1.6 x C. The electron was the first possible candidate for a building block of atoms. The study of cathode rays led Roentgen to discover yet another type of radiation.
He noticed that a fluorescent material would glow when placed near his covered cathode-ray tube. Cathode rays could not travel through air, but this new radiation did. Because they were unknown, Roentgen called this new radiation X-rays.
X-rays are produced by collisions of the cathode rays (electrons) with the walls of the tube or with the anode. The strongest X-ray beams are produced by placing the metal anode at a 45 o angle to the beam, such as in an X- ray tube used in a diagnostic X-ray machine. The discovery of X-rays led to the discovery of natural radioactivity...
Radioactivity and the Discovery of the Nucleus When Becquerel placed a piece of phosphorescent material on a covered photographic plate, the developed plate showed a silhouette of the sample. Radiation apparently was passing from these materials to expose the film.
The penetrating radiation did not seem to be connected with the phosphorescence. Becquerel named this new radiation natural radioactivity because it seemed to be produced continuously by compounds containing uranium or thorium. Where did these rays come from? How could rays continue to be emitted when no energy was being added to the samples? Was this radiation somehow a property of the atoms themselves? Is more than one type of radiation involved?
Rutherford noticed when the beam of radiation from a uranium sample passes through a magnetic field, it splits into three components. Alpha deviates slightly to the left, indicating positively charged particles. Beta is bent in the opposite direction, indicating negatively charged particles. Beta is also bent much more, indicating less massive particles than those in the alpha beam. Further study indicated that these beta rays were electrons. The gamma rays were undeviated by the magnetic field. These are electromagnetic waves similar to X-rays but with shorter wavelengths.
The alpha particles were established to be helium atoms stripped of their electrons. Rutherford realized that they would make effective probes for studying the structure of the atom. A beam of alpha particles was scattered from a thin gold foil. The size of the gold atoms would affect the angle of the scattered alpha particles. Most of them went straight through the foil, as expected. This was consistent with the plum-pudding model of the atom. BUT, a few particles scattered back at much larger angles.
The backward scattering was as if someone had fired bullets into a piece of tissue paper and the bullets bounced back. If most of the alpha particles go through, but a few are scattered through large angles, there must be very dense but small centers somewhere within the atoms. The nucleus of the atom had been discovered! The nucleus is the very dense center of the atom, containing most of its mass and all of its positive charge. The electrons are responsible for most of the atom’s size, but very little of its mass.
Atomic Spectra and the Bohr Model of the Atom The atom has a positively charged nucleus that contains most of the mass, and negatively charged electrons are arranged somehow around this nucleus. Could the atom be arranged like a tiny solar system, with the electrons in orbit around the nucleus? Why didn’t the electron spiral into the nucleus as it lost energy due to radiating electromagnetic waves? Why are the electromagnetic waves emitted only with particular wavelengths?
If a substance is heated and the emitted light is observed through a prism, each substance produces characteristic colors or wavelengths, called the atomic spectrum of that substance. The spectrum of hydrogen is simple, with just four wavelengths in the visible portion. Balmer discovered that the wavelengths of these four lines could be computed from a simple formula: where n and m are both integers and R is the Rydberg constant: R = x 10 7 m -1
Quantization of light energy A blackbody radiator is represented by a hole in a metal or ceramic box. The hole appears black at room temperatures. When the box is heated, the hole emits a continuous spectrum of electromagnetic radiation. The average wavelength depends on the temperature. To complete the picture of the atom, there was yet another piece of the puzzle that needed to be discovered... Both Planck and Einstein contributed to the idea of light quanta.
Quantization of light energy Planck derived a formula that predicted the distribution of wavelengths emitted, depending on the temperature. His formula required that light could only be absorbed or emitted in discrete chunks or quanta, whose energy depended on the frequency or wavelength. where h = x J s is called Planck’s constant. This idea was indeed radical. Einstein showed that the quantization of light energy explains a number of other phenomena. The idea of light quanta (photons) having energies E = hf prepared the way for a new model of the atom.
Bohr’s model of the atom Bohr combined all these ideas: the discovery of the nucleus knowledge of the electron the regularities in the hydrogen spectrum the new quantum ideas of Planck and Einstein He pictured the electron as orbiting the nucleus in certain quasi-stable orbits. Light is emitted when the electron jumps from one orbit to another.
Bohr’s model of the atom The energy between the two orbits determines the energy of the emitted light quantum.
Bohr’s model of the atom The hydrogen spectrum can be explained by representing the energies for the different electron orbits in an energy-level diagram. The blue Balmer line is produced by the indicated jump.
What is the wavelength of the photon emitted in the transition from n = 4 to n = 2? ∆E = E 4 - E 2 = eV - (-3.4 eV) = 2.55 eV E = hf where h = x J s = 4.14 x eV s f = E / h = (2.55 eV) / (4.14 x eV s) = 6.16 x Hz = c / f = (3 x 10 8 ) / (6.16 x Hz) = 487 nm
Particle Waves and Quantum Mechanics Bohr’s model of the atom generated intense activity in physics. Unanswered questions included why only certain orbits were stable. Quantum mechanics provided the answer. If light waves sometimes behave like particles (as shown by Planck and Einstein), could particles sometimes behave like waves?
De Broglie suggested that certain things traditionally thought of as particles, such as the electron, might sometimes behave like waves. The frequency and wavelength of the wave depend on the particle’s energy and momentum:
If the electron in Bohr’s model of the atom were pictured as a standing wave wrapped around a circular orbit, de Broglie showed that its wavelength could only take on certain values. These values yield the quasi-stable orbits predicted by Bohr. Schrödinger developed a theory of the atom that used three-dimensional standing waves to describe the orbits of the electron about the nucleus. Quantum mechanics was born.
In quantum mechanics, the orbits are not simple curves as in the Bohr model. Instead, they are three-dimensional probability distributions centered on the nucleus. These describe the probability of finding electrons at certain distances and orientations about the nucleus. The waves cannot tell us exactly where the particle is located. The more precisely we know the particle’s momentum, the less we know about the particle’s location. This limitation was introduced by Heisenberg: Heisenberg Uncertainty Principle
Quantum mechanics successfully predicts the structure and spectra of atoms with many electrons. Quantum numbers describe the various possible stable orbits. No two electrons in an atom can have the same set of quantum numbers. Once an orbit is filled, other electrons must take on higher values for at least one of the quantum numbers. The regularities of the periodic table can be explained by the way the electrons fill the available orbits. The theory also predicts the ways different elements combine to form compounds. Quantum mechanics has become the fundamental theory of chemistry as well as atomic, nuclear, and condensed-matter physics.