Biochem LEC - Chapter 2.pdf

CHAPTER 2: Atomic Theory (LECTURE)  Lesson Outline Early Views of Atom Dalton’s Atomic Theory Discovery of Subatomic Particles Atomic Models Modern Atomic Model  Early Views of Atom 5th century B.C. Democritus - All matter is composed of small, finite particles and called it atomo...

CHAPTER 2: Atomic Theory (LECTURE)  Lesson Outline Early Views of Atom Dalton’s Atomic Theory Discovery of Subatomic Particles Atomic Models Modern Atomic Model  Early Views of Atom 5th century B.C. Democritus - All matter is composed of small, finite particles and called it atomos – “indivisible” or “uncuttable.” Atoms as moving particles that differed in shape and size, could join together. Aristotle – Matter is consisted of various combinations of the four “elements” — fire, earth, air, and water — and could be infinitely divided.  Dalton’s Atomic Theory 19th century – John Dalton laid for an atomic theory that linked the idea of elements with the idea of atoms. His atomic theory was based on four postulates: 1. Each element is composed of extremely small particles called atoms. 2. All atoms of a given element are identical, but the atoms of one element are different from the atoms of all other elements. 3. Atoms of one element cannot be changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions. 4. Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.  Dalton’s Atomic Theory PROBLEMS WITH DALTON’S ATOMIC THEORY Matter is composed of tiny indivisible atoms. atoms are divisible and are composed of smaller, subatomic particles called electrons, protons, and neutrons Atoms of an element are identical in mass. all elements have isotopes – atoms with this same proton number but different numbers of neutrons atoms of an element do not have to have the same mass  Discovery of Subatomic Particles As scientists developed methods for probing the nature of matter, the supposedly indivisible atom began to show signs of a more complex structure, and today we know that the atom is composed of subatomic particles. Particles with the same charge repel one another, whereas particles with unlike charges attract one another.  Discovery of Subatomic Particles CATHODE RAYS AND ELECTRONS J.J. Thomson – observed that cathode rays are the same regardless of the identity of the cathode material. Experiments showed that cathode rays are deflected by electric or magnetic fields in a way consistent with their being a stream of negative electrical charge. Cathode rays are streams of negatively charged particles and these negatively charged particles are called electrons. Discovery of Subatomic Particles  Discovery of Subatomic Particles OIL DROP EXPERIMENTS Robert Millikan – succeeded in measuring the charge of an electron by performing the oil-drop experiment in 1909. The electron has a charge of 1.602 x 10-19 C.  Discovery of Subatomic Particles RADIOACTIVITY Radioactivity – spontaneous emission of radiation. Radiation – the emission of energy as electromagnetic waves or as moving subatomic particles, especially high-energy particles that cause ionization  Discovery of Subatomic Particles RADIOACTIVITY Henri Becquerel – discovered that a compound of uranium spontaneously emits high-energy radiation in 1896. Marie and Pierre Curie – began experiments to identify and isolate the source of radioactivity in the compound at Becquerel’s suggestion.  Discovery of Subatomic Particles RADIOACTIVITY Ernest Rutherford – revealed three types of radiation: alpha α, beta β, and gamma γ. Most of the mass of each gold atom and all of its positive charge, which he called proton, reside in a very small, extremely dense region and called it nucleus. Discovery of Subatomic Particles  Discovery of Subatomic Particles ISOTOPES AND NEUTRONS During the early 1900s, scientists identified several substances that appeared to be new elements, isolating them from radioactive ores. For example, a “new element” produced by the radioactive decay of thorium was initially given the name mesothorium. However, a more detailed analysis showed that mesothorium was chemically identical to radium (another decay product), despite having a different atomic mass.  Discovery of Subatomic Particles ISOTOPES AND NEUTRONS Frederick Soddy – discovered an element could have types of atoms with different masses that were chemically indistinguishable. And he called these isotopes—atoms of the same element that differ in mass.  Discovery of Subatomic Particles ISOTOPES AND NEUTRONS The nucleus was known to contain almost all of the mass of an atom, with the number of protons only providing half, or less, of that mass. Different proposals were made to explain what constituted the remaining mass, including the existence of neutral particles in the nucleus. As you might expect, detecting uncharged particles is very challenging.  Discovery of Subatomic Particles ISOTOPES AND NEUTRONS James Chadwick – discovered neutrons: uncharged, subatomic particles with a mass approximately the same as that of protons.  Atomic Models DALTON’S ATOMIC MODEL  Atomic Models PLUM-PUDDING MODEL OF ATOM J.J. THOMSON – electrons contribute only a very small fraction of an atom’s mass they probably are responsible for an equally small fraction of the atom’s size. The atom consists of a uniform positive sphere of matter in which the mass is evenly distributed and in which the electrons are embedded like raisins in a pudding or seeds in a watermelon.  Atomic Models NUCLEAR MODEL OF ATOM E. RUTHERFORD – most of the volume of an atom is empty space in which electrons move around the nucleus.  Modern Atomic Model MODERN ERA’S VIEW OF ATOM Following the work of Ernest Rutherford and his colleagues in the early twentieth century, the picture of atoms consisting of tiny dense nuclei surrounded by lighter and even tinier electrons continually moving about the nucleus was well established. This picture was called the planetary model, since it pictured the atom as a miniature “solar system” with the electrons orbiting the nucleus like planets orbiting the sun.  Modern Atomic Model BOHR’S ATOMIC MODEL N. BOHR – attempted to resolve the atomic paradox by ignoring classical electromagnetism’s prediction that the orbiting electron in hydrogen would continuously emit light. Furthermore, he adopted Planck’s idea that energies are quantized and made three postulates: 1. Only orbits of certain radii, corresponding to certain specific energies, are permitted for the electron in a hydrogen atom. 2. An electron in a permitted orbit is in an “allowed” energy state. An electron in an allowed energy state does not radiate energy and, therefore, does not spiral into the nucleus. 3. Energy is emitted or absorbed by the electron only as the electron changes from one allowed energy state to another. This energy is emitted or absorbed as a photon that has energy E = hν.  Modern Atomic Model BOHR’S ATOMIC MODEL N. BOHR – assumed that the electron orbiting the nucleus would not normally emit any radiation (the stationary state hypothesis), but it would emit or absorb a photon if it moved to a different orbit.  Modern Atomic Model BOHR’S ATOMIC MODEL Limitations to Bohr’s Model of Atom: 1. Bohr model explains the line spectrum of the hydrogen atom, it cannot explain the spectra of other atoms. 2. Bohr also avoided the problem of why the negatively charged electron would not just fall into the positively charged nucleus, by simply assuming it would not happen. 3. There is a problem with describing an electron merely as a small particle circling the nucleus because electron exhibits wave- like properties, a fact that any acceptable model of electronic structure must accommodate.  Modern Atomic Model WAVE BEHAVIOR OF MATTER Macroscopic objects act as particles. Microscopic objects (such as electrons) have properties of both a particle and a wave. Their exact trajectories cannot be determined. Radiation appears to have either a wave-like or a particle-like (photon) character.  Modern Atomic Model WAVE BEHAVIOR OF MATTER L. DE BROGLIE – one of the first people to pay attention to the special behavior of the microscopic world. He asked the question: If electromagnetic radiation can have particle- like character, can electrons and other submicroscopic particles exhibit wavelike character? An electron moving about the nucleus of an atom behaves like a wave and therefore has a wavelength. the wavelength of the electron, or of any other particle, depends on its mass, m, and on its velocity, v: λ = h/mv  Modern Atomic Model WAVE BEHAVIOR OF MATTER If an electron is viewed as a wave circling around the nucleus, an integer number of wavelengths must fit into the orbit for this standing wave behavior to be possible.  Modern Atomic Model WAVE BEHAVIOR OF MATTER W. HEISENBERG – considered the limits of how accurately we can measure properties of an electron or other microscopic particles. He determined that there is a fundamental limit to how accurately one can measure both a particle’s position and its momentum simultaneously. The more accurately we measure the momentum of a particle, the less accurately we can determine its position at that time, and vice versa. It is fundamentally impossible to determine simultaneously and exactly both the momentum and the position of a particle.  Modern Atomic Model QUANTUM MECHANICAL DESCRIPTION OF ATOM In 1926, Erwin Schrödinger, proposed an equation that incorporates both the wave- like and particle-like behaviors of the electron (Schrödinger equation). ෡ 𝛙 = 𝐄𝛙 𝐇 - His work opened a new approach to dealing with subatomic particles, an approach known as quantum mechanics or wave mechanics.  Modern Atomic Model QUANTUM MECHANICAL DESCRIPTION OF ATOM