Atom Oxtoby hapter p - Basic Building Block Atom from the Greek a-tom or indivisible (Democritus circa 5 B) Four elements (earth, air, water, fire) Dalton (88) proposed modern atomic theory: All matter consists of indivisible atoms All atoms of the same element are identical Different elements have different atoms Atoms retain their identity in reactions Atoms combine in whole number ratios to form compounds Electronic structure Z electrons e - ~ -5 m Nucleus Z protons N neutrons A typical atomic radius is:.5 x - m.5 nm.5 Å (Angstrom) nucleus is a tiny fraction of the volume of the whole atom with a radius of only ~ -5 m. Mass number A = Z N - m Although this is the classical picture of the atom, we will learn later that electrons in fact do not move about the nucleus in planetary orbits. But the nucleus contains almost all the mass of the whole atom because it contains the massive particles (the protons and neutrons). nucleus thus has an enormous density.
ow Do We Know This? From Rutherford s alpha particle scattering experiments: α-particles are helium atom nuclei or doubly charged helium atoms, 4 e Rutherford fired α-particle beams at a gold foil and detected the angles through which the α-particles were deflected. α particle beam Gold foil Results: Most particles undeflected Some show minor deflections A very small number are even scattered backwards Rutherford s Interpretation Most go straight through Ernest Rutherford Rutherford Backscattering Spectrometer New Zealander with Scottish Father Studied in ambridge under JJ Thomson (Nobel prize 9 for discovery of electron). Nobel Prize for hemistry in 98 for work on radioactivity. Only later did he start his α particle scattering work. Rutherford's partner in the initial phase of this work was ans Geiger, who later developed the Geiger counter to detect and count fast particles.
Rutherford Backscattering Spectrometer Explain why e will not scatter backwards from or e atoms in a sample. backscattered energy of these ions is related to the mass of the target element from which the ion backscatters. Newton s radle Rutherford Recounts the Discovery "I had observed the scattering of alpha-particles, and Dr. Geiger in my laboratory had examined it in detail. e found, n in I thin remember pieces two of heavy or three metal, days that later the Geiger scattering coming was to usually me in great small, excitement of the order and of saying one degree. "We have One been day Geiger able to came get some to of and the said, alpha-particles "Don't you think coming that young Marsden, backward whom " It I am was training quite the in most radioactive incredible methods, event that ought ever to happened begin a small to me research?" in my life. Now It was I had almost thought as that, incredible too, so I as said, if you " Why fired not a 5-inch let him shell see if at any a piece alphaparticles tissue paper can be and scattered it came through back and a large hit you." angle?" I may of tell you in confidence that I did not believe that they would be, since we knew the alpha-particle was a very fast, massive particle with a great deal of energy, and you could show that if the scattering was due to the accumulated effect of a number of small scatterings, the chance of an alpha-particle being scattered backward was very small. Fundamental Particles Proton Neutron Electron harge (). x -9 -. x -9 harge (e) - Mass (kg).7 x -7.75 x -7 9.9 x - All atoms are electrically neutral (i.e., they have no net charge). ence in an atom: number of protons = number of electrons Mass (u).78.87.55
Atomic Number, Z hemical elements differ from one another in their atomic number, Z, the number of protons in their nucleus: element Z e Li Be 4 B 5 Iron, Fe (Z=), lead Pb (Z=8), Uranium U (Z=9) I ask you to learn by heart the first ten elements. N 7 O 8 F 9 Ne etc. Every atom of the same element has the same atomic number, which equals the number of electrons. latter determines the chemical properties and leads to the periodic table: See also http://www.chemsoc.org/viselements/pages/periodic_table.html Mass Number, A mass number, A, of an atom is the number of heavy particles in the nucleus: i.e., A = Z N where Z is the atomic number (number of protons) and N is the number of neutrons e.g., All nitrogen atoms have Z = 7 (making them nitrogen) Most nitrogen atoms also have 7 neutrons (N = 7) mass number for most nitrogen atoms is 4 and we write: A, mass number 4 N Element label (Z=7) Not all the Atoms of an Element are Necessarily the Same; Breakdown of Dalton s Law; Isotopes Most elements have several different isotopes which differ only in the number of neutrons in the nucleus Example: isotopes of common elements Only a few e.g., niobium, Nb rhodium, Rh, manganese, Mn cobalt, o are monoisotopic. 4 Z N 7 8 A 4 % abund. 99.985.5 Rad. 98.9. Rad. 4
Atomic Mass and Mass Number mass number, A, is always an integer. Atomic masses, are not exactly integers because: Example: Isotopes Natural chlorine, l, has two common isotopes: 5 l, 7 l. (i.e., 7 protons either 8 or neutrons). exact mass of a proton or neutron is not exactly u.. energy binding the nucleus together is so enormous that its mass is less than the mass of its constituent particles.. atomic mass of an element is an average over the masses of all isotopes of the element. se occur in the ratio 75.5% 5 l : 4.5 % 7 l exact atomic mass of each is 4.99 u :.9 u ence the natural atomic mass is calculated by a weighted average of the two according to their abundance: = (.755 x 4.99 u) (.45 x.9 u) = 5.45 u Of course. no single atom has a mass of 5.45 u 5