The Atom Oxtoby Chapter 1 p 1-22

The Basic Building Block Atom from the Greek a-tom or indivisible (Democritus circa 500 BC) Four elements (earth, air, water, fire) Dalton (1808) 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 ~10-15 m e- The Nucleus Z protons N neutrons Z electrons Mass number A = Z + N 10-10 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.

A typical atomic radius is: 1 – 2.5 x 10-10 m 1 – 0.25 nm 1 – 2.5 Å (Angstrom) The nucleus is a tiny fraction of the volume of the whole atom with a radius of only ~10-15 m. But the nucleus contains almost all the mass of the whole atom because it contains the massive particles (the protons and neutrons). The nucleus thus has an enormous density.

How Do We Know This? From Rutherford’s alpha particle scattering experiments: a-particles are helium atom nuclei – or doubly charged helium atoms, 4He2+ Rutherford fired a-particle beams at a gold foil and detected the angles through which the a-particles were deflected. a- 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 New Zealander with Scottish Father Studied in Cambridge under JJ Thomson (Nobel prize 1906 for discovery of electron). Nobel Prize for Chemistry in 1908 for work on radioactivity. Only later did he start his a – particle scattering work.

Rutherford Backscattering Spectrometer Rutherford's partner in the initial phase of this work was Hans Geiger, who later developed the Geiger counter to detect and count fast particles.

The backscattered energy of these ions is related to the mass of the target element from which the ion backscatters.

Rutherford Backscattering Spectrometer Explain why He2+ will not scatter backwards from H or He atoms in a sample. Newton’s Cradle

Rutherford Recounts the Discovery Then I remember two or three days later Geiger coming to me in great excitement and saying "We have been able to get some of the alpha-particles coming backward …" It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." "I had observed the scattering of alpha-particles, and Dr. Geiger in my laboratory had examined it in detail. He found, in thin pieces of heavy metal, that the scattering was usually small, of the order of one degree. One day Geiger came to me and said, "Don't you think that young Marsden, whom I am training in radioactive methods, ought to begin a small research?" Now I had thought that, too, so I said, " Why not let him see if any alpha-particles can be scattered through a large angle?" I may 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.

The Fundamental Particles Charge (C) (e) Mass (kg) (u) The Proton + 1.602 x10-19 +1 1.673 x 10-27 1.00728 Neutron 1.675 x 10-27 1.00867 Electron - 1.602 x10-19 -1 9.109 x 10-31 0.00055 All atoms are electrically neutral (i.e., they have no net charge). Hence in an atom: number of protons = number of electrons

The Atomic Number, Z Chemical elements differ from one another in their atomic number, Z, the number of protons in their nucleus: element H He Li Be B C N O F Ne etc. Z 1 2 3 4 5 6 7 8 9 10 Iron, Fe (Z=26), lead Pb (Z=82), Uranium U (Z=92) I ask you to learn by heart the first ten elements. Every atom of the same element has the same atomic number, which equals the number of electrons. The latter determines the chemical properties and leads to the periodic table:

See also http://www.chemsoc.org/viselements/pages/periodic_table.html

The Mass Number, A The 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) The mass number for most nitrogen atoms is 14 and we write: 14N Element label (Z=7) A, mass number

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, Co are monoisotopic. Z N A % abund. 1H 1 99.985 2H 2 0.015 3H 3 Rad. 12C 6 12 98.9 13C 7 13 1.1 14C 8 14

Atomic Mass and Mass Number The mass number, A, is always an integer. Atomic masses, are not exactly integers because: The exact mass of a proton or neutron is not exactly 1 u. The energy binding the nucleus together is so enormous that its mass is less than the mass of its constituent particles. The atomic mass of an element is an average over the masses of all isotopes of the element.

Example: Isotopes Natural chlorine, Cl, has two common isotopes: 35Cl, 37Cl. (i.e., 17 protons + either 18 or 20 neutrons) These occur in the ratio 75.5% 35Cl : 24.5 % 37Cl The exact atomic mass of each is 34.969 u : 36.966 u Hence the “natural” atomic mass is calculated by a weighted average of the two according to their abundance: = (0.755 x 34.969 u) + (0.245 x 36.966 u) = 35.45 u Of course. no single atom has a mass of 35.45 u