2-1 2-2 Radioactivity AS II (background info) Rutherford Model • high energy radiation • released by some natural materials • Scattering Experiment ( U, Ra, ...) and many man-made elements (e.g., Co-60) (Co-59 is the natural non-radioactive isotope) • Subatomic Particles • primary components: 2+ α radiation, pos. heavy particles, He • Chem. Elements β radiation, neg. light particles, electrons –13 Ref 2: 4 - 6 γ radiation, emr w/ λ ≈ 10 m (= 0.1 pm) 21 ν ≈ 10 Hz (note: X-rays might be secondary components) FUP: 2: 4 EofC: 2 : 36 – 50 see Pet. Fig 2.10 Adv Rdg 7 : 1 2-3 2-4 Pet. Fig. 2.10 Radioactivity Rutherford Scattering Experiment for exp. set-up, see Pet Fig. 2.11 & 2.12; HT 2.1 & 2.2; SB 2.7 consists of: • α source • target metal foil (thin sheet; e.g. Au, ... • screen/detection system expected: minor bending of α rays (due to e– ) surprise: some α particles “bounced back” 2-5 2-6 Pet. Fig. 2.11 Experimental Set-up HT Fig. 2.1 Experimental Sep-up HT Fig. 2.2 Explanation Pet. Fig. 2.12 Explanation “Plum Pudding” “Rutherford” 2-7 2-8 Evaluation of Rutherford Exp. Universal Subatomic Particles atom = mostly empty space relative relative w/ tiny, “dense”, pos. center mass (u) charge = “nucleus” & e– ~ 0 – 1 dispersed, ~ weightless, neg. particles = “electrons” p+ ~ 1 + 1 approx. sizes: atom ~ 100 pm o n ~ 1 0 nucleus ~ 0.01 pm “like a pea in a stadium” for exact absolute values see: further investigation: Pet. Table 2.1 or SB Table 2.2 nucleus = protons (p+) & neutrons (no) 2-9 2-10 Elements • differ by # of p+’s in increments of 1: 1 ........ H 2 ........ He ........ 92 ......... U • # of no is variable (but close to # of p+) • many elements = mixture of isotopes (same # of p+ , diff # of no) e.g., Cl atoms 75.5% have 17 p+, 18 no (Cl-35) 24.5% have 17 p+, 20 no (Cl-37) 2-11 2-12 Isotopic Notation Ions A cations: pos., lost e–’s Z X – anions: neg., gained e ’s (a neg. ion) X ... atomic symbol – A ... mass number ( p+’s + no’s) e.g., 1) Cl , gained 1 e– 3+ – Z ... atomic number (p+’s (= e–’s, if neutral)) 2) Al , lost 3 e ’s has 13 p+ and 10 e–’s also: X - A 12 e.g., 6 C or C - 12 NB.(remember) Natural C consists of ≈ 99% C-12 and ≈ 1% C-13 2-13 2-14 Atomic Mass of Isotopes Isotopic Mixtures • Most elements exist as mixtures • means “relative atomic mass” • units: “atomic mass units” = “u” • on earth, composition nearly const. • by definition: • Atomic masses of elements (in P.T.) atomic mass of C-12 = 12 u exactly = weighted avg. of natural isotopic mix. (12.00000000...) 1 Ex. Ag is 51.84% Ag-107 (106.905 u) & • ∴ 1 u = 12 of mass of C-12 + o 48.16% Ag-109 (108.905 u) • ∴ atomic mass of H-1, p , n . approx., but not exactly 1 u ∴ atomic mass of Ag = + o 0.5184 × 106.905 u + 0.4816 × 108.905 u = NB.: as p ’s and n ’s combine in nuclei 107.87 u their masses change slightly (nuclear physics) Ex. isotope subatomic particles atomic mass(u) NB. mass number ≠ atomic mass (average mass number is not normally used ) H-2 1p 1n 1e 2.0141 He-4 2p 2n 2e 4.0026 General Formula: x1 I1 + x2 I2 + ... = E C-12 6p 6n 6e 12.0000 where, x ...fraction, abundance; x1 +x2 + ... = 1 I atomic mass of isotope Notes: E ... atomic mass of “element” even though the 3 of subatomic particles are whole-number multiples, the atomic masses are not; (xs mass as you go from lower to higher atoms converted to energy) 2-15 2-16 Periodic Table (P.T.) Pet. Fig. 2.15 or SB Fig 2.9 • see Pet. Fig. 2.15 • used to organize elements • elements in same group (column) have similar properties etc. ...... • groups 1 ............ 18 for the time being, learn names & symbols of first 36 elements (H - Kr) 2-17 Summary of Lesson AS II • radioactivity: describe the 3 components and their properties • Rutherford experiment experimental set-up: source, foil, screen experimental results: “bounce-back” description of nucleus & e–’s • subatomic particles: p+, no, e– • elements characterized by # of p+’s (= Z) • isotopic notations • elements as isotopic mixtures • definition of (relative) atomic mass • atomic masses of isotopes and elements • rudimentary knowledge of P.T. • HMWK 1.6 - 1.8 .