Download AS II Rutherford Model

2-1
2-2
Radioactivity
AS II
Rutherford Model
(background info)
• Scattering Experiment
•
high energy radiation
•
released by some natural materials
( U, Ra, ...) and
many man-made elements (e.g., Co-60)
(Co-59 is the natural non-radioactive isotope)
• Subatomic Particles
•
• Chem. Elements
primary components:
α radiation, pos. heavy particles, He2+
β radiation, neg. light particles, electrons
Ref
2: 3 - 6
Prob
2 : 23 -27, 33 - 39, 43, 47 odds
γ radiation, emr w/ λ ≈ 10–13 m (= 0.1 pm)
ν ≈ 1021 Hz
(note: X-rays might be secondary components)
Adv Rdg 8 : 1, 3
see Pet. Fig 2.10
2-3
Pet. Fig. 2.10 Radioactivity
2-4
Rutherford Scattering Experiment
for exp. set-up, see
Pet Fig. 2.11 & 2.12
also HT 2.1 & 2.2
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
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”
2-6
“Rutherford”
2-7
Evaluation of Rutherford Exp.
2-8
Universal Subatomic Particles
atom = mostly empty space
w/ tiny, “dense”, pos. center
= “nucleus”
&
dispersed, ~ weightless, neg. particles
= “electrons”
approx. sizes:
atom
relative
mass (u)
relative
charge
e–
~0
–1
p+
~1
+1
no
~1
0
~ 100 pm
nucleus ~ 0.01 pm
“like a pea in a stadium”
further investigation:
nucleus = protons (p+) & neutrons (no)
for exact absolute values see:
Pet. Table 2.1
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
A
Z
X
X ... atomic symbol
A ... mass number ( p+’s + no’s)
Z ... atomic number (p+’s (= e–’s, if neutral))
also: X
e.g.,
-A
12
6
C
or
C - 12
NB.(remember)
Natural C consists of
≈ 99% C-12 and ≈ 1% C-13
Ions
cations:
pos., lost e–’s
anions:
neg., gained e–’s (a neg. ion)
e.g.,
1)
Cl– , gained 1 e–
2)
Al3+, lost 3 e–’s
has 13 p+ and 10 e–’s
2-13
2-14
Atomic Mass of Isotopes
•
Isotopic Mixtures
means “relative atomic mass”
units: “atomic mass units” = “u”
•
by definition:
atomic mass of C-12 = 12 u exactly
•
(12.00000000...)
•
∴
•
∴
+
Most elements exist as mixtures
•
on earth, composition nearly const.
•
Atomic masses of elements (in P.T.)
= weighted avg. of natural isotopic mix.
Ex. Ag is 51.84% Ag-107 (106.905 u) &
48.16% Ag-109 (108.905 u)
1 u = 121 of mass of C-12
atomic mass of H-1, p+, no
approx., but not exactly 1 u.
o
NB.: as p ’s and n ’s combine in nuclei
their masses change slightly (nuclear physics)
Ex. isotope subatomic particles
H-2
1p 1n 1e
He-4
2p 2n 2e
C-12
6p 6n 6e
•
atomic mass(u)
2.0141
4.0026
12.0000
Notes:
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)
∴ atomic mass of Ag =
0.5184 × 106.905 u + 0.4816 × 108.905 u =
107.87 u
NB. mass number ≠ atomic mass
(average mass number is not normally used )
General Formula:
x1 I1 + x2 I2 + ... = E
where, x ...fraction, abundance; x1 +x2 + ... = 1
I atomic mass of isotope
E ... atomic mass of “element”
2-15
Periodic Table (P.T.)
• 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)
2-16
Pet. Fig. 2.15
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