View
216
Download
1
Category
Tags:
Preview:
Citation preview
Tro, Chapter 2 (primarily)
Websites in this Powerpoint
• http://www.colorado.edu/physics/2000/applets/nforcefield.html
• http://phet.colorado.edu/simulations/sims.php?sim=Rutherford_Scattering
• http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/ [show first?]
• http://atom.kaeri.re.kr/
Copyright © Houghton Mifflin Company. All rights reserved. 2–1
Plan
• Law of Definite Proportion (revisited)– use of ratio as a conversion factor
• Law of Multiple Proportions• Charge and Coulomb’s Law• Model of atom circa 1890’s (Thomson)
– Atoms are not “indestructible”
• Rutherford’s Gold Foil Expt and interpretation• Subatomic Particles (characteristics); “amu”• Isotopes, Complete Symbols, “Game”
Copyright © Houghton Mifflin Company. All rights reserved. 2–2
The Law of Definite Proportion
• For any sample of any compound, the mass ratio of the elements that combine chemically to form it (or are formed from it upon chemical separation) is constant– i.e., the masses are in a fixed (definite) ratio
(proportion)
• NOTE: Applies to ONE compound– i.e., one type of compound
• NOTE #2: The ratio need NOT be a ratio of small whole numbers (it just is “constant”)– E.g., the mass ratio of Pb : Cl in lead(II) chloride
is (always) 2.92 (i.e., 2.92 g Pb / 1 g Cl)
Copyright © Houghton Mifflin Company. All rights reserved. 2–3
Reminder (Data illustrating the Law of Definite Proportion)
For a compound (red powder, here):
The (best estimate for the) mass ratio of Hg : O is 12.6 (i.e., 12.6 g Hg / 1 g O) in any sample of this compound.
Mass ratio of Hg : O 12.568… 12.6101… 12.575
Copyright © Houghton Mifflin Company. All rights reserved. 2–4
Applying the Law of Definite Proportion
If a sample of this compound contains 40.0 g of oxygen (i.e., 40.0 g of oxygen would be produced if the sample were separated
chemically into its elements), how much mercury would the sample contain? (Hint: Use the mass ratio like a
conversion factor)
40.0 g O x 201.1 g Hg
16.0 g O502.75 503 g Hg
12.6 g Hg
1 g O = 504 g Hg
Copyright © Houghton Mifflin Company. All rights reserved. 2–5
The Law of Multiple Proportions
• When two elements form multiple (different) compounds, the mass ratios are in a ratio of small whole numbers– i.e., the masses of the second element that
combine with 1 g of the first element in each compound can be reduced to a ratio of small whole numbers
• NOTE: Applies to a SERIES of compounds (i.e., more than one!)
• E.g., the N : O mass ratios in three separate “nitrogen oxide” compounds are: 1.750, 0.8750, and 0.4375.– These ratios are themselves in a 4 : 2 : 1 ratio
Copyright © Houghton Mifflin Company. All rights reserved. 2–6
Example
• Samples of two compounds, both containing only carbon and oxygen, are separated into their component elements with the following results:– A sample of Compound A yields 25.9 g O and 9.7
g C– A sample of Compound B yields 20.3 g O and
15.3 g C.• Are these data consistent with the Law of
Multiple Proportions?• What could be the formulas of the two
compounds?
Copyright © Houghton Mifflin Company. All rights reserved. 2–7
Atomic Theory Explains Law of Multiple Proportions
Copyright © Houghton Mifflin Company. All rights reserved. 2–8
For a fixed amount (mass OR atoms) of C, there is twice as much mass of O b/c 2x as many atoms (per C). Must be a small whole number ratio since ratio of ATOMS must be whole number.
Charge, and Coulomb’s Law
• Charge—a property that, for example, a comb “acquires” when it is rubbed through hair (not a definition!)– “positive”, “negative”, or “neutral” (none)– Can have a “greater” or “smaller” amount
of charge
• To “experience” Coulomb’s Law: http://www.colorado.edu/physics/2000/applets/nforcefield.html
Copyright © Houghton Mifflin Company. All rights reserved. 2–9
Coulomb’s Law (semiquantitative)
The force between two charged particles depends on three things:
1. The “nature” (signs) of the two charges determines the nature of the force Opposite charges attract; Like charges repel
[The following two variables determine the magnitude of the force (how strong it is)]
2. The magnitude of the charges The larger the (product of) charges, the stronger the force
3. The distance between the (centers) of the particles The larger the distance, the weaker the force
NOTE: Tro discusses Coulomb’s Law on pp. 319-320
Copyright © Houghton Mifflin Company. All rights reserved. 2–10
Copyright © Houghton Mifflin Company. All rights reserved. 2–12
Figure 2.8 Deflection of Cathode Rays by an Applied Electric Field
Recall: Atoms appear to be composed of some “light weight” negatively charged particles
Copyright © Houghton Mifflin Company. All rights reserved. 2–14
Called “electrons” (e-’s)
Copyright © Houghton Mifflin Company. All rights reserved. 2–15
Tro, p. 51 The Plum Pudding Model of the Atom (Thomson)
(Embedded in the “mush”)
(Positive “mush”; most of the mass)
Copyright © Houghton Mifflin Company. All rights reserved. 2–16
Rutherford’s Experiment
• particles– Emitted from radioactive samples– Positively charged (bent away from + plate)– High energy (charged “missile”)
• Aimed beam at a thin metal foil– Maybe 100 atoms thick (small!)– Pudding model predicted all would go
straight through (no force strong enough to alter their path)
Copyright © Houghton Mifflin Company. All rights reserved. 2–17
Rutherford’s Observations
(Note: “flash” = an particle hitting screen)
• Most particles went straight through(flashes were directly opposite foil)
• BUT small fraction had their paths altered (deflected)(Flashes appeared at various angles, even
some back behind the source!)
Copyright © Houghton Mifflin Company. All rights reserved. 2–18
Like Figure 2.6 in Tro. Rutherford's Experiment On -Particle Bombardment of Metal Foil
Copyright © Houghton Mifflin Company. All rights reserved. 2–19
Figure 2.7 (a) Expected Results of the Metal Foil Experiment if
Thomson's Model Were Correct (b) Actual Results (with Rutherford’s “explanation” model*)
*NOTE: Rutherford’s initial model (1911) did not include neutrons! Only in ~1920 did he propose this (and even then, his concept of “neutron” was a “proton-electron pair”. See: http://en.wikipedia.org/wiki/Neutron
Copyright © Houghton Mifflin Company. All rights reserved. 2–20
To see a simulation:
• http://phet.colorado.edu/simulations/sims.php?sim=Rutherford_Scattering – NOTE: This animation only shows an extreme close-up of
the nucleus. It would be a much better animation if you could “zoom out” and see a) one entire atom, and b) several layers of atoms in the foil
– Set the p and n values to their minimum to best minimize the problem noted above.
• Low tech: magnets demo!• Also see (link opens in Firefox, but not IE right now):
http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/ to get perspective on the size of nucleus vs. atom
Copyright © Houghton Mifflin Company. All rights reserved. 2–21
Figure 2.14 (Zumdahl) Cross Section of a Nuclear Atom
NOTE: On this scale, the nucleus would be so tiny as to not be visible!!!! Its diameter is ~100,000 times smaller!!
Last Words on Rutherford
• Expt in 1910
• Nucleus was “all the positive charge and most of the mass” in tiny space
• Model did NOT include neutrons! – Wasn’t until 20 years later (your life thus
far?) that observations led to hypothesis of neutrons (Chadwick, 1932)
Copyright © Houghton Mifflin Company. All rights reserved. 2–23
Copyright © Houghton Mifflin Company. All rights reserved. 2–24
The amu is a (tiny!) unit of mass
• Amu = “atomic mass unit”
• Recall: 1 fg = 10-15 g (one quadrillionth of a g)
• 1 amu ~ 1.67 x 10-24 g (a billion times smaller than a fg!)
• 1 amu is approximately the mass of a single proton or neutron
Copyright © Houghton Mifflin Company. All rights reserved. 2–25
Subatomic Particles (Basics) [Like Table 2.1 in Tro]
Particle Charge Mass (g) Mass (other)
Proton +1 ~1.67 x 10-24 ~ 1 amu
Electron -1 ~1/1800 th of an amu
Neutron 0 (neutral)
~1.67 x 10-24 ~1 amu
Copyright © Houghton Mifflin Company. All rights reserved. 2–26
Atomic terms/concepts(Note: Some definitions are on the Week 2 sheet)
A. Atomic number, Z
B. Mass number, A
C. Isotope (and isotopes)• Isotopic mass ≠ mass number!!
D. Complete symbol (for an isotope)
A. Atomic Number, Z
• A number (e.g., a counting number: 1, 2, 3, etc.)
• The # of protons in the nucleus• Tells you “who you are” if you’re an atom
– If Z = 8, you must be O; if Z = 12, you are Mg• Regardless of the number of n’s or e-’s!!
– NO EXCEPTIONS!!!
• The counting number (not the “decimal” #) in the box for an element on the periodic table!– No need to memorize these!
Copyright © Houghton Mifflin Company. All rights reserved. 2–27
B. Mass Number, A (initial thoughts/insights)
• What do you think?
Copyright © Houghton Mifflin Company. All rights reserved. 2–28
– No, it’s not the “mass of an atom”!• Mass has units. A is unitless
– No, it’s not that “decimal” number in the box on the periodic table!
• A, like Z, is a number (a counting number)– The masses of atoms are generally not
(exact) whole numbers of any mass unit!
B. Mass Number, A (continued)
& C. Isotopes
• the number of protons + neutrons in the nucleus– It does not tell you who you are– It is not as fundamental as Z
Copyright © Houghton Mifflin Company. All rights reserved. 2–29
• C. Consider three atoms: (done on board; all have 6 protons, but one has 8 neutrons, one
has 6 neutrons, and one has 7 neutrons)
A, identity, and approximate mass for each?
How can they all be C atoms? Aren’t they all the same? Nope. DALTON WAS WRONG!
Consider Br-80 and Se-80
• What do atoms of these two isotopes have in common?
• What is different?
• Are the masses the same?
Copyright © Houghton Mifflin Company. All rights reserved. 2–30
Mass number ≠ mass!
• Mass of an atom of Br-80 (“isotopic mass”):– 79.918530 amu
• Mass of an atom of Se-80 (“isotopic mass”):– 79.916522 amu
• Mass numbers are the same, but not masses– “Close” is not the same as “identical”!!
• http://atom.kaeri.re.kr/ (click on any spot on the “plot” to get to a place where you can enter in
info about a given element or isotope [in the “nuclide” box])
Copyright © Houghton Mifflin Company. All rights reserved. 2–31
NOTE:
• “(Average) Atomic Mass” (which is discussed in Tro, Section 2.8) has not yet been defined or discussed in this PowerPoint. It may have been addressed verbally in class, but will be formally addressed later in another PowerPoint.
Copyright © Houghton Mifflin Company. All rights reserved. 2–32
D. Complete Symbol
• (On board, plus see next slide)
Copyright © Houghton Mifflin Company. All rights reserved. 2–33
A
ZX
Mass Number
Atomic Number Element Symbol
Net charge, if not neutral (more later)
Example Problem(s)
• Fill in the blanks! (Only do 1st row now)
#p #n #e Overall Charge
Complete Symbol
55 78 1+
126
16 18 3-
4521Sc
___ 2___ Pb
***You should also be able to do problems in which mass number is also one of the columns in the table (or asked about separately).***
21 45 – 21 = 24
Mass number
021 (same as p)
Copyright © Houghton Mifflin Company. All rights reserved. 2–35
Ions: The 3rd Kind of Nanoscopic Entity
• Atom: Neutral nanoscopic entity with one nucleus (spherical “thing”)
• Molecule: Group of atoms somehow linked together in some manner (also neutral)
Now add:
• Ion: Nanoscopic entity (derived from an atom or molecule) having an overall charge.– NOT neutral total # of e-’s ≠ total # of p’s
Copyright © Houghton Mifflin Company. All rights reserved. 2–36
Two kinds of ion with respect to sign of charge.
• Cation (KAT-ion): an ion having a positive charge [fewer e-’s than p’s]
– Na+, Mg2+, Fe3+, NH4+
• Anion (ANN-ion): an ion having a negative charge [more e-’s than p’s]
– Cl-, N3-, O2-, OH-, NO3-, PO4
3-, CN-
Copyright © Houghton Mifflin Company. All rights reserved. 2–37
Two kinds of ion with respect to “makeup”
• Monatomic: (can be thought of as being) derived from a single atom (with electrons either added or removed)
– Na+, Mg2+, Fe3+, Cl-, N3-, O2-
– Like an “atom with a charge”
• Polyatomic: (can be thought of as being) derived from a single molecule (with electrons either added or removed)
– NH4+, OH-, NO3
-, PO43-, CN-
– Like a “molecule with a charge”
“2-” means two electrons were added to an O atom
“3+” means three electrons were removed from an Fe atom
“3-” means three electrons were added to a PO4 molecule
Copyright © Houghton Mifflin Company. All rights reserved. 2–38
Example Problem(s)
• Fill in the blanks! (Do other rows now)
#p #n #e Overall Charge
Complete Symbol
55 78 1+
126
16 18 3-
4521Sc
___ 2___ Pb
***You should also be able to do problems in which mass number is also one of the columns in the table (or asked about separately).***
54 (one fewer than p)
Cs13355
55 + 78 = 133
Pb =>
82208822+80
(two fewer than p)
15 (three fewer than e)
33115P
Copyright © Houghton Mifflin Company. All rights reserved. 2–39
Recommended