tro, chapter 2 (primarily) websites in this powerpoint

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

http://phet.colorado.edu/simulations/sims.php?sim=Rutherford_Scattering

http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/

http://atom.kaeri.re.kr/

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”


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)


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


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


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


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?


Atomic Theory Explains Law of Multiple Proportions


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


http://www.colorado.edu/physics/2000/applets/nforcefield.html

http://www.colorado.edu/physics/2000/applets/nforcefield.html

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


Coulomb’s Law

+10 +1



Figure 2.8 Deflection of Cathode Rays by an Applied Electric Field


Figure 2.7 A Cathode-Ray Tube

Recall: Atoms appear to be composed of some “light weight” negatively charged particles


Called “electrons” (e-’s)


Tro, p. 51 The Plum Pudding Model of the Atom (Thomson)

(Embedded in the “mush”)

(Positive “mush”; most of the mass)


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)


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!)


Like Figure 2.6 in Tro. Rutherford's Experiment On -Particle Bombardment of Metal Foil


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


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






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!!


Atomic Nucleus

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)



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


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


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!


B. Mass Number, A (initial thoughts/insights)

• What do you think?


– 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


• 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?


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])


http://atom.kaeri.re.kr/

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.


D. Complete Symbol

• (On board, plus see next slide)


A

ZX

Mass Number

Atomic Number Element Symbol

Net charge, if not neutral (more later)


Figure 2.15 Two Isotopes of Sodium

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)


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


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-


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


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


PS2 “sign posting”

• You should now be able to complete problem set 2 up through problem #18 (out of 21 [total] on set).


tro, chapter 2 (primarily) websites in this powerpoint

Documents

g of oxygen

mass ratio of hg

ratio of small

mass ratio of pb

o mass ratios

law of definite proportionfor

law of definite proportionif

multiple different compounds