Intersection 2

9/05

Reading: 1.8-1.11 p17-28

2.1-2.2 p 40-44

Questions

• Turning in course pack

• Lab reports

• Calendar

• Registration for CHEM 126

Outline

• Review• Concept questions• Representation and Scale in Chemistry• History of the atom

– Dalton– Thompson (Plum Pudding)– Millikan– Rutherford– Bohr

Review of Week 1

ScienceChemistry

Fact

Hypothesis

Theory

Model

Law

Safety

Lab equipment

Graph

Calculation

Significant Figures

Quantitative

Qualitative

Teamwork Data Gathering

and Analysis

W1S1

W1S1W1L2

W1S1

W1IS1

W1L2

W1L1

NAS Building in Washington, DC

“The investigation of truth is in one way hard and in another way easy. An indication of this is found in the fact that no one is able to attain the truth entirely, but everyone says something true about the nature of things, and by the union of all a considerable amount is amassed.”

-Aristotle in “Metaphysics”

Pictures from: http://images.google.com/imgres?imgurl=http://antiquescientifica.com/anatomical_model__head_Pichlers_Witwe_and_Son.jpg&imgrefurl=http://antiquescientifica.com/archive34.htm&h=652&w=501&sz=100&tbnid=aT8PH6LpljMJ:&tbnh=135&tbnw=103&hl=en&start=4&prev=/images%3Fq%3Danatomical%2Bmodel%26svnum%3D10%26hl%3Den%26lr%3DAnd http://images.google.com/imgres?imgurl=http://www.picture-newsletter.com/modelplanes/model-airplane-fg6e.jpg&imgrefurl=http://www.picture-newsletter.com/modelplanes/&h=779&w=1200&sz=120&tbnid=_vVnAtgUr6wJ:&tbnh=97&tbnw=150&hl=en&start=1&prev=/images%3Fq%3Dmodel%2Bairplane%26svnum%3D10%26hl%3Den%26lr%3D

Why do chemists use models?

• macroscale– physical properties that can be observed by the

unaided human senses

• microscale– samples of matter that have to be viewed with a

microscope

• nanoscale– samples that are at the atomic or molecular

scale where chemical reactions occur

Macroscale, Microscale, and Nanoscale

Models of microscale and nanoscale because we can’t see with our naked eye. (Visual learners)

• How are atoms represented?

• How are elements represented?

• (http://www.webelements.com/)

Models and representations

      exactly 12 amu

12C

6 protons6 neutrons

        

Concept Question 1

Antoine Lavoisier, the "father of chemistry", listed lime as a chemical element in his table of 33 known elements. Which of the following observations best shows that lime cannot be an element?

(a) Lime reacts with water, generating a large amount of heat.

(b) Lime and carbon dioxide are produced when limestone is roasted.

(c) When a certain soft metal is burned in oxygen, lime is produced (with no other products).

(d) Lime melts at a temperature of 2572°C.

Elements • Element: A substance that cannot be decomposed into

two or more new substances by chemical or physical means

• The diatomic elements:– H, O, N, and the halogens  (H2, O2, N2, F2, Cl2, Br2, I2)

• Allotropes are different forms of the same element in the same physical state at the same temperature and pressure.– O2 vs. O3

– diamond, graphite, C60 buckyballs

• http://www.bris.ac.uk/Depts/Chemistry/MOTM/diamond/diamond.htm• http://www.chem.yorku.ca/hall_of_fame/essays98/buckyball/bucky1/bucky.htm

IrReSPONSiBILiTiEs is the longest word that can be spelled entirely using chemical symbols without reusing any element...

Ir - iridiumRe - rheniumS - sulphurP - phosphorusO - oxygenN - nitrogenSi - siliconBi - bismuthLi - lithiumTi - titaniumEs – einsteinium

Concept Question 2

Which of the following samples could be methane, CH4?  A sample that contains:

(a) 25% hydrogen by weight; 75% carbon by weight

(b) 4.0 g of H atoms and 1.0 g of C atoms

(c) 0.40 mole of H atoms and 1.0 x 1023 C atoms

(d) 0.40 mole of H2 molecules and 0.20 moles of C atoms  

Models of Molecules

H

O

H

H2O

Chemical Formula for water

Chemistry Teacher: “Johnny, what is the chemical formula of water?”

Johnny: “HIJKLMNO.”

Chemistry Teacher: “That’s wrong!”

Johnny: “But yesterday you said it was H to O…”

Nanoscale representations of the three states of matter

• The circle on the left shows a magnified view of a very small portion of liquid water in a closed container.

• What would the magnified view show after the water evaporates?

Concept Question 3

History of the Atom

A Swiss alchemy lab from:

http://www.rosicrucians.org/salon/swiss/swiss.html

Early history of the atom• The word atom dates to 420 BC.

• Democritus and his teacher Leucippus proposed the idea that space was either empty (as in a vacuum) or occupied by atoms that were eternal, invisible, and so small that they could not be further diminished.  

• Aristotle and Plato disagreed, claiming that the four basic elements of earth, wind, fire, and water made up the material world and that all things could be derived of some combination of the four

Picture from http://www.npp.hu/tortenelem/atomosok-e.htm

Alchemy

• ~300 BC-1650• Based on the idea that everything was made by

some combination of earth, air, fire, and water• Attempts to transmute material to gold: the perfect

substance• Sought universal solvent• Looking for an Elixir to extend life 17th century

Phlogiston

• Late 17th century phlogiston was put forth by Beecher and advocated by Stahl.  They claimed when wood is burned, it releases "phlogiston" into the air.  If the wood is burned in a jar, the flame eventually goes out when the air is saturated with "phlogiston."  

• Once scientists were better able to study gases and carry out quantitative research, they discovered that oxygen was the critical component of all of the reactions involving phlogiston.    Phlogiston became the opposite of oxygen.  

Dalton’s Theory

In the early 1800s, Dalton began to formulate his theory and model. His theory had five main principles:1. Chemical elements are made of atoms.

2. The atoms of an element are identical in their masses

3. Atoms of different elements have different masses

4. Atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3 and so on.

5. Atoms can be neither created nor destroyed

Dalton on Elements

• Atoms are the smallest units of matter

• Chemical elements are made of atoms.

Dalton on Compounds

- compounds are combinations of different elements, and that in these compounds there is a constant ratio of atoms

- changing their physical state could not separate these compounds

- chemical reactions occurred due to a rearrangement of combinations of atoms

Flaws in Dalton’s Model• What makes the atoms of each element different?

• Why do atoms combine to form compounds?

• Why do they combine only in integer ratios?

• Why are specific ratios of atoms observed in compounds?

• Why do groups of elements have such similar properties and reactivities?

Electricity is Key to Atomic Structure• Charges of the same type repel one another; charges of the opposite

type attract one another• 1891 -G. Johnstone Stoney term "electron" coined for the unit of

electrical charge found when current was passed through chemicals • 1896 -Henry Bacquerel discovers that uranium ore emits rays that

exposed a photographic plate through protective black paper• 1898 -Marie and Pierre Curie isolate polonium and radium which emit

the same rays.  Radioactive elements emit three types of radiation (alpha (+), beta (-), and gamma (neutral) rays) which can be separated by passing them through electrically charged plates.  Alpha and beta rays have mass.  

• Conclusion:  Radioactive elements have atoms which are made of something smaller (alpha and beta particles.)

Radioactivity

JJ Thomson & the Cathode Ray Tube

Picture from : www.chem.uiuc.edu/ clcwebsite/cathode.html

Thompson’s Experiments

1) Thompson showed that the charge and the rays were inseparable. 

2)  The rays (carrying negative charge) bent towards the positive electrode.

3)  By adjusting the strength of the electric and magnetic fields, he was able to make their effects cancel out and the cathode rays follow their original path. By measuring how much the rays were bent by a magnetic field and how much energy they carried, Thompson calculated the ratio of the mass of the particles to their electric charge 5.6x10-9 g/coulomb

4) Using different metals as cathodes, all cathode rays consisted of particles with the same mass to charge ratio.

Thompson Adds to Atomic Theory

1) Cathode rays are charged particles called corpuscles (today's called electrons)

2) Corpuscles are constituents of the atom

3) Corpuscles are the only constituents of the atom

Millikan Finds the Charge of e-

In 1909 Robert Millikan determined the charge of the electron in his "oil drop experiment“

Charge was always a multiple of -1.6 x 10 -19 C.  He proclaimed that this value was the correct value for the charge of an electron.

Millikan’s Experiment

How did Millikan determine the charge on one electron if he didn't know how many electrons an oil drop acquired?

Your challenge: to determine the number of pennies in this beaker without counting the pennies.

The rules:1. Pennies may be removed from the beaker, but they cannot be counted at any time.2. All pennies have to be returned to the beaker.

What is your method?

• Need 15-20 volunteers to grab a handful of pennies and weigh them:

• Sort the data from smallest to largest and find the difference between neighboring measurements.

Plum Pudding Model of an Atom

Thompson described an atom as consisting of small, negatively charged corpuscles situated inside a positively charged field by electric static forces.

http://nobelprize.org/physics/educational/quantised_world/structure-1.html

Rutherford’s Hypothesis

Alpha () particles are positively charged particles emitted by certain radioactive atoms. If particles are shot at a thin gold foil……

Rutherford’s Data

In Rutherford's own words, "It was almost as if you fired a 15-inch shell into a piece of tissue paper and it came back and hit you."

Flash demo

Rutherford’s Conclusions

• Most of his particles passed through the foil without encountering the atomic nucleus, but a few came near enough to the nucleus to be deflected by the repulsion by a like charged nucleus.

• Nucleus was only 1/10,000th the size of the entire atom, but contained nearly all the mass.  

Bohr Model

• This model was a planetary model, in which the electrons orbit around protons and neutrons that occupy a central space. In Bohr’s model, there are different electron levels, each holding a different number of electrons. The first level holds 2 electrons, the second holds 8, and the third holds 18. The farther the electron level is from the “nucleus” of the atom, the more energy it has. The electrons usually fill the electron levels from closest to furthest out.

Bohr Model Flaws

Ineffective explanation for bonding between atoms.

Little ability to predict molecular shapes.

Could not predict how many bonds a particular atom is likely to form

Atomic Orbital: Today’s Working Model of an Atom

Atomic orbital theory places the electrons in specific regions of space called orbitals. These orbitals can be mathematically derived through quantum mechanics.

Subatomic Particles

Mass (g) Charge

electron 9.10938188 x10-28 -1

proton 1.6762158 x10-24 +1

neutron 1.67492716 x10-24 0

Summary

• Models are used to give chemists a macroscopic view of a nono- and microscopic world

• Atomic models have evolved as scientists discovered more facts

As You Go…• No sandals (cooler weather is on the way to help…)

• Scientific report due today in discussion• HW 2 Due next Tuesday• Chemical Scholarship assignment due next Wednesday

in discussion

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