Fall 2004 - Topic 7

ATOMS:

Dalton and Beyond

Dr. Donna Brestensky, Chemistry

*** Please pick up a handout as you come in. ***

Animation Reference: www.tvgreen.com

Start of Modern Era of Atoms:Dalton’s Atomic Theory

John Dalton

(1766-1844)

British chemist,

lecturer, and

meteorologist

Dalton’s Atomic Theory (1803) - 1

1) All matter is made up of indivisible and indestructible basic particles called atoms.

2) All atoms of a given element are identical, both in mass and in properties. Atoms of different elements have different masses and properties.

3) Compounds are formed when atoms of different elements combine in the ratio of small whole numbers.

Dalton’s Atomic Theory (1803) - 2

4) Elements and compounds are composed of definite arrangements of atoms.

Chemical change occurs when the atomic arrays are rearranged.

Significance of Dalton’s Atomic Theory

• Continued to break down earlier views of “elements”

• Bridged gap between lab data and hypothetical atom.

- way of calculating relative atomic weights.

• Explained Law of Definite Proportions [Proust 1799]

- All samples of a compound contain same weight proportions of constituent elements.

• Explained Law of Conservation of Mass

- “Initial Mass = Final Mass”

- Only reorganizing of unchangeable atoms occurs in chemical reaction.

Dalton: inconsistencies uncovered…

1) The basic state of an element = one atom?

Perhaps… basic natural state of an element may be a molecule made of 2 or more atoms.

2) Dalton: “Thou knows…no man can split the atom.” No: radioactivity, atomic particles.

3) Atoms of given element have same mass and properties? Not exactly: isotopes exist…

Thinking about Atoms…

Current Definitions: Matter Classification • Element:

- pure substance

- made of unique, (nearly) identical atoms

- cannot be broken down into simpler substances by a

chemical reaction.

• Compound:

- pure substance

- made of atoms of at least 2 different elements

- can be broken down into simpler substances by a

chemical reaction.

Identification of Elements

• Physical properties

• Chemical properties

• Relative atomic weights (better values)

• Flame test for solids/solutions

• Interaction with light:line-absorption spectrumline-emission spectrum

Flame Test for Element Identification

(From left) Sodium, potassium, lithium; strontium, barium, potassium.

Spectroscopes: Seeing Atomic Light

Original 1859Bunsen-Kirchhoff spectroscope

Typical setup for viewing aline-emissionspectrum

Elements: Ages of Discovery

Classification of the Elements:Development of the Periodic Table

• Dobereiner 1817: “Triads”, group properties

• Newlands 1863: row “Octaves”, group properties

• Mendeleev 1869: first-published “Period” definition (see next slides)

• Meyer 1870: 2nd-published “Period” definition; volume/properties

DmitriMendeleev(1834-1907)

“Creator of the Periodic Table”

(probably formulated periodic idea at same

time as Meyer)

Mendeleev’s early notes

for the Periodic Table

(1869)

Mendeleev’s table, as orig.

published

• Formatted sideways compared to modern table

• ? instead of a name: element was predicted to exist but not known yet

Characteristics of Mendeleev’s Table• Organized 60+ known elements…

- by similar properties in each vertical family (group)

- by valence = “combining number”

(split out elements with multiple valence)

- by roughly increasing atomic weight within each horizontal row (moved 17 elements

based on properties rather than weight)

• Used to predict existence of new elements (of 10, found 7; other 3 do not exist)

Comparison of eka-silicon’s predicted properties

and known Group 4 properties

Eka: “one beyond”

1880s Revision of Mendeleev’s Table

Contains “rare gases” and 3 elements unknown at time of first version, though their properties were predicted:

germanium (Ge), formerly eka-silicon;gallium (Ga), formerly eka-aluminum; scandium (Sc), formerly eka-boron.

Modern Periodic Table Organization

• Elements are NOW placed in order of increasing atomic number (# of + protons).

- Why? Gives absolute order... atomic weights not characteristic

(different-mass atoms called isotopes exist!)

• A relationship between nuclear charge and arrangement of elements in the Table was finally discovered in 1914 (Henry Moseley).

• In 1860s, Mendeleev could NOT have predicted a relationship to subatomic particles!

Discovery of Atomic Structure;Sub-atomic Particles

• Thomson: 1897 electron mass-to-charge ratio

• Millikan: 1909 electron charge

• Rutherford: 1910-11 mass & charge of nucleus

• Chadwick: 1932 neutron

• Bohr: 1913 electron energy levels

• Gell-Mann/Zweig: 1964 quark theory

Joseph John Thomson(1856-1940)

British physicist and mathematician

Nobel Prize in 1906 (existence of electrons)

1897: calc’d electron’s mass-to-charge ratio in cathode-ray experiment

Thomson’s Cathode-Ray Experiment

Known before: • atoms are normally neutral (neither positive nor negative charge)

• When cathode rays are made, remaining atoms are positively charged (ions)

Schematic ofactual 1897apparatus(vacuum inside):

Cathode-Ray Experiment:Thomson (1897)

• Undeflected => Point 1

• Rays can be attracted to + plate (hit Point 3) or deflected by magnetic field (hit Point 2).

• Rays have negative charge, which can’t be separated from rays!

Vacuum tube w/fluorescent end coating, electrodes, and high-voltage passing through.

Thomson’s Cathode-ray Results

• Calculated mass-to-charge ratio (using math and known field strengths) and energy of ray particles

• Mass-to-charge ratio for cathode rays was over 1000 times smaller than that of a charged hydrogen atom (a proton), suggesting– either cathode rays carried huge charge,– or they were amazingly light relative to their

charge => supported in future

Thomson’s conclusions/questions

• “I can see no escape from the conclusion that [cathode rays] are charges of electricity carried by particles of matter.”

but...

• “What are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision? - J. J. Thomson

•“We have, in the cathode rays, matter in a new state...a state in which all matter...is of one and the same kind; this matter being the substance from which all the chemical elements are built up."

Thomson’s “plum pudding” atom model*

* Never had plum pudding? Think of a blueberry muffin.

Cathode rays (electrons) are...

• tiny “corpuscles”

of negative charge

• surrounded by a sort of “cloud” of positive charge

Robert Millikan (1868-1953)

U.S. physicist

Nobel Prize in 1923 (charge of electron:

1909 oildrop expt.)

With Thomson’sresult, this allowed

calculation of electron mass. Millikan’s experimental apparatus.

Millikan’s Oil-Drop Experiment (1909)

• Spray oil... droplets go thru plate’s hole

• Hit air molecules with X-rays... knock off electrons.

• Electrons on oil drops… now, charged.

• Adjust voltage... a drop is held stationary.

• Use drop’s mass, voltage to calculate drop’s charge (always whole multiple of 1.60 x 10-19 C).

Diagram of apparatus -electrical field betweenplates is adjustable.

Ernest Rutherford(1871-1937)

nuclear physicist,Thomson’s student,

New Zealander teaching in Great Britain

Nobel Prize in 1908(radioactive decay)

1910-11: Gold foil experiments

Rutherford’s Experiments (1910-11) (done by undergrad Ernest Marsden/physicist Hans Geiger)

• Fired beam of alpha particles at very thin gold foil.• Alpha particles = positive-charged helium ions,

mass 4 amu [He+2]

Rutherford’s Experiment: prediction

By Thomson’s model,

mass and + charge of gold atom are too dispersed to deflect the positively-charged alpha particles,

so...

particles should shoot straight through the gold atoms.

Rutherford’s Experiment: prediction

pass through like this …

Rutherford’s experiment: what actually happened

Rutherford’s results, response in amazement

Most alpha particles went straight through, andsome were deflected,

BUT

a few (1 in 20,000) reflected straight back to the source!

“It was quite the most incredible event that has ever happened to me. It was almost as incredible as if you had fired a fifteen inch shell at a piece of tissue paper and it came back and hit you.”

Rutherford’s Model of the Atom

Expt. Interpretation:

• gold atom has small, dense, positively-charged nucleus surrounded by “mostly empty” space in which the electrons must exist.

• like tiny solar system

+

Also, calculated nuclear mass as mass of positively-charged protons. Protons only half of actual mass: suggests neutral particles of same mass as proton?

How the Nucleus Repels Alpha Particles

+

How much of an atom is empty space?

+

How much of an atom is empty space?

+

Most of it!

How much of an atom is empty space?

In fact, if the nucleus of an atom were the size of a large room, the outermost electrons (far edge of the electron cloud) would be in:

• The room next door• The far side of campus• Downtown Olean• New York City

+

(click for the right answer)

Most of it!

James Chadwick(1891-1974)

Rutherford student English nuclear physicist

Nobel prize in 1935(existence of neutron)

Chadwick’s subatomic particle: neutron• Made rays of different atomic particle

• Not deflected by electric fields, so no charge (neutral) => neutron

• Collide neutron with different-weight gases...measure their deflections

=> calculate neutron mass: similar to + proton’s

• Neutrons penetrate and split various heavy atoms, b/c not repelled by nucleus (unlike alpha)=> atomic bomb

Actual 1932 apparatus:Alpha particles from polonium source (right)hit beryllium target (left),making new rays

Known Properties of Subatomic Particles

Property

Particle

Mass (amu),

Mass (g)

Relative

Charge

Electron 0.00055

9.1093897 x 10-28

- 1

Proton 1.00728

1.6726231 x 10-24

+ 1

Neutron 1.00866

1.6749286 x 10-24

0

Niels Bohr(1885-1962)

Danish physicist

Revised Rutherford’smodel of atom (1913)

Bohr Looks at Emission Spectrum: Hydrogen’s Fingerprint

The line-emission spectrum of hydrogen gas (the bands visible to humans)

Observation:

when hit with electricityhydrogen gives off light of specific wavelengths, NOT continuous range!

Bohr’s Model of Atom (1913)

Hypotheses:

• Circling electron maintains orbit ONLY at specific distances from nucleus (containing protons and neutrons).

• Only way electron could exist for long time w/o giving off radiation.

• Electron is more stable as distance r from nucleus decreases.

The first three allowed energy levels,at distances r1, r2, and r3 from nucleus.

r1

r2

r3

H's electron

H's nucleus containing 1 proton

Ongoing Study of Subatomic Structure

• Other ways to study atoms and atomic pieces:

in cloud chamber (Wilson 1911) or bubble chamber

One of first photographs ofalpha particle trails, in water mist

Ongoing Study of Subatomic Structure• typical coiled motion of electron in cloud chamber,

under influence of varying magnetic field

Electron generated on left.

Note tighter spiral afterelectron gives off light

Ref: The Particle Odyssey, p. 37

Ongoing Study of Subatomic Structure

• So...

there’s evidence that protons (+), neutrons (neutral), and electrons (-) exist in the atom.

End of the story?

NO! Still more to see and learn!

More new particles: antimatter!• Rare simultaneous generation of an electron and a positron when certain high-energy light passes through chamber: energy converts to mass => Einstein’s equation E=mc2

(Note: positrons – i.e, antielectrons- are not found in atoms.)

Fermi National Accelerator Lab: *6-km Tevatron ring and 3-km Main Injector

• Chicago site for study of sub-subatomic particles

• proton and antiproton beams used

*contrast to world’s-largest machine: CERN 27-km LEP collider (1989-2000)

Proton and neutron are not fundamental!• 1960s - Gell-Mann and Zweig - proposed protons and

neutrons are made of smaller particles they named quarks (refers to term in James Joyce’s Finnegans Wake)

• Need to use 2 different quarks (UP and DOWN) held together by gluon particles

• UP quark has +2/3 charge, DOWN quark has –1/3

Quark Evidence from Particle Destruction?

• (CERN) after collision of electron and positron... evidence of quarks?

• (DESY-PETRA) quark and anti-quark evidence?

Computer modelling of other new particles?• Beginning about 2006,

CERN’s new LHC (Large Hadron Collider) particle accelerator will search for clues to the Big Bang and the origin of mass.

• Does proposed Higgs particle really exist?

• Simulated tracks from proton-proton collision: decay of Higgs particle

Ref: The Particle Odyssey, p. 15