Early Atomic Theories

• History• Theories• Scientists• Models

What’s inside the box?

• How can you determine the identity of the items in the boxes?

Early History

• Democritus (Greek)– world is made up of::

• empty space• tiny particles (atomos)

• Aristotle (Greek)– world is composed of continuous matter (hyle)

• accepted until 17th Century

John Dalton

• agreed with Newton and Boyle– atoms were the basis

(no proof)

• English• studied Lavoisier and

Proust (both were French)

Lavoisier

• Chemical change in a closed system has equal mass before and after the change, matter is neither created nor destroyed– Law of Conservation of Mass

Proust

• Law of Definite Proportions

– Specific substances always contain elements in the same ratio by mass

• example: H2O has a ratio of 1:8 (H:O)

Dalton’s Law

• Law of Multiple Proportions– certain elements can combine to form two or

more different chemical compounds

• Hydrogen and Oxygen can to form water (1:8) and peroxide (1:16)

Dalton’s Atomic Theory

All matter is composed of extremely small particles called atoms.

Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. (*)

Atoms cannot be subdivided, created, or destroyed. (*)

Atomic Theory con’t

• Atoms of different elements can combine in simple, whole-number ratios to form chemical compounds.

• In chemical reactions, atoms are combined, separated, or rearranged.

What does the * mean?

• These tennets are no longer true today!– #2 because of isotopes– #3 because of subatomic particles

~1832: FARADAY:PROPOSED EXISTANCE OF ELECTRONPROPOSED ELECTRICITY WAS CARRIED BY CHARGED ATOMS ----IONS

~1879: CROOKES: INVENTED GAS DISCHARGE TUBE (CRT)

RAY FROM - “POLE” (CATHODE) TO + “POLE (ANODE)

1895: ROENTGEN: CRT HIT TARGET, GET LOWER ENERGY EMISSIONS ---- X-RAYS

1896: BEQUEREL: DISCOVERED RADIOACTIVITY!

1897: JJ THOMPSON USED CRT AND EXPLORED NATURE OF THESE “RAYS”

Subatomic Particles: Electron

• J. J. Thomson (Eng)– cathode ray tube

experiment proved that the atom is divisible

– cathode (negative electrode)

– anode (positive electrode)

ZnS- +

NO CHARGE ON PLATES

-

+

1. RAY DEFLECTED BY ELECTRIC & MAGNETIC FIELDNOT LIGHT; THEREFORE, PARTICLES

2. DEFLECTION TOWARD POSITIVE PLATEPARTICLES NEGATIVELY CHARGED

3. LARGE DEFLECTIONDETERMINED CHARGE/MASS (q/m) RATIO

q/m < 1/1000 THE MASS OF HYDROGEN ATOM!!!!

THOMPSON:“FOUND” FARADAY’S ELECTRON

DETERMINED THE ATOM WAS NOT THE SMALLEST PARTICLE

1909: MILLIKEN DETERMINED THE EXACT CHARGE AND MASS OF THIS ELECTRON

ALL DATA INTEGRAL VALUES OF SAME NUMBER

q = -1.6 x 10-19 C

m = 9.1 x 10-31 kg

ABOUT 1/1800th OF THE HYDROGEN ATOM

TODAY: 1.60219 x 10-19 C9.10940 x 10-31 kg

Subatomic Particles: Electron

• Robert Milliken (USA)– Oil Drop Experiment

• first to measure the mass of an electron– 9.109 x 10-28g

• first to measure the charge of an electron– (-1)

Subatomic Particles: The Nucleons

• What is a nucleon?– A nucleon is a particle that is found within the

nucleus of an atom.

• What are the major nucleons?– Proton and the Neutron

Ernest Rutherford (New Zealand)

• Gold Foil Experiment– hit a thin piece of gold

foil with a beam of alpha radiation (positively charged)

• some of the beam went through uninerrupted

• some of the beam was deflected to the side or totally reflected

1911 -- RUTHERFORD’S “GOLD FOIL” EXPERIMENT

ZnS COATEDSCREEN

GOLDFOIL

STREAM OF (ALPHA)PARTICLES

1. MOST PASS THRU UNDEFLECTEDMOST OF ATOM VOLUME IS

EMPTY SPACE

2. SOME POSITIVE PARTICLES DEFLECTED SLIGHTLY

NEAR COLLISIONS WITH MASSIVE, POSITIVELY CHARGED PARTICLE

3. 1 OF 20000 DEFLECT ACUTELY

CROSS SECTION OF MASSIVE, POSITIVELY CHARGED PARTICLE IS 1/20000th THAT OF ATOM

RUTHERFORD FOUND THE NUCLEUS!!!

Rutherford’s Results

• Since the positively charged radiation was repelled in certain areas, there was evidence for a positive entity inside of the foil– Proton

• This led to the idea of a central core that is very dense (nucleus)

• Since some of the radiation passes through unharmed the foil must not be totally positive

ATOM MUST BE A VERY DENSE, POSTIVELY CHARGEDNULCLEUS SURROUNDED BY VERY LIGHT, NEGATIVELY

CHARGED ELECTRONS

QUANDRY:HEAVY PROTON (+ CHARGE) IN NUCLEUSLIGHT ELECTRONS ON OUTSIDE

COMBINED, ACCOUNT FOR ~ 1/2 THE ATOMIC MASS

AND THE ATOM IS NEUTRAL!

1932: CHADWICK ISOLATED THE NEUTRON

IN NUCLEUSO CHARGEMASS ~ SAME AS PROTON

Chadwick

• If the nucleus is the home of the majority of the mass, and the atom is electrically neutral there must be a neutral particle with a mass: neutron

Comparison of the Major Nucleons

• Proton– +1 Charge– Mass:1.673 x 10-24g– number of protons

must equal the number of electrons for the atom to be neutral

– p+

• Neutron– No charge– Mass:1.675 x 10-24g– no

What holds it all together?

• nuclear force holds the particles together in the nucleus

What are isotopes?

• Isotopes are atoms of the same element that have different masses (different numbers of neutrons).

What do the numbers mean?

• Atomic Number– Z– number of protons

• Mass Number– A– number of protons

plus the number of neutrons

Where do you find the numbers?

• Using the periodic table locate the symbol for the element that you are looking for. Inside the element’s square will be the numbers.

Writing Nuclides

X• Copper

• Oxygen

• Silver

The Isotopes of Hydrogen

Name of Isotope Atomic # Mass #Protium 1 1

Deuterium 1 2Tritium 1 3

The Isotopes of Hydrogen

How many electrons, protons, and neutrons are found in a copper atom of

mass # 65?

• Z = protons therefore protons = 29

• Protons = Electrons therefore electrons = 29

• A - Z = neutrons so 65 - 29 = 36, there are 36 neutrons

• Now try Oxygen and Calcium:

Ions

• Ions have charge due to an imbalance in the number of protons and electrons. Atoms can either gain or lose electrons. If they gain electrons the ion is negative, where is they lose electrons the charge is positive.

Try these:

• O 2-

• Ca 2+

Isotopes of Beanium Lab

• Purpose: to determine the average atomic mass of a new element called Beanium

• Beanium has 3 isotopes: white, brown, and speckled

• If you are given a sample of Beanium, what do you need to know in order to calculate the average atomic mass.

• REMEMBER, that means the average mass of all three isotopes!

• Number of each type, mass of each type, and then total to get the average mass per atom (bean)

Data Table for Beanium Lab

Type of Isotope

Mass of Isotope

(g)

Number of

Isotope

Average Mass of Isotope

% of Each

Present

White

Brown

Speckled

Total 100