Chapter 4: Atomic Structure

History of the Atom

-Atomic History song

Size of Atom Video Atoms and elements

400 B.C.- Democritus Believed matter was made of empty

space and tiny particles he called “atoms”

He said that different things were made of diff. types of atoms

Leuccippus & Epicurus agreed w/ him

335 B.C. - Aristotle Said matter was continuous, what

you see is what you get Called it “hyle” Identified 4 elements: earth, wind,

fire, water

Late 1500’s - Galileo 1st person to make extensive use of

the experimental method to study natural phenomena

1st to disagree w/ Aristotle Agreed w/ Copernicus (sun center of

universe)

Late 1600’s – Sir Issac Newton Brilliant English scientist In his 23-24th year he was stuck at home

during the Bubonic plague During this time he:

Invented calculus Did 100’s studies on light Developed laws of motion Developed laws of gravity

Atom idea resurfaced- said there were small particles (offered no proof)

1789- Antoine Lavoisier French scientist who developed the

Law of Conservation of Mass

1799- Joseph Proust French scientist who developed the

Law of Definite Proportions

1803- John Dalton English scientist Published series of 3 papers, based

on the findings of Proust and Lavoisier

1. Dalton’s Atomic TheoryA. All matter is composed of small

particles called atoms that can’t be broken down

B. Atoms of the same element have identical properties (same mass,etc)

C. Atoms of different elements have different properties

D. Atoms unite in simple ratios to form compounds

2. Developed Law of Multiple Proportions

3. Dalton’s Model of the Atom Used solid wooden balls to show

matter was made of indivisible particles

1876- Eugen Goldstein German scientist who introduced the

name “cathode ray” and its positive counterpart “canal rays”

1895 – Wilhelm Roentgen German scientist who discovered X-

rays

1896 – Antoine Henri Becquerel French scientist who discovered

radioactivity Radioactivity: the phenomenon in

which the unstable nuclei of certain elements spontaneously emit particles or energies

Forms of Radiation

1. Alpha particle ( ) [Rutherford]-helium nucleus (2 neutrons & 2 protons)- can be stopped by sheet of paper

2. Beta Particle (ß) [Rutherford]- high speed electron formed at the moment of decay-can be stopped by sheet of aluminum or layers of clothing

3. Gamma Rays ( ) [Villard]-high energy x-ray-stopped by few feet of concrete or few

inches of lead

1897- J.J. Thomson Did research on cathode rays, built a CRT

(cathode ray tube), now found in TV’s and computer screens (originally built by William Crookes)

CRT is a glass tube filled w/ a gas and it has 2 electrodes (anode=positive electrode, cathode=negative electrode)

Passed a voltage between the electrodes, saw a light beam called a cathode ray

Subjected the beam to electric and magnetic field, found the beam to be negatively charged

Called those negative particles -electrons

Thomson’s Model of the Atom Called the “plum pudding model” Disproves Dalton’s theory (atom

can’t be broken down)

1911- Robert Millikan American physicist Did the “oil drop experiment”

In this experiment he had oil drops that were charged pass thru charged plates, he offset the voltage on the plates to get the drop to suspend briefly, wrote down the voltage

He found the exact charge on an electron(e-) to be (-1)

Also using Thomson’s work he found the mass of the e- to be 1/1897th of a Hydrogen atom

1911-1912 –Lord Ernest Rutherford New Zealand physicist Worked w/ Bohr, Geiger and

Marsden on the “Gold Foil Experiment”

Credited w/ the discovery of the atomic nucleus

Gold Foil ExperimentSet-Up

Gold Foil Movie

Gold Foil ExperimentSet-Up

Lead box w/ pin hole In box- piece of radioactive material

which gave off alpha particles (positive particles)

Used a very thin sheet of gold foil as a target (few atoms thick)

Surrounded the foil w/ a screen coated in zinc sulfide that sparked when alpha particles hit it

Observations

Observations

A. 99% of particles went straight thru the foil like it wasn’t there

B. .9% went thru foil but passed thru at a slight angle

C. .1 % hit the gold foil like a brick wall & bounced right back

Conclusions

A. Atoms made of mostly empty space

B. Some sort of (+) charge is concentrated in the nucleus to repel the alpha particles (must have a charge of (+1) to offset the electron-because atoms are neutral)

C. All the mass of the atom is concentrated in one place

Rutherford’s Model of the Atom Atoms consist of a small, dense,

positively charged center (the nucleus)-surrounded by empty space

Electrons travel in that empty space orbiting around the positive nucleus

1914- Discovery of Proton

Thomson predicted the existence of a positive particle w/ CRT experiments

Rutherford coined term “proton” in 1919 (he is credited w/ the discovery w/ Gold Foil experiment)

Found positive particle, that is larger and the exact opposite charge of the electron, 1800 times heavier than the electron

1914- Atomic Number Henry Mosley Studied x-rays produced by different

metals (used the metals as anodes) Saw that each metal produced a

different wavelength- because each had a different number of protons

Atomic Number = # of protons

# protons = # electrons

# of protons determines the identity of the element

1922- Bohr’s Model of the Atom Niels Bohr worked on gold foil Had electrons in specific orbits or

shells

1932 – Discovery of the Neutron James Chadwick found high energy

particles w/ no charge and a mass equal to a proton

Call neutrons

Nucleons Particles in nucleus Also known as hadrons Consist of protons and neutrons Each proton is made of 3 quarks (2

ups and 1 down), each neutron is made of 3 quarks (2 downs and 1 up)

Proton Electron Neutron

Mass (compared to H)

1 1/1900th 1

Charge +1 -1 0

Mass Number

=# of protons + # of neutrons

# of neutrons = mass # - atomic #Ex. Carbon

Mass #= 12Atomic # = 6# protons =# e- = 6# of neutrons = 6

Atomic notation

Mass #222Rn

86

Atomic#

(#p=#e-)

#protons = 86

# electrons =86

#neutrons =136

Isotopes JJ Thomson discovered Atoms of the same element that

have a different mass due to the fact that have different # of neutrons

Disproves Dalton’s theory that atoms of the same element are the same

Average Atomic Mass (# on the periodic table)

Scientists found the masses of all the isotopes and their percent abundance with a mass spectrometer

A mass spectrometer compares each element to Carbon-12 atom

Measured in atomic mass unit (amu) The average atomic mass was calculated

using those numbers (get a decimal #, not a whole #, because it’s an average)

Ex. Find average atomic mass (from isotopes)

Cu-63 62.930 amu 69.17% in the sample

Cu-65 64.928 amu30.83% in the sample

1. Find mass contribution of each isotope (mass) x ( %)Cu-63 62.930 x .6917 = 43.53Cu–65 64.928 x .3083 = 20.02

2. Find average mass by adding the isotopes mass contributions together

Average atomic mass =43.53 + 20.02 = 63.55 amu

History of the atom- part 1 History of the atom- part 2