HISTORY OF THE ATOM

DEMOCRITUS – 400B.C.

A.He believed matter was made up of empty space

and tiny particles he called “atomos”

B.Said different things were made with different

types of atoms.

ARISTOTLE – 335 B.C.

A.Matter was continuous – “what you see is what you

get”

B.Identified four elements – earth, wind, fire, and

water

GALILEO – LATE 1500’S

A.First person to make extensive use of the

experimental method to study natural phenomena

B.First to agree with Aristotle

C.Agreed with Copernicus that sun was center of the

universe.

SIR ISAAC NEWTON – LATE 1600 ’S

A.English Scientist

B.When he was 23-24 years old he was stuck at home during

the Bubonic Plague. At this time he invented the following:A. CalculusB. Laws of MotionC. Laws of GravityD. Did lots of studies of lights

C.The atom idea resurfaced- he said that there were small

particles but he offered no proof.

ANTOINE LAVOISIER -1789

A.Law of Conservation of Mass

B.Matter cannot be created or destroyed!!!!!

JOSEPH PROUST - 1799

A.Law of Definite Proportions

B.Compounds made of the same elements in the

same ratios!!

JOHN DALTON - 1803

A. Published a series of papers on the findings of Proust and Lavoisier.

B. Dalton’s atomic theoryA. 1. Elements are made of tiny particles called atoms.B. 2. All atoms of a given element are identical.C. 3. The atoms of a given element are different from those of any

other element; the atoms of different elements can be distinguished from one another by their respective relative weights.

D. 4. Atoms of one element can combine the atoms with atoms of other elements to form chemical compounds; a given compound always has the same relative number of types of atoms.

E. 5. Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together.

JOHN DALTON - 1803

Law of Multiple Proportions

Dalton’s model of the atoms – he used solid

wooden balls to show matter was made of indivisible

particles.

EUGENE GOLDSTEIN - 1876

A.Introduce the term cathode ray and its positive

counterpart the canal ray.

WILHELM ROENTGEN - 1895

A.Discovered x-rays

ANTOINE HENRY BECQUERAL - 1896

A.Discovered Radioactivity

B.The phenomena of the unstable nuclei of certain

elements spontaneously emitting particles of energy.

ANTOINE HENRY BECQUERAL - 1896

C. Forms of radiation

1. Alpha particle (α)[Rutherford]1. Helium nucleus (2 neutrons and 2 protons)2. Do not penetrate solids3. Very large, has a + charge4. Because of high KE, they can damage surface

tissue

ANTOINE HENRY BECQUERAL - 1896

2. Beta Particle (β) [Rutherford]A. High speed electron is formed at the moment of

decayB. Hard to stopC. Penetrates light materials; clothing, paper, etc.D. Cannot penetrate most metals

ANTOINE HENRY BECQUERAL - 1896

3. Gamma Ray ( γ) [Villard]A. High energy x-raysB. Have no (-) charge and no massC. Pure energyD. Can penetrate through most materials (metals,

walls, etc.)E. Cannot penetrate through dense materials like

lead.

JJ THOMSON - 1897

A. Did research on cathode rays and built a (CRT) or cathode ray

tube [we us these tubes in TV’s and computer screens]A. A CRT is a glass tube filled with a gas with 2 electrodes

attached (anode = positive electrode and cathode = negative electrode)

B. Passed a voltage between the electrodes, saw a light beam

which he called a cathode ray.

C. Subjected the beam to electrical and magnetic fields, found

the beam to be negatively charged (he called the particles

electrons)

JJ THOMSON - 1897

JJ THOMSON - 1897

D. HIS MODEL OF THE ATOM1. “Plum Pudding Model”2. Negative particles with a positive core.

ROBERT MILLIKAN -1911

A. American physicist

B. Did the “Oil Drop Experiment”A. In this he had oil drops that were charged pass

through charged plates, he offset the voltage on the plates to get the oil drop to suspend briefly and wrote down the voltage.

B. He then found the exact charge on an electron to be -1

C. He also found the mass of the electron to be 1/1897th of a hydrogen atom.

LORD ERNEST RUTHERFORD

A. A New Zealand Physicist

B. In 1911-1912 worked with Bohr, Geiger, and

Marsden on the gold foil experiment.

LORD ERNEST RUTHERFORD

C. Gold Foil Experiment Set-upi. Lead box with pin holeii. Put a piece of radioactive material inside which

gave off alpha particles (positive particles)iii. Used a very thin sheet of gold foil as a targetiv. Surrounded the foil with a screen coated in zinc

sulfide that sparked when alpha particles hit it

LORD ERNEST RUTHERFORD

LORD ERNEST RUTHERFORD

1. Observations he made:1. 99% of the particles went straight thru the gold

foil like it wasn’t even there.2. 0.9% went thru but passed thru at a slight angle3. 0.1% hit the gold foil like a brick wall and

bounced right back.

LORD ERNEST RUTHERFORD

2. Conclusionsa. Atoms made up mostly of empty spaceb. Something must repel the positive alpha

particles – must be a positively charged nucleusc. All the mass of the atom is concentrated in one

place- the nucleus which is surrounded by empty space

d. Stated that the + charged nucleus must be (+1) to offset the (-1) of the electron. (Atoms are neutral)

LORD ERNEST RUTHERFORD

d. Rutherford’s model of the atom

1. Electrons travel in space around nucleus

like the planets around the sun.

NEILS BOHR MODEL -1913

a. Worked on gold foil

1914 DISCOVERY OF THE PROTON

a. Thomson is working with CRT again and finds a

positive particle that is larger and the exact

opposite charge of the electron.

b. 1800 times heavier than the electron

HENRY MOSELEY -1913

a. Studied x-rays produced by different metals (used the

metals as anodes)

b. Saw that each metal produced a different wavelength

because each had a different number of protons

c. From this development the atomic numbera. Tells the number of protonsb. # of protons = # of electronsc. # of protons determine the identity of the element

JAMES CHADWICK - 1932

a. Found high energy particles with no charge and a

mass equal to a proton.

b. Called them neutrons

c. Nucleons – particles found in the nucleus of an

atom, also known as hadrons; these particles are

protons and neutrons which are each made up of

three quarks.

MASS NUMBER

Mass # = # of protons + # of neutrons

# of neutrons = mass # - atomic #

Ex. Carbon

Mass #= 12

Atomic # = 6

# protons =# e- = 6

# of neutrons = 6

#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

AV E R A G E AT O M I C M A SS ( # O N T H E P E R I O D I C TA B L E )

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 atomMeasured 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. F IND AVERAGE ATOMIC MASS (FROM ISOTOPES)

Cu-63 62.930 amu =69.17% in the

sample

Cu-65 64.928 amu=30.83% in the

sample

1. Find mass contribution of each isotope

(mass) x ( %)

Cu-63 62.930 x .6917 = 43.53

Cu–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