Democritus – Greek philosopher 4th Century BC
First to come up with “atom”
Matter is composed of tiny particles called atoms
These atoms are invisible, indestructible fundamental units of matter
Antoine Lavoisier (France 1782)
Law of Conservation of Mass
In a chemical reaction mass is conserved.
Joseph Proust (France 1799)
Law of Definite Proportions:
The elements that comprise a compound are always in a certain proportion by mass.
John Dalton (England 1766-1844)
School teacherStudied the ratios in
which elements combine in chemical reactions
Formulated first modern Atomic Theory
Dalton’s Atomic Theory
1. All matter is made of atoms.
2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.
Dalton’s Atomic Theory
3. Atoms of different elements can chemically combine in simple whole number ratios to form compounds.
Example: CO2
Dalton’s Atomic Theory4. Chemical reactions occur when
atoms are rearranged.
Chemical reactions do not change atoms of one element to another.
Atomic Theory
Much of Dalton’s theory still holds today
However, we now know that
atoms are not indivisible
The Size of the Atom
If you placed 100,000,000 Cu atoms side by side they would form a line only 1 cm long.
Radius of most atoms is about 5x10-11 to 2x10-10m.
History
Electron means “amber” in GreekProperties discovered by the Greek
Thales of Miletos 600 BC. Rubbed the mineral amber with cat fur and attracted feathers.
Benjamin Franklin (America 1740’s)
Law of conservation of charge.
Saw electricity as a flowing fluid and called the flow direction positive.
J(oseph) J(ohn) Thomson (England 1897)
He discovered the electron while experimenting with cathode rays.
Thomson’s Discovery
He determined that the cathode ray was made of negatively charged particles – electrons.
Cathode Rays
Thomson also was able to estimate that the mass of the electron was equal to about 1/1840 of the mass of a hydrogen atom.
His discovery of the electron won the Nobel Prize in 1906.
Cathode Rays
Thompson showed that the production of the cathode ray was not dependent on the type of gas in the tube, or the type of metal used for the electrodes.
He concluded that these particles were part of every atom.
Ions
Positively charge atom (cation) Atom lost electrons.
Negatively charged atom (anion) Object gained electrons.
Electron is the basic quantity of charge.
Electric charges always exist in whole number multiples of a single basic unit, the electron.
A particle with a positive charge must be present in the atom to balance each negatively charge electron.
The Discovery of the Proton
Discovered by Eugen Goldstein (German) in 1886.
He observed “Canal rays” and found that they are composed of positive particles – protons.
Canal Rays
Canal Rays passed through holes, or channels, in the reverse direction as the cathode ray.
Ernest Rutherford (Born in New Zealand 1871-1937)
University of Manchester, England
Tested Thomson’s theory of atomic structure with the “gold foil” experiment in 1910.
Gold Foil Experiment
Bombarded thin gold foil with a beam of ‘alpha’ particles.
If the positive charge was evenly spread out, the beam should have easily passed through.
Rutherford's Gold Foil Experiment
Rutherford and coworkers aimed a beam of alpha particles at a sheet of gold foil surrounded by a florescent screen.
Rutherford's Experiment
Most particles passed through with no deflection, while some were highly deflectedRutherford concluded that most particles passed through because the atom is mostly empty space.
Rutherford’s Conclusions
All of the positive charge, and most of the mass of an atom are concentrated in a small core, called the nucleus.
The Discovery of the Neutron
Chadwick bombarded alpha particles(helium nuclei) at Beryllium.
Neutrons were emitted and in turn hit parafin and ejected protons from the parafin.
Properties of Subatomic ParticlesParticle Symbol Relative
ChargeMass Relative to the Proton
Mass (g)
Electron
e- 1- 1/1840 9.11 x 10-28 g
Proton p+ 1+ 1 1.67 x 10-24g
Neutron n0 0 1 1.67 x 10-24g