HISTORY OF THE ATOM
460 BC Democritus develops the idea of atoms
he pounded up materials in his pestle and
mortar until he had reduced them to smaller
and smaller particles which he called
ATOMOS
(greek for indivisible)
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Atomic structure -atom: definition and structure of the atom
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460 BC Democritus develops the idea of atoms he pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he called
ATOMOS
( greek for indivisible )
- Chemistry has been practiced for a very long time, even if its practitioners were much more interested in its applications than in its underlying principles.
- Before the end of the eighteenth century, the principal gases of the atmosphere nitrogen and oxygen had been isolated
- Natural laws had been proposed describing the physical behavior of gases.
- Yet chemistry cannot be said to have entered the modern age until the process of combustion was explained.
- we explore the direct link between the explanation of combustion and Dalton s atomic theory.
1808 John Dalton SOLID SHPERE MODEL
- suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity and called them
- He visualized the atom as a solid individual particle incapable of subdivision
ATOMS
Dalton s Atomic Theory From 1803 to 1808 , John Dalton, an English schoolteacher, used the two funda- mental laws of chemical combination just described as the basis of an atomic theory. His theory involved three assumptions:
- Eachchemicalelementiscomposedofminute,indivisibleparticlescalled atoms. Atoms can be neither created nor destroyed during a chemical.
- All atoms of an element are alike in mass(weight) and other properties, but the atoms of one element are different from those of all other elements.
- In each of their compounds, different elements combine in a simple numerical ratio, for example, one atom of A to one of B (AB), or one atom of A to two of B(AB 2 ).
Sub-Atomic Particles
- At the end of the nineteenth century several experimental evidences show that the atom is made of still smaller particles.
- These subatomic particles are called the fundamental particles.
- The three most important subatomic particles are the proton, neutron and electron
- The main landmarks in the evolution of atomic structure are:
- 1896 J. J. Thomson’s discovery of the electron and the proton
- 1909 Rutherford’s Nuclear Atom
- 1913 Bohr Atom
- 1932 Chadwick’s discovery of the neutron
JJ ’ s Experiments (1898) 8
- Experiment 1 – Using a cathode ray tube, he wanted to test if the rays were charged particles by seeing if they could be deflected by and electrical field. - Success!
- Because he concealed the rays in a vacuum, the experiment worked.
ELECTRON
- found that atoms could sometimes eject a far smaller negative particle which he called an
J. J. Thomson – summary of findings
10
- Postulated the existence of electrons using cathode-ray tubes_._
- Determined the charge-to-mass ratio of an electron.
- The atom must also contain positive particles that balance exactly the negative charge carried by particles that we now call electrons.
- Thomson’s experiments with cathode rays later lead to his discovery of Isotopes and Electrons.
Thompson developed the idea that an atom was made up of electrons scattered unevenly within an elastic sphere surrounded by a soup of positive charge to balance the electron's charge 1904 like plums surrounded by pudding. PLUM PUDDING / RAISIN BUN MODEL
He fired Helium nuclei at a piece of gold foil which was only a few atoms thick. They found that while most of the helium nuclei passed through the foil, a small number were deflected and, to their surprise, some helium nuclei bounced straight back.
Plum Pudding model of an atom.
Actual Results.
A nuclear atom viewed in cross section.
Rutherford’s new evidence allowed him to propose that:
- Atom has a dense central core known as the nucleus which contains practically the entire mass of the atom, leaving the rest of the atom almost empty
- The entire positive charge of the atom is located on the nucleus while electrons were distributed in vacant space around it
- The electrons were moving in orbits around the nucleus like planets around the sun However, this was not the end of the story.
Millikan s oil-drop experiment
- The fall of a droplet in the electric field between the condenser plates is speeded up or slowed down, depending on the magnitude and sign of the charge on the droplet.
- By analyzing data from a large number of droplets, Millikan concluded that the magnitude of the charge, q, on a droplet is an integral multiple of the electric charge, e.
- That is, q = ne(where n = 1 , 2 , 3 ,………….).
- All electrons have the same mass About 1 / 2000 atomic mass unit All electrons have the same charge - 1. 602 x 10 - 19 Coulomb The atom was still thought to resemble Plum Pudding
- Ions (charged atoms or molecules) are produced by energetic radiation, such as X-rays (X).
- Some of these ions become attached to oil droplets, giving them a net charge.