democritus: believed all matter consisted of extremely small particles that could not be divided....
TRANSCRIPT
Democritus: Believed all matter consisted of extremely small particles that could not be divided.
Called these particles “Atomos”- Greek word meaning uncut or indivisible
Ancient Greek Models
Aristotle: Believed there was no limit to the number of times matter could be divided
Believed all substances were built from only four substances: earth, air, fire, and water.
Ancient Greek Models
Remember, this was all just speculation!!!
Through experimentation, Dalton discovered that elements in a compound always join the same way.
He proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided. His theory is as follows:
1. All elements are composed of atoms2. All atoms of the same element have the same
mass3. Compounds contain atoms of more than one
element4. In a compound atoms of different elements
combine the same way.
Dalton’s Atomic Theory
Dalton’s theory explained data from many experiments for a while, but over time scientists found all of Dalton’s ideas about atoms were not correct. They did not discard his theory, but they revised it.
A Theory Revised
John Dalton (born 1766)
Investigations in the 1800’s proved that atoms were in fact divisible.
ATOM- smallest particle of an element that retains the chemical properties of that element.
HISTORY OF THE ATOM
Thomson’s Model of the Atom
1898 Joseph John Thomson
• Thompson’s experiments provided the
first evidence that atoms are made of
even smaller particles.
•Experiments using electric currents
were carried out in tubes know as
cathode-ray tubes.
Cathode Rays and Electrons (Thomson’s Experiment)
Cathode-Ray Experiment
• Observed that cathode rays were deflected by a magnetic field (like wire carrying electric current) which had a negative charge.
•Rays were also deflected away from a negatively charged object and attracted to the positive plate inside the tube.
•CONCLUSION: Cathode Rays are negatively charged!
Thomson- found evidence that there were negative charges scattered throughout an atom!
HISTORY OF THE ATOM
Thompson developed the idea that an atom was made up of
“electrons,” which are negatively charged. They are
scattered unevenly within an sphere surrounded by a soup
of positive charge to balance the electron's charge like
plums surrounded by
pudding.
1904
PLUM PUDDING
MODEL
Negative charges scattered throughout an atom filled with a positively charged mass.
HISTORY OF THE ATOM
1910 Ernest Rutherford
Disproved the PLUM PUDDING model
Rutherford and his team fired alpha
particles at a piece of gold foil which was
very thin.
This experiment led to the discovery of a
Nucleus, where all of the atoms positive
charge is concentrated.
HISTORY OF THE ATOM
gold foil
helium nuclei
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.
helium nuclei
HISTORY OF THE ATOM
Rutherford’s new evidence allowed him to
propose a more detailed model with a central
nucleus that was densely packed.
He suggested that the positive charge was
all in a central nucleus, holding the electrons
in place by electrical attraction
However, this was not the end of the story.
HISTORY OF THE ATOM
1913 Niels Bohr
Studied under Rutherford at the
Victoria University in Manchester.
Bohr refined Rutherford's idea by
adding that the electrons were in
orbits, moving in many directions, like
planets orbiting the sun, with each
orbit only able to contain a set number
of electrons.
Bohr’s Atom
electrons in orbits
nucleus
Each electron in an atom has a specific amount of energy, and an atom can gain or lose energy.
The possible energies that electrons in an atom can have are called energy levels.
An electron cannot exist between energy levels. An electron in an atom can move from one
energy level to another when the atom gains or loses energy.
**Bohr was correct in assigning energy levels to electrons, but incorrect in assuming that electrons moved like planets in a solar system. We will discuss this further later on.
Bohr’s Model
HELIUM ATOM (according to Bohr’s Model)
+N
N
+-
-
proton
electron
neutron
Shell
Proton has positive charge equal in magnitude of negative charge of electron= neutral atoms
nucleus