What is matter?
Matter is anything that has mass and
takes up space
What is matter made of??
Atoms.
All matter is made of atoms.
Atoms are the building blocks of Matter
Remember????
• The Cell theory - 3 basic principles:
• All organisms are made of one or more
cells
• The cell is the basic unit of organization
in organisms
• All cells come from cells
There is also an Atomic Theory
Different discoveries made by different
scientists let to today’s atomic model:
-Greeks
-Dalton
-Crookes
-JJ Thomson
-Rutherford
-Niels Bohr and the Atomic Cloud Model
-Schrodinger ( not in the book)
First Thoughts
• Greeks- believed matter is made of tiny
particles.
• Geeks named the tiny particles atom
• Atom = “uncuttable” in Greek
• now we know that the atom can be
divided into smaller particles by nuclear
reactions
**Science today and in the past
• Past – no means to perform experiments
• Discoveries were based in discussions,
observations and reasoning
• There were philosophers instead of
scientists
**Science and Technology today
• Technology –allows us to perform
experiments and to develop and prove
theories
• That is why knowledge in constantly
evolving
• Technology gives us the tools to do
Science
Dalton’s Concept – early 19th century
• Matter is made up of atoms - correct
• Atoms can’t be divided into smaller parts -
wrong
• All the atoms of an element are exactly alike –
almost correct – isotopes are different
• Different elements are made of different atoms –
correct – Element gold has only atoms of gold
• His model: Dalton pictured an atom as a hard
sphere that was the same throughout, like a
marble.
(textbook fig. 3)
Crooke - brought some evidence of Dalton’s ideas.
Late 19th century
BatteryCRT – Cathode ray tube with two metal electrodes
connected to a battery
An electrode is a piece of metal that can conduct
electricity.
When connected to the battery – generates cathode rays
Cathode Rays are a stream of particlesthat traveled from the cathode to the anode.
The cross was blocking the way of the particles and created a shadow
Since Crookes removed all the air from the CRT, he did not know what the particles were.
Thomson experiment(1897)
used the CRT to understand what were those
particles – the particles were attracted by a
positive plate
Thomson conclusions: Cathode Rays
Light is energy in form of waves, not particles and does not have charge, therefore cannot be bent by a magnet.
The cathode rays couldn’t be light.
Thomson conclusions: Cathode Rays
Thomson concluded that the beam must be made up of charged particles of matter that came from the cathode.
The charges were attracted by the + plate, so the
particles had a negative charge.
Thomson and The Electron
Thomson repeated the experiment with other metals, with the same results.
He concluded that these particles were present in all materials.
These negatively charged particles are now called electrons.
Thomson’s Atomic
Model
If atoms contain negatively charged particles, then all matter, should be negatively charged as well.
But all matter isn’t (–) charged.
Matter and atoms are neutral
*So the atom must have + and - regions
Rutherford’s Experiments ( 1906)
• Scientists needed more evidence to support Thompson’s model
• Rutherford fired fast-moving, positively charged bits of matter, called alpha particles, at a thin film of gold.
Rutherford’s Experiment – most particles passed
through the foil, some were deflected and some did not
pass at all.
Results: He discovered the nucleus ( positive charge)
The deflected particles passed close to a + area
The repelled ones hit a positive area :+ and + repel
The ones that passed ( majority) hit a - area
Results: He discovered the nucleus
The deflected particles passed close to a + area: nucleus
The repelled ones hit a positive area: nucleus
The ones that passed ( majority) hit a – area: electrons
Rutherford’s Model
The Nucleus
• He hypothesized that almost all the mass of the atom and all of its
positive charge are
crammed into an
incredibly small region at the center of the atom called the nucleus.
The Proton
• Rutherford’s experiment was done in 1906 and in 1920 scientists called the positive charges in the nucleus protons.
• A proton is a positively charged particle present in the nucleus of all atoms.
The discovery of the neutron and the
revised atomic model
Rutherford described the atom as a massive positive center surrounded by an empty space occupied by electrons, that have almost no mass.
How was the Neutron discovered?
Scientists knew at that time that:
1) Atoms have electrons and protons
2) Electrons have very little mass, so…..
3) The mass of an atom should have been approximately equal to the mass of its protons, but…..
4) The mass of most atoms is at least twice as much as the mass of its protons.
WHERE is the rest of the mass coming from?
• There must be something else in the
nucleus to account for the extra mass.
• This third particle, which was later call
the neutron would have the same mass
as a proton and be electrically neutral.
The model of the atom was revised :
A central nucleus with
protons and neutrons and
electrons surrounding
the nucleus moving in orbits.
Niels Bohr (1913) and the Atomic Cloud Model
Soon after Niels Bohr model, scientists soon learned that electrons are in constant, unpredictable motion and can’t be described by moving in an orbit.
*Erwin Schrodinger: The Electron Cloud
Model ( 1926) – current model
• Electrons travel in a region surrounding the nucleus, which is called the electron cloud.
• The electrons can be anywhere, most likely close to the nucleus, attracted by the + forces