Title: Atomic Structure
1Atomic Structure
2Democritus (460 - 370 BC)
- Was the first person to come up with the idea of
atom - Believed that all matter was composed of
indivisible particles he called ATOMS - Which is derived from the Greek word Atomos
meaning indivisible - He also believed that different atoms
- Are different sizes
- Have different properties
- Other philosophers of that time did not agree
with his theories.
3John Dalton (1766-1844)
- Dalton is the Father of Atomic Theory
- Daltons ideas were so brilliant that they have
remained essentially intact up to the present
time and has only been slightly corrected.
4Daltons Atomic Theory (1803)aka 5 Postulates
- All matter is composed of extremely small
particles called atoms. (I agree with Democritus!)
2. All atoms of a given element are identical,
having the same - size - mass - chemical
properties.
3. All atoms of different elements are different.
5Daltons Atomic Theory (1803)aka 5 Postulates
- Atoms cannot be created, divided into smaller
particles, or destroyed.
In a chemical reaction, atoms of different
elements are separated, joined or rearranged.
They are never changed into the atoms of another
element. We will learn more later
5. Atoms combine in definite whole number ratios
to make compounds (you cant have a ½ of a Carbon
bonding with Oxygen its a whole atom or no atom)
6Daltons Atomic Model
- Based on Daltons Atomic Theory (5 postulates),
most scientists in the 1800s believed that the
atom was like a tiny solid ball that could not be
broken up into parts. - Dalton was credited for the three Atomic Laws
that were proven after his time.
7JJ Thomson (1856-1940)
- Used cathode rays to prove that Daltons
Solid-ball model could be broken into smaller
particles - Thomson is credited with discovering electrons
8Cathode Ray Tubes
- Cathode rays had been used for some time before
Thompsons experiments. - A cathode ray is a tube that has a piece of
metal, called an electrode, at each end. Each
electrode is connected to a power source
(battery).
- When the power is turned on, the electrodes
become charged and produce a stream of charged
particles. They travel from cathode, across the
tube to the anode.
9Cathode Ray Tubes
- Thomson put the tube in a magnetic field. He
predicted that the stream would travel in a
straight path. - Instead, he found that the path curved away from
a negatively charged plate and toward a
positively charged plate - Why?
- Like charges repel each other, and objects with
unlike charges attract each other, Thomson
concluded that the stream of charged particles
had electrons in them.
10Cathode Ray Tube Experiment
- Thompson Concluded
- Cathode rays are made up of invisible, negatively
charged particles called Electrons.
- These electrons had to come from the matter
(atoms) of the negative electrode. - Since the electrodes could be made from a variety
of metals, then all atoms must contain electrons!
11Thomsons Plum Pudding Model
- Thomsons Plum Pudding model is a charge sphere
that has (- )charged electrons scattered inside,
like raisins in plum pudding. - Overall, the atom is neutral atom because the
atom had the same number of positive and negative
charges.
- From Thomsons experiments, scientists concluded
that atoms were not just neutral spheres, but
somehow were composed of electrically charged
particles. - The balance of positive and negative charge
supports the neutral atom.
12Rutherford (1871-1937)
- Took Thomsons Plum Pudding Model and added to it
- Used the Gold Foil Experiment to discover the
existence of - Protons
- Nucleus
- You must be able to explain the Gold Foil
Experimentit will be on the CST
13Gold Foil Experiment
- Rutherford directed a narrow beam of alpha
particles ( charges) at a thin piece of gold
foil - Based on the Plum Pudding Model, he predicted
that the charges would go through the foil - Because the atom overall was neutral, he thought
the electrons would travel straight through
14The Gold Foil Experiment
15Conclusions from Gold Foil
- Rutherford found that every once and a while, a
particle was deflected bounced back - Why?
- Because the charge hit a central mass of
positive charge and was repelled.
16Conclusions from Rutherfords Gold Foil
Experiment (memorize this!)
- The atom contains a positively charged nucleus
- This nucleus contains almost all of the mass of
the atom, but occupies a very small volume of the
atom.
- The negatively charged electrons occupied most of
the volume of the atom.
- The atom is mostly empty space.
17Rutherfords Planetary Model
- To explain his observations, Rutherford
developed a new model - The electrons orbit the nucleus like the planets
revolve around the sun.
18Bohr (1885-1962)
- Worked in Rutherfords lab
- Wondered why electrons are not attracted to the
nucleus and cluster around it - Disproved Rutherfords Planetary Model
- Designed and proved the way electrons are
arranged in the atom
19Bohrs Energy Level Model
- Energy Level Model Electrons are arranged in
circles around the nucleus. Each circle has a
different energy. - Electrons are in constant motion, traveling
around the circle - Electrons can jump from one circle to the next
- But they cant go to the nucleus they traveling
too fast to be fully attracted.
20Bohrs Energy Level Model
He proposed the following
1. Protons and neutrons are in the nucleus
2. Electrons can only be certain distances from
the nucleus.
3. The electrons orbit the nucleus at fixed
energy levels.
4. The electrons must absorb or emit a fixed
amount of energy to travel between these energy
levels
21Electron Cloud Model
- As a result of continuing research throughout the
20th century, scientists today realize that
energy levels are not neat, planet-like orbits
around the nucleus of an atom.
- Instead, they are spherical regions of space
around the nucleus in which electrons are most
likely to be found. - Electrons travel in lanes like a runner on a
track. Electrons can be anywhere in their lane.
The lanes are called electron clouds.
22Electron Cloud Model
- Electrons themselves take up little space but
travel rapidly through the space surrounding the
nucleus.
- These spherical regions where electrons travel
may be depicted as clouds around the nucleus.
- The space around the nucleus of an atom where the
atoms electrons are found is called the electron
cloud.
23Electron Cloud Model
No electrons here.
Nucleus
Electron clouds
Electrons will be found here 90 of the time.
- Each cloud is determined by the probability of an
electrons location. An electron will be found in
its cloud about 90 of the time.