Atomic TheoryWhat is it?Who figured it out?When did they do it?How did they do it?Why do we believe it?

What can you tell from this picture?

What is an ATOM?

An atom is the smallest particle of an element that retains the identity of the element.

ZZZZZZZZZ

ATOM

Atoms combine to form compounds

The Ancient Greeks

DemocritusLived 450 B.C.Proposed that all

the stuff in the world is “atomos”

Tiny, indivisible particles

AristotleQuestioned theory

of DemocritusRejected it for lack

of proof

Roger Bacon

Lived in 13th centuryBelieved that science should be based on

experimental evidence.

ALCHEMISTS

Tried to turn base metals into precious metals.

Developed knowledge and techniques

Not true scientists

Copper into Gold Laboratory

Alchemists tried to turn normal copper into gold coins!

If they succeeded, they would never need to work!

Prove that the coins are GOLD

How is copper different than gold?How can you test it?What techniques are you using…that

Democritus did not use?

Antoine Lavoisier

1743-1794 French Father of Chemistry Sadly, beheaded in

French Revolution

Lavoisier

LAW OF CONSERVATION OF MATTERMatter, like energy, is neither created nor

destroyed in a chemical reaction.

This concept established modern chemistry.

What was different?

Experimentation He used a balance to

study the role of oxygen in rusting and burning.

Other people did it, too!

Priestly did similar experiments.

He believed a false theory of Phlogiston.

Kept his mind closed to new idea.

PROUST

1799 Law of Constant

Composition A given compound

always contains the same elements in the same proportion by mass.

WATER11% Hydrogen

WATER89% Oxygen

JOHN DALTON

LAVOISIER PROUST

DEMOCRITUSBACON

ATOMIC THEORY OF MATTER

DALTON’S ATOMIC THEORY OF MATTER

1. All matter is composed of submicroscopic (extremely small) indivisible particles called ATOMS.

2. All atoms of a given element are identical. The atoms of any one element are different from those of any other element.

DALTON’S ATOMIC THEORY OF MATTER

3. Atoms are neither created nor destroyed in a chemical reaction. Chemical reactions occur when atoms are separated, joined, or rearranged. However, atoms of one element are never changed into atoms of another element as a result of a chemical reaction.

DALTON’S ATOMIC THEORY OF MATTER

4. Atoms of different elements can mix physically or can combine chemically with one another in simple whole number ratios to form compounds.

What does this mean?

Atomic Theory of Matter

Not perfect...but a workable theory to build on.

Scientists had been doing many experiments with electricity since Ben Franklin flew his kite.

Faraday suggested that electricity might explain the atom

English physicist, J.J. Thomson 1856-1940

Discovered electrons in 1897

Thomson experimented with a “cathode ray tube”

Cathode Ray Tube

Cathode Ray Tube

Thomson’s Cathode Ray Tube Experiment

Flow of electric current through gases. Sealed gas in glass tube with metal plates at the end. Connected plates to high voltage source: Anode + and Cathode –

This ray could be deflected toward a positive charge…

It has a negative charge.

This ray could move things…

It was made of particles of matter.

This ray acted the same no matter what materials were used…

It was not atoms…it must be part of all atoms

Cathode Ray Tube

Cathode ray is composed of very small negatively charged particles that are part of

atoms

ELECTRONS

Cathode Ray Tube

Diagram of CRT

Atomic Theory

Millikan devised experiments to determine the mass and charge of the electron.

Protons : discovered in 1886

Positively charged particles

Also discovered with cathode ray tube…these particles went the other direction.

The Nuclear Atom

RutherfordIdentified three types of radiation

Alpha…positive particleBeta…negative particleGamma…high energy

Devised the GOLD FOIL EXPERIMENT

Now…where were we….?

Rutherford’s Nuclear Atom

Most of the mass of an atom is in the center…the nucleus…with electrons moving around it.

Nuclear atom

By 1932, neutron was discovered, too.The nucleus is the central core of the

atom, composed of protons and neutrons. Because, protons and neutrons have much greater mass than electrons, almost all of the mass of the atom is concentrated in a tiny nucleus…a dime in a football stadium!

What next?

We know the parts of the atom.We know about the nucleus.What about the electrons?

Bohr’s Model

Light provided the next clues for the structure of the atom

Light has a Dual Nature

Like this picture…

Young lady or Old lady?

Light is a wave

Electromagnetic radiation travels in waves.

As the wavelength increases, the frequency decreases.

The greater the frequency the greater the energy.

Light acts like Particles

Electromagnetic radiation also has the properties of particles.

Bohr suggested that energy is emitted and absorbed in discrete quantities called

Quanta or Quantumpackets or pieces of energy

“Jumps”

Demonstrate Quantum

Dual Nature of Light is the next tool for understanding the atom

Energy is directly proportional to frequency…wave nature.

Einstein proposed that light consists of quanta of energy that behave like particles of light…he called these photons.

Example: photoelectric effect

This is the DUAL NATURE OF LIGHT

Continuous Spectrum vs. Line Spectrum

What does this have to do with ATOMS?

Atoms give off energy as photons of light.Line spectra of elements reveal that

electrons exist in quantized (discrete) energy levels.Rainbow…continuous spectrumLine…discontinuous spectrum

The color allows us to calculate the energy higher frequency = higher energy… purple higher than red

Emission Spectra

Line Spectra of elements show discrete or quantized energy levels in the atom.

These energy levels are different for each element.

The color or wavelength of light shows the energy level because energy can be calculated from frequency of light emitted.

Bohr’s Model of the Atom Bohr applied Quantum

Theory to the structure of the hydrogen atom.

Quantum theory means that electrons jump from level to level.

Bohr pictured the electrons spinning around set orbits, like planets around the sun.

Planetary Model

Quantum Leaps!

Electrons exist at low energy…ground state

Add energy to go to higher energy…excited state• Electron drops back down to ground

state as soon as it can • It releases the exact energy it needed to

jump up as a photon of a frequency or color.

Schroedinger’s Quantum Mechanical

(Today’s) ModelWe still have energy levels, like BohrHeisenberg Uncertainty Principle

It is impossible to know the velocity and position of a particle at the same time

We can not know the exact location of an electron.

Any effort to do so, will change the position.We can only figure the probability of finding it

in a certain region.

Orbitals

Area where electrons are most likely to be found.

Example: If your mom wants you to do some work, she can find you in your room most of the time, or the house, or the yard…but you could be at the mall.And sometimes her efforts to find you, make you move!

Today’s Atom

Dense, small,nucleus (with protons and neutrons) is surrounded by a fuzzy cloud shapes where electrons (that act more like waves) are most likely to be found… orbitals.

Today’s Model of the Atom

akaSchroedinger’s

or

Quantum Mechanical

Model

Links

Go to Holt On-Line for a nice review of all this. my.hrw.com/

Go to this site to see some great atomic atrwork!www.chemsoc.org/.../orbital/images/elev3.jpg

The Crazy World of Quantum Mechanics“If you aren’t shocked by it, you don’t

understand it.” Bohr