1. Atomic Theory

2. Origin
The earliest recorded idea about the existence of atoms comes from a Greek philosopher named BC
He theorized that if you were to cut an object in half, then cut it in half again and again over and over…
Eventually you would be left with something too small to cut any more.
This would represent the smallest individual unit of matter.
He gave it the name, “ATOMOS,” which means UNCUTTABLE.

3. Atomic Theory
An Englishman named John Dalton is credited with developing the first modern ideas about atoms in 1808:
All matter is composed of atoms.
The “building blocks of the universe”
Atoms of each element are the same size, mass, etc., but are different from atoms of other elements.
Each element has a unique type of atom
Atoms can’t be subdivided (cut) or destroyed.
Atoms are the smallest individual unit of matter
Has since been falsified by evidence (nuclear reactions)

4. Discovery of Electrons
Tiny negatively charged particles that surround the nucleus (charge = -1)
Discovered in 1897 when J.J. Thomson placed a magnet near a cathode ray tube and bent the beam of particles.

5. Revising the Atomic Model
Dalton’s tiny, solid ball model no longer works
It doesn’t account for these negative particles (electrons)
Thomson proposed a new model, where the “ball” became a positively-charged sphere with negatively- charged “electrons” embedded within it.
“PLUM PUDDING”
“MUFFIN MODEL”

6. Proposing the Nucleus
The idea of a nucleus was first proposed in 1904 by Japanese physicist, Hantaro Nagaoka
Inspired by the solar system, he envisioned electrons as “planets” orbiting the “Sun”, a positively charged nucleus at the center of the atom.

7. Finding Evidence of the Nucleus
In 1911, Ernest Rutherford set out to provide evidence for JJ Thomson’s model
Shot positively charged alpha particles at thin gold foil
If Thomson’s model was correct, then the alpha particles would pass right through the gold foil like bullets through tissue paper.
Instead…some BOUNCED BACK!

8. Proposing a New Model
Why would the alpha particles bounce off the gold foil?
Rutherford reached two conclusions:
Most of the atom is empty space, allowing most of the alpha particles to pass right through the gold foil
If you remove the empty space in all the atoms in your body, you would shrink to a size smaller than a grain of salt…with the same mass you currently have.
If the nucleus is the size of a soccer ball, the nearest electron would be in orbit around the soccer ball a half-mile away, with only empty space in between.

9. Proposing a New Model
Why would some alpha particles bounce off the gold foil?
Rutherford reached two conclusions:
At the center of the atom is a very small, very dense, positively charged nucleus. When alpha particles pass too close to this nucleus, they deflect at an angle; when they hit the nucleus directly, they bounce back.

10. Valence Electrons
Electrons are more likely to reside in certain areas of the electron cloud.
Different areas are referred to as energy levels
Think of these as concentric “shells”
Proposed by Niels Bohr as a way to explain the behavior of atoms
VALENCE ELECTRONS are electrons that reside on the outer “shell” of the atom.
They determine chemical bonds

11. Discovering Protons
Positively charged particles which reside in the nucleus
Mass = 2000 times greater than the mass of electrons
Charge = +1 (equal but opposite to electrons)
The number of protons in the nucleus is what makes each element unique
“Discovered” by Ernest
Nucleus of a hydrogen atom (alpha particles)

12. A Problem with Atomic Mass…
The mass of an atom could be predicted by the number of PROTONS and ELECTRONS it had…
It could be measured by comparing the weights of equal volumes of gasses (Atomic Mass Units)
THE PROBLEM…
The predictions did not match the measurements
The observed mass of atoms was always at least double what the prediction said it should be
Atomic physicists around the world struggled with how to explain the “extra mass” in atoms…

13. Neutrons Neutral particles which reside in the nucleus
Mass equal to the mass of a proton
No electrical charge
Discovered by James Chadwick in 1932
Neutrons explained the “extra mass”
Since they had no charge, their mass is the only indication they are there.

14. Electron Cloud Model NUCLEUS ELECTRON CLOUD
Contains PROTONS and NEUTRONS
Overall positive charge (“+” & “Ø”)
Contains nearly all the mass of the atom
Extremely small and dense
ELECTRON CLOUD
ELECTRONS orbit the nucleus
Contributes almost no mass to the atom
Electrons can only be predicted to be in certain areas based on probability
Most of the atom is empty space

15. Elements A Substance made of only one kind of atom ATOMIC NUMBER
Atoms of each element have a unique number of protons, which differentiates that element from other elements
ATOMIC MASS
The average number of particles in the nucleus (protons & neutrons) for atoms of that element

16. Isotopes
The number of neutrons in an atom can vary without changing the identity of the element
ISOTOPES are atoms of an element with different numbers of neutrons

17. Ions Atoms are usually neutral
They have equal numbers of protons (+) and electrons (-)
IONS are atoms that have gained or lost valence electrons and as a result have either a positive (+) or negative (-) charge

18. What if… CARBON12 …we start with a neutral atom?
…we change the number of…?
It will result in…
CARBON12
6 protons +
6 neutrons Ø
6 electrons -
PROTONS
add one proton
New Element
NITROGEN
7p+ 6n0 6e-
NEUTRONS
add one neutron
Isotope
Carbon13
6p+ 7n0 6e-
ELECTRONS
add one electron
Ion
Carbon-
6p+ 6n0 7e-