1. Unit 2: History of the Atom

2. An element is matter with a fixed composition – it is made of the same atoms
An atom is the smallest piece of matter that still has the properties of its element.

3. THE ATOM: the smallest particle of an element that still has properties of that element.

6. Democritus; Greek philosopher who lived around the year 430 B.C.
He proposed the idea that matter is formed of small pieces that could not be cut into smaller parts.
Atomos: Greek word meaning ‘uncuttable’. This is where we get the word Atom.

7. JOHN DALTON: ~1800s
John Dalton, English chemist, proposed an atomic theory and model for atoms.
Dalton thought that atoms were solid spheres that could not be broken into smaller pieces.
Studied the ratios in which elements combine in chemical reactions
Developed a theory to explain his findings

8. *Wrong! Dalton didn’t know about isotopes.
DALTON’S ATOMIC THEORY:
All matter is made up of atoms.
*True, atoms are the smallest unit of matter
2. Atoms are indivisible/indestructible.
*Wrong! Dalton didn’t know about subatomic particles (protons, neutrons, electrons).
3. All atoms of the same element have the same mass
*Wrong! Dalton didn’t know about isotopes.
4. Atoms mix or combine in whole-number ratios to form compounds (1:1, 2:1, 1:3, etc).
*True! An example is H2O

9. Thomson and the smaller parts of atoms
J.J. Thomson, British scientist, 1897.
Discovered that atoms contain negatively charged particles.
Since scientists knew that atoms had no electrical charge he inferred that atoms must also have some sort of positive charge.

10. The idea An atom is neutral,
therefore if there are negative particles (electrons)
there must also be positive particles (protons).

11. Thomson and smaller parts of atoms continued....
He proposed the plum pudding model.
This model described an atom similar to a chocolate chip cookie. The chocolate chips (negative) are scattered throughout a ball of positive charge.
The negatively charged particles later became known as electrons.

12. Thomson’s PLUM-PUDDING atomic model:
A positively-charged mass with electrons dispersed throughout
(like a chocolate chip cookie)

13. Rutherford and the Nucleus
Ernest Rutherford, student of Thomson, 1911.
Known for the Gold Foil experiment
He used a thin sheet of gold foil and a particle beam of positive particles to show that an atom’s positive charge must be clustered in a tiny region in its center.
We now call that region of positive charge at the center of an atom the nucleus.

14. Rutherford’s Gold Foil Experiment

15. Scientists knew that electrons had almost no mass.
Rutherford continued...
Scientists knew that electrons had almost no mass.
They reasoned that nearly all of an atom’s mass must also be located in the tiny, positively charged nucleus.
Rutherford named these positively charged particles, found in the nucleus, protons.

16. Discovered that atoms were mostly empty space, but had a concentrated positively- charged space called the “nucleus”
Negative electrons were scattered around the outside of the nucleus.

17. Rutherford’s Atomic Model
Rutherford’s atomic model featured a tiny, very dense, positive nucleus surrounded by negative electrons that are far apart. - - - - - + - - -

19. Bohr’s Model
Niels Bohr, Danish scientist, 1913; student of both Thomson and Rutherford.
Bohr showed that electrons could have only specific amounts of energy, leading them to move in certain orbits.
Proposed a model of the atom in which the electrons are in fixed orbits
Set paths around the nucleus
Each orbit corresponds to a particular energy level where an electron can exist
These orbits resemble planets orbiting a sun.

20. Bohr’s Atomic Model
According to Bohr’s model, electrons are never found between orbits.

21. Bohr Model: electrons orbit the nucleus in well-defined energy levels or orbits
Electrons fill from the lowest energy level (closest to the nucleus) outward – each level can hold up to 2n2 electrons, where n is the number of the level.
the first level can contain up to 2e, 2(12) = 2
the second up to 8e, 2(22) = 8
the third up to 18, 2(32) = 18; and so on.
Only seven energy levels are needed to contain all the electrons in an atom of any of those elements now known.

22. Modern Atomic Theory
Developed by Louis de Broglie and Erwin Schrödinger
Electrons move in indefinite paths, and their exact locations can only be predicted based on their energy level.
Called the “wave model,” or quantum mechanical model
Simply put, we say the electrons move in “Clouds”

24. Cloud of Electrons
In the 1920’s scientists determined that electrons do not orbit the nucleus like planets. Instead, they can be found anywhere in a cloudlike region around the nucleus. (the cloud is a visual model)
An electron’s movement is related to its energy level. or specific amount of energy it has.
Electrons of different energy levels are likely to be found in different places of the ‘cloud’.

25. Summary
Dalton’s Theory- all matter is made up of atoms, which can’t be divided
Dalton’s Solid Sphere
Thomson’s Model- discovered atoms were made up of smaller particles (these smaller particles are charged)
Thomson’s Plum Pudding
Rutherford’s Theory- discovered the positively charged nucleus
Rutherford’s Positive Nucleus