1. Chpater Review
Atomic Theory
Atomic structure
Valence electrons

2. Topic 3
The development of the Atomic theory
Democritus concluded that matter could not be divided into smaller and smaller pieces forever.
Democritus named this small, indivisible piece, an atom; atom comes from the Greek word atomos, which means “not to be cut” or “indivisible.”

3. Topic 3 Philosophical idea
The development of the Atomic theory
Philosophical idea
Aristotle was part of the generation that succeeded Democritus.
Aristotle did not believe in atoms, instead, he thought that all matter was composed of 4 basic elements, water, fire, earth, and air. His opinion was accepted for nearly 2000 years.

4. The first atomic theory
Topic 3
The development of the Atomic theory
The first atomic theory
In the early 1800’s, the English chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.
In 1803, Dalton combined the results of his experiments with other observations about matter and proposed an atomic theory.

5. Foundations of Atomic Theory
Topic 3
The development of the Atomic theory
Foundations of Atomic Theory
The transformation of a substance or substances into one or more new substances is known as a chemical reaction.
Law of conservation of mass: mass is neither created nor destroyed during ordinary chemical reactions or physical changes.

6. Foundations of Atomic Theory, continued
Topic 3
The development of the Atomic theory
Foundations of Atomic Theory, continued
Law of definite proportions: a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound.
Law of multiple proportions: if two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers.

7. Law of Conservation of Mass
Topic 3
The development of the Atomic theory
Law of Conservation of Mass

8. Law of Multiple Proportions
Topic 3
The development of the Atomic theory
Law of Multiple Proportions
The ratio of the masses of one element combined with a certain mass of a second element is always a ratio of small whole numbers.

9. Dalton’s Atomic Theory
Topic 3
The development of the Atomic theory
Dalton’s Atomic Theory
All matter is composed of extremely small particles called atoms.
Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
Atoms cannot be subdivided, created, or destroyed.

10. Dalton’s Atomic Theory, continued
Topic 3
The development of the Atomic theory
Dalton’s Atomic Theory, continued
Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
In chemical reaction, atoms are combined, separated, or rearranged.

11. Topic 3
The development of the Atomic theory
Modern Atomic Theory
Not all aspects of Dalton’s atomic theory have proven to be correct. We now know that:
Atoms are divisible into even smaller particles (by a nuclear change)
A given element can have atoms with different masses (isotopes)

12. Topic 3 BUT, Some important concepts remain unchanged.
The development of the Atomic theory
Modern Atomic Theory
BUT, Some important concepts remain unchanged.
All matter is composed of atoms.
Atoms of any one element differ in properties from atoms of another element.

13. Table of Contents The structure of the atom Chapter 3
Section 1 Developing the atomic theory
Section 2 The structure of the atom
Section 3 Counting atoms

14. Structure of the atom
Scientific investigations showed that atoms are actually composed of several basic smaller particles.
Atom: Defined as the smallest particle of an element that retains the chemical properties of that element.
All atoms consist of two regions.
- The nucleus is a very small region located at the center of an atom (protons + neutrons)
- Surrounding the nucleus, is a region occupied by negatively charged particles called electrons.

15. Discovery of the electron
In 1897, J.J. Thomson provided the first hint that an atom is made of smaller particles.
When current is passed through a cathode ray tube, a stream of particles travel from the cathode(-) to the anode (+), producing a beam or ray.
When a magnetic or an electric field was placed around the tube, the rays were deflected away from the negatively charged end.

16. Discovery of the electron
Cathode Ray Tube
experiment
The mass of the electron
9.10·10-28 g
Millikan’s Experiment (1909)
Charge to mass ratio of the electron
-1.76·10 8 C/g

17. Discovery of the electron
Thomson concluded: All cathode rays are composed of identical negatively charged particles, electrons.
He also found the charge to mass ratio of the electrons.
Millikan measured the charge of the electrons.
Scientists used the charge of electrons and the charge to mass ratio of the electron to determine the mass of an electron.
The mass of an electron was found to be about 1/2000! the mass of a hydrogen atom.

18. Thomson’s plum pudding model
Based on what was learned about electrons:
1. Because atoms are electrically neutral, they must contain a positive charge to balance the negative electrons.
2. Because electrons have so much less mass than atoms, atoms must contain other particles that account for most of their mass.

19. Thomson’s plum pudding model
Thomson proposed a model for the atom that is called the plum pudding model,
The negative electrons were spread evenly throughout the positive charge of the rest of the atom.

20. Discovery of the atomic nucleus
After Thomson proposed his plum pudding model of the atom, new experiments disproved this model.
In 1911, Ernest Rutherford and his associates Hans Geiger and Ernest Marsden bombarded a thin piece of gold foil with fast positively charged alpha particles.
Most of the alpha particles passed through the foil. However, a some particles were deflected and some were deflected back to the source.

21. Section 2 The Structure of the Atom
Chapter 3

22. Rutherford's experiment
 particles experienced some powerful force within the atom.
This force occupies a very small amount of space (few  particles had been deflected.
He concluded: The force must be caused by a very densely packed matter with a positive charge (The nucleus)
EUREKA!!!!!!! Rutherford discovered that size of the nucleus was very small compared to the size of the atom.

23. Rutherford’s atomic Model
Dense, positive nucleus at the center of the atom.
Most of atom is empty space.
But, can electrons just float around in empty space?

24. The Bohr Model
Rutherford’s model proposed that electrons were held in an atom by the attraction between them and the positive nucleus.
In 1913, scientist Niels Bohr proposed a model in which he placed each electron in a specific energy level.
Electrons move in definite orbits around the nucleus, like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus.

25. Bohr’s atom
Electrons don’t just float around, they have a distinct orbit.
There is a certain amount of energy in these levels. (Energy to pull the electron away)

26. The Wave Model Louis De Broglie - 1924
Bohr’s model just worked well in explaining the simplest hydrogen atom.
Today’s atomic model is based on the principles of wave mechanics.
Electrons do not move in a definite orbit. It is impossible to determine its exact location.
Scientists can only predict where an electron is most likely to be found.

27. Modern atomic model
Schrödinger
An atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make the atom neutral.
The region of space around the nucleus where an electron is most likely to be found is called an orbital.

28. Orbitals
Orbitals have different shapes and maximum numbers at any level.
s (sharp) - spherical (max = 1)
p (principal) - dumb-bell shaped (max = 3)
d (diffuse) - four-lobe-shaped (max = 5)
f (fundamental) - six-lobe shaped (max = 7)

29. Exit Slip
Determine whether each of the following statements is true or false. If false, correct the statement.
(a) The nucleus has most of the mass and comprises most of the volume of an atom.
(b) Every atom of a given element has the same number of protons.
(c) The number of electrons in an atom equals the number of neutrons in the atom.