1. The Structure of the Atom
Chapter 4
The Structure of the Atom

2. Early Ideas about Matter and the Atom
Very early in history, Greek philosophers thought that matter was composed of four different elements: earth, water, air, and fire.

3. Democritus ( B.C.)
First person to propose the idea that matter was not infinitely divisible. He came up with the thought that matter was made of tiny particles called atomos. His atom could not be further divided.

4. Aristotle ( B.C.)
Aristotle was a big supporter of the idea that matter was made of earth, wind, fire, and air.

5. John Dalton ( )
Atomic theory didn’t come back until John Dalton proposed that matter was made of extremely small particles called atoms.

6. Dalton’s Atomic Theory
1) All matter is made of atoms. Atoms are indivisible and indestructible.
2) All atoms of a given element are identical in mass and properties
3) Compounds are formed by a combination of two or more different kinds of atoms.
4) A chemical reaction is a rearrangement of atoms.

7. J.J. Thomson
JJ Thomson introduces the concept of the electron using an experiment involving a “cathode ray”. From his experiment, he developed the “Plum Pudding Model”

10. JJ Thomson’s Plum Pudding Model

11. Ernest Rutherford (1911)
Rutherford came up with the idea of the nucleus, where most of the mass of an atom lies, as well as all of the positively charges particles. He later called the
particles “protons”.

12. Rutherford’s Gold Foil Experiment

14. Thomson vs. Rutherford

15. Niels Bohr (1913)
Bohr concluded that the electrons in an atom travel in a fixed path, called “orbits” around the nucleus.

16. James Chadwick (1932)
Chadwick proved the existence of neutrons in the nucleus, which improved upon Bohr’s Model.

17. The current Model
It is now accepted that electrons do not travel in fixed orbits, rather they travel in what is known as an “electron cloud”. These are areas where the probability of finding an electron are high.

18. Modern Atomic Model

19. Anatomy of an atom Atoms are made of 3 basic subatomic particles
Protons (+)
Neutrons (0)
Electrons(-)

20. Masses of Subatomic Particles
Electron
Proton
Neutron
9.11 X g
1.673 X 10-24g
1.675 X 10-24g

21. Size of an Atom

22. Size of an Atom
If a nucleus was increased to the size of an orange, then comparatively an orange would be the size of the earth

23. Atomic Number
The atomic number is equal to the number of protons and the number of electrons in an atom.

24. Atomic Mass
Atomic Mass is the equal to the number of protons plus the number of neutrons.
AM= p + n

25. Number of Neutrons
What’s the formula for the number of neutrons?

26. Warm Up
SYMBOL
Protons
Electrons
Neutrons Rb Fe Al Cs

27. Something interesting
How we can see atoms: Scanning Tunneling Microscope

28. HOW STM WORKS

29. Image from a STM

30. STM
With a STM, scientists can actually move atoms around to form shapes and even simple machines.
This promoted the birth of a branch of science known as “nanotechnology”.
It is even thought that we may some day be able to make machines the size a single molecule.

31. Isotope
Isotopes are atoms that have the same number of protons, but have a different number of neutrons. -If two atoms have a different number of protons, can they be the same element?

32. Examples of Isotopes Carbon 12 Carbon 13 Carbon 14 Protons Electrons
Neutrons

33. Behavior of Isotopes
Despite having different numbers of neutrons, isotopes behave in generally the same way as one another. -Chemical behavior depends mainly on the number of electrons, not the neutrons.

34. Atomic Mass unit
Scientists needed to come up with a standard mass to measure the masses of atoms.
The decided to say that carbon weighs 12 amu and that 1 amu would be the mass of 1/12 a carbon atom.
The mass of 1 amu is very nearly the mass of 1 proton or 1 neutron.

35. Practice Today Page 105: 5 Page 114: 7, 8, 10 Page 121: 21, 22
If you do not finish it in class, it will be due for homework tomorrow.

36. Atomic Mass
Atomic Mass is the weighted average mass of the isotopes of an element.

37. Computing Average Atomic Mass
Look at the Atomic Masses on the periodic table. Are there any whole numbers? The reason is because of isotopes.

38. Average atomic mass
The average atomic mass is the averaged mass of all the isotopes of a certain element. It depends largely on the abundance of each isotope.

39. Three isotopes of Silicon
Isotope name
Isotope mass (amu)
Relative Abundance
Silicon-28
27.98
92.21
Silicon-29
28.98
4.70
Silicon-30
29.97
3.09

40. Calculating A.A.M
To calculate the A.A.M, you need to first calculate the contribution of each isotope.
(AM of Isotope A)(% of Isotope A) + (AM of Isotope B)(% of Isotope B)…
AM = atomic mass of the isotope
% = the percent abundance of the isotope.

41. CALC. AAM for silicon