1. Chapter 2 Atoms, molecules, and ions
The Atomic Theory
History
The Structure of the Atom
Proton, Neutron, Electron
Atomic Number, Mass Number
Isotopes
The Periodic Table
DAB/CAS/CSU/2010

2. Matter

3. Brownian Motion

4. Atomic theory Atoms are incredibly small.
What we know about them is based on indirect evidence.

5. History of atomic theory
500 BC Democritus proposed the idea of atom but had no experimental data to support
Believed that all matter consists of very small, indivisible particles, which he named atomos (meaning uncuttable or indivisible).
1800 Dalton proposed an atomic theory supported by an experimental data

6. Dalton’s atomic theory: Support
Support for his theory came when he began looking at the masses of elements that combined with each other
He found that elements combine by simple ratio by mass

7. Dalton’s atomic theory:
All matter is composed of tiny indivisible particles called atoms.
All atoms of the same element are identical (having same size, mass, and chemical properties). The atoms of one element are different from the atoms of all other elements.
Atoms of two or more elements can combine in small whole number ratios to form compounds.
A chemical reaction involves only the separation, combination, or rearrangement of atoms; it does not result in their creation or destruction.

8. Dalton’s atomic theory: Support
Law of Definite Proportion
Different samples of the same compound always contain its constituent elements in the same proportion
Law of Multiple Proportions
If two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers.
Example: CO & CO2

9. Dalton’s atomic theory: Support
Law of Conservation of Mass
Matter can neither be created nor destroyed
The mass of atoms before and after reaction is conserved

10. Dalton’s atomic theory: Support
Law of Conservation of Mass

11. Dalton’s atomic theory:
Problems with Daltons Theory
Atoms of the same element have different masses
Atoms have subatomic particles (protons, electrons, neutrons.)
So, Dalton’s theory is not completely correct!

12. The structure of the atom

13. Subatomic Particles
Particle Symbol Charge Relative Mass (amu) Electron e Proton p+ + 1 Neutron n 0 1

14. Subatomic Particles: Discovery
Electron was discovered by JJ Thomson in ～1900 using his cathode ray tube.
Electron charge was found by American Robert Millikan ～ 1910 by an oil-drop experiment.
Proton was found by the adaptation of cathode ray tube ～ 1900.
Neutron was discovered by Chadwick in 1932.

15. The Electron Discovery
1890’s discovery of radiation lead to discovery of electron
Radiation – the emission and transmission of energy through space in the form of waves
Used cathode ray tube
Cathode – negatively charged plated, emits an invisible ray
Anode – positively charged tube

16. Thomson’s plum pudding model of the atom

17. Subatomic Particles: Discovery
Rutherford’s Gold Foil Experiment:
Plum pudding model: atom has a weak positive charge.
Rutherford tested the idea by positively charged particles at gold foil.
He expected particles to pass through.
His results surprised him.

18. Movie on Gold Foil Experiment

19. Subatomic Particles: Discovery
Rutherford’s Gold Foil Experiment: Results
Atoms is mostly empty space.
Protons are found in the dense nucleus in the center of the atom.

20. Location of Subatomic Particles
10-13 cm
electrons
protons
neutrons
10-8 cm
nucleus

21. Rutherford’s Model: Problem:
Could not explain why electrons would not spiral into the nucleus.

22. Implications of Rutherford’s Exp’t.
Nucleus – at the center of the atom
Dense central core within the atom
Contains positively charged particle (protons)
Protons carry the same charge as that of electron
Mass of protons is 1840 times that of electron
Neutrons – neutral particles within the nucleus having a mass slightly greater than that of protons

23. Particle
Mass (g)
Coulumb
Charge unit
Electron x
x 10-19 -1
Proton x
x 10-19 +1
Neutron x

25. ATOMS TO ELEMENTS What makes elements different from one another?
The number of protons, electrons, and neutrons in them.
Example:
Gold and Mercury

26. Counts the number of protons in an atom
Atomic Number, Z
Counts the number of protons in an atom

27. Atomic Number on the Periodic Table11 Na
Atomic Number
Symbol

28. All atoms of an element have the same number of protons11 Na
11 protons
Sodium

29. Learning Check
State the number of protons for atoms of each of the following: A. Nitrogen 1) 5 protons 2) 7 protons 3) 14 protons B. Sulfur 1) 32 protons 2) 16 protons 3) 6 protons C. Barium 1) 137 protons 2) 81 protons 3) 56 protons

30. Counts the number of protons and neutrons in an atom
Mass Number, A
Counts the number of protons and neutrons in an atom
Mass Number, A = p + n
n = A - p

31. Atomic Symbols Show the mass number and atomic number
Give the symbol of the element
mass number
23 Na sodium-23
atomic number 11

32. Number of Electrons Number of protons = Number of electrons
An atom is neutral
The net charge is zero
Number of protons = Number of electrons
+ 2 + (-2) = 0
Atomic Number =Number of protons
Atomic number = Number of electrons

33. Subatomic Particles in Some Atoms
O P Zn
8 p+
8 n
8 e-
15 p+
16 n
15 e-
30 p+
35 n
30 e-

34. Subatomic Particles in Some Atoms
O P Zn
8 p+
8 n
8 e-
15 p+
16 n
15 e-
30 p+
35 n
30 e-

35. What happens when atoms losses electrons?
When Na losses 1 electron?
11 p+
11 e-
2311Na+
2311Na
+11
-10 1
11 p+
11 e-
12 n
11 p+
10 e-
12 n

36. What happens when atoms gains electrons?
When Cl gains 1 electron?
17 p+
17 e-
3517Cl-
3517Cl
+17
- 18
- 1
17 p+
17 e-
18 n
17 p+
18 e-
18 n

37. What happens when atoms gains electrons?
When O gains 2 electron?
8 p+
8 e-
168O
168O
+ 8
- 10
- 2
8 p+
8 e-
8 n
8 p+
10 e-
8 n

38. Isotopes
Atoms with the same number of protons, but different numbers of neutrons.
Atoms of the same element (same atomic number) with different mass numbers
Isotopes of chlorine
35Cl 37Cl
chlorine chlorine - 37
17 p+
18 n
17 e-
17 p+
20 n
17 e-

39. Learning Check 6 6 6 #p _______ _______ _______
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.
12C C 14C
#p _______ _______ _______
#n _______ _______ _______
#e _______ _______ _______ 6 6 7 6 8

40. Learning Check
An atom of zinc has a mass number of 65. A. Number of protons in the zinc atom 1) 30 2) 35 3) 65 B. Number of neutrons in the zinc atom C. What is the mass number of a zinc isotope with 37 neutrons? 1) 37 2) 65 3) 67

41. Learning Check
Write the atomic symbols for atoms with the following: A. 8 p+, 8 n, 8 e- ___________ B. 17p+, 20n, 17e- ___________ C. 47p+, 60 n, 47 e- ___________
16O 8
37Cl 17
107Ag 47

42. Atomic Mass on the Periodic Table11 Na
22.99
Atomic Number
Symbol
Atomic Mass

43. Atomic Mass
Atomic mass is the weighted average mass of all the atomic masses of the isotopes of that atom.

44. Example of an Average Atomic Mass
Cl-35 is about 75.5 % and Cl-37 about 24.5% of natural chlorine. 35 x 75.5 = x 24.5 = 9.07