1. Pre-AP Chemistry
Atoms:
The Building
Blocks of Matter

2. The Development of the Atomic Theory
Democritus~ 400 BC
Possibly first to propose the “atom”.
Atom in Greek means indivisible
Aristotle ~ 300 BC
Thought matter was continuous and made of 4 basic elements:
Fire
Wind
Earth
Water

3. The Development of the Atomic Theory
ALCHEMY – based on Aristotle’s idea of continuous matter
Attempt to “transmutate” one type of matter to another
EX: Lead  Gold
Studied for ~ 2000 years!
IT DIDN’T WORK!!!!!
Helped lead to the development of the SCIENCE of Chemistry

4. Law of Conservation of Mass
Mass is neither created nor destroyed during chemical or physical reactions.
Total mass of reactants =
Total mass of products
Antoine Lavoisier

5. Laws of Proportions Law of Definite Proportions:
A compound will always contain the same ratio (mass) of elements
EX: H2O will always have 2 H and 1 O
Law of Multiple Proportions:
If the same elements combine in different ratios (mass) the result will form different compounds
EX H2O and H2O2

6. Dalton’s Atomic Theory (1808)
All matter is composed of extremely small particles called atoms
Atoms of a given element are identical in size, mass, and other properties
RIGHT!
John Dalton
WRONG!
Atoms cannot be subdivided, created, or destroyed
Atoms of different elements combine in simple whole-number ratios to form chemical compounds
In chemical reactions, atoms are combined, separated, or rearranged
WRONG!
RIGHT!
RIGHT!
WAS HE CORRECT????

7. Modern Atomic Theory
Several changes have been made to Dalton’s theory.
Dalton said:
Atoms of a given element are identical in size, mass, and other properties;
Modern theory states:
Atoms of an element have a characteristic average mass which is unique to that element. = ISOTOPES

8. Modern Atomic Theory #2 Dalton said:
Atoms cannot be subdivided, created, or destroyed
Modern theory states:
Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

9. Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to the presence of a negatively charged particle with a very LARGE charge and very SMALL mass.
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

10. Discovery of the Electron
In 1909, Robert Milikan confirmed Thomson findings of a negative particle in the cathode tube.. He was even able to determine the mass of the electron compared to a hydrogen atom.

11. Some Modern Cathode Ray Tubes

12. Conclusions from the Study of the Electron
All elements must contain electrons because they all produced beams.
Electrons are negative b/c they move from the negative to the positive area in the tube.
Electrons have mass b/c they moved the wheel placed in the beams path.
Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons
Electrons have so little mass that atoms must contain other particles that account for most of the mass

13. Thomson’s Atomic Model
Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

14. Rutherford’s Gold Foil Experiment
Alpha particles are helium nuclei (+ charged)
Particles were fired at a thin sheet of gold foil
Particle hits on the detecting screen (film) are recorded

15. Try it Yourself!
In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

16. The Answers
Target #1
Target #2

17. Rutherford’s Findings
Most of the particles passed right through
A few particles were deflected
VERY FEW were greatly deflected
“Like howitzer shells bouncing off of tissue paper!”

18. Rutherford’s Findings
Conclusions: Nucleus
The nucleus is small
Reason: Very few particles repelled.
The nucleus is dense
Some particles didn’t pass through
The nucleus is positively charged
Reason: Positive particles were repelled
Conclusions: Electron Cloud
The electron cloud is large
Reason: Most particles passed through.
The electron cloud is not dense
Reason: Particles passed through.
The electron cloud is negatively charged
Reason: Atom is neutral, so e- cloud must balance out charge of the + nucleus.

19. Atomic Structure
Atoms are made of 3 basic particles which have a characteristic charge, mass, and location inside the atom.
Particle
Charge
Mass (g)
Location
Electron -1
9.109 x 10-28
Electron cloud
Proton +1
1.673 x 10-24
Nucleus
Neutron
1.675 x 10-24

20. Atomic Number
Atomic number of an element is the number of protons in the nucleus of each atom of that element.
Element
# of protons
Atomic #
Carbon 6
Phosphorus 15
Gold 79

21. Mass Number Nuclide p+ n0 e- Mass #
Mass number is the number of protons and neutrons in the nucleus of an isotope.
Mass # = p+ + n0
Nuclide p+ n0 e-
Mass #
Oxygen - 10 - 33 42
- 31 15 18 8 8 18
Arsenic 75 33 75
Phosphorus 16 15 31

22. Isotopes Elements occur in nature as mixtures of isotopes.
Isotopes are atoms of the same element that differ in the number of neutrons

23. Composition of the nucleus
Atomic Masses
Atomic mass is the average of all the naturally isotopes of that element.
Carbon =
Isotope
Symbol
Composition of the nucleus
% in nature
Carbon-12
12C
6 protons
6 neutrons
98.89%
Carbon-13
13C
7 neutrons
1.11%
Carbon-14
14C
8 neutrons
<0.01%

24. Isotopes…Again (must be on the test)
Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.
Isotope
Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium) 1
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium) 2

25. The Mole 1 dozen = 12 1 gross = 144 1 ream = 500 1 mole = 6.022 x 1023
THIS IS A CONVERSION FACTOR!!!!!!
There are exactly 12 grams of carbon-12 in one mole of carbon-12.

28. I didn’t discover it. Its just named after me!
Avogadro’s Number
6.022 x 1023 is called “Avogadro’s Number” in honor of the Italian chemist Amadeo Avogadro ( ).
I didn’t discover it. Its just named after me!
Amadeo Avogadro

29. # of atoms or # of molecules
Atomic Conversions
There are two basic steps that can be performed for atomic conversions.
# of atoms or # of molecules
# of moles
Mass = # grams
6.022 x 1023___ = 1 mol
1 mol = atomic mass (from pt)
Conversion Factors

30. Calculations with Moles: Converting moles to grams
How many grams of lithium are in 3.50 moles of
lithium?
1 mol = atomic mass (from pt)
3.50 mol Li
6.94 g Li
= g Li
24.3
1 mol Li

31. Calculations with Moles: Converting grams to moles
How many moles of lithium are in 18.2 grams of
lithium?
1 mol = atomic mass (from pt)
18.2 g Li
1 mol Li
= mol Li
2.62
6.94 g Li

32. Calculations with Moles: Using Avogadro’s Number
How many atoms of lithium are in 3.50 moles of
lithium?
6.022 x 1023___ = 1 mol
3.50 mol Li
6.022 x 1023 atoms Li
= atoms Li
2.11 x 1024
1 mol Li

33. Calculations with Moles: Using Avogadro’s Number
How many atoms of lithium are in 18.2 g of lithium?
# of atoms or
# of molecules
# of moles
# grams
1 mol = atomic mass (from pt)
1 mol Li
18.2 g Li
= mol Li
2.62
6.94 g Li

34. Calculations with Moles: Using Avogadro’s Number- step 2
How many atoms of lithium are in 18.2 g of lithium?
# of atoms or
# of molecules
# of moles
# grams
6.022 x 1023___ = 1 mol
2.62 mol Li
6.022 x 1023 atoms Li
1.58 x 1024
= atoms Li
1 mol Li

35. Calculations with Moles: Converting moles to grams in Compounds
How many grams of CO2 are in 3.50 moles of CO2?
1 mol = atomic mass (from pt)
C = g
O = g
CO2 = g
Just add the masses of all the atoms together
3.50 mol CO2
44.01 g CO2
= g CO2
154
1 mol CO2

36. The Atomic Scale
Most of the mass of the atom is in the nucleus (protons and neutrons)
Electrons are found outside of the nucleus (the electron cloud)
Most of the volume of the atom is empty space
“q” is a particle called a “quark”

37. About Quarks… Protons and neutrons are NOT fundamental particles.
Protons are made of two “up” quarks and one “down” quark.
Neutrons are made of one “up” quark and two “down” quarks.
Quarks are held together
by “gluons”