1. Chemistry Chapter 4
Atoms:
The Building
Blocks of Matter

2. 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

3. Proust’s Law of Definite Proportions
A given compound always contains
the same proportions (by mass)
of elements regardless of the
source
For example water H2O must
Always have 8 g oxygen
Combining with 1 g Hydrogen
Hydrogen Peroxide H2O2 must
Have 16 g Oxygen combining with
1 g Hydrogen

4. Law of Multiple Proportions
In compounds atoms combine in simple whole number
ratios. The mass ratios will be different for
different compounds

5. Law of Definite Proportions
H2O
2 atoms H to 1 atom O
2 gram H to 8 gram O
20% Hydrogen by mass and
80% Oxygen by mass
Law of Multiple Proportions
H2O
2 atoms H to 1 atom O
2 gram H to 8 gram O
H2O2
2 atoms H to 2 atoms O
2 gram H to 32 grams O

6. If it takes 1. 000 g of S for every 1
If it takes g of S for every g F to make a compound SF what must be the mass ratio
to make the compound SF2 ?
A: g S to g F
b) What must be the mass ratio for SF3 ?
A: g S to g F
If it takes g N to combine with g O to form NO, what is the mass ratio for NO2 ?
A: g N to g O

7. It takes 24. 000g C to combine with 4
It takes g C to combine with 4.000g H to form C2H4 what must be the mass of Hydrogen to combine with g Carbon?
A; g H
Hydrogen sulfide (H2S) is composed of two elements: hydrogen and sulfur. In an experiment, g of hydrogen sulfide is fully decomposed into its elements. If g of hydrogen is obtained in this experiment, how many grams of sulfur must be obtained?
What law of chemical behavior does this experiment demonstrate?
A: 6.500g g= 6.116g S
Law of conservation of mass

8. Assume that we used 5. 000g of Iron to heat up with 10
Assume that we used 5.000g of Iron to heat up with g sulfur to make our pyrite (Fool’S gold). What is the mass of pyrite made?
A; grams pyrite
What law does this illustrate?
Law of conservation of mass

10. 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; atoms of different elements differ in size, mass, and other properties
John Dalton
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

11. Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce that the electrons are the subatomic negatively charged particles carrying electricity in a cathode ray tube.
Thompson measured the mass of cathode rays, showing they were made of particles that were around 1800 times lighter than the lightest atom, Hydrogen
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

12. Some Modern Cathode Ray Tubes

13. Conclusions from the Study of the Electron
Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.
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

14. 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.

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

16. 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?

17. The Answers
Target #1
Target #2

18. Rutherford’s Findings
Most of the particles passed right through
A few particles were deflected
VERY FEW were greatly deflected
“Like bullets bouncing off of tissue paper!”
Conclusions:
The nucleus size is small compared to
the atomic size
The nucleus is dense
The nucleus is positively charged

19. 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”

20. Atomic Mass Scale 1 atomic mass unit (u) has a mass of 1/12 of the
mass of carbon-12 atom or *10-24g
(1g= *1023 u)
Particle
Charge
Mass (u)
Proton
Positive
(1+)
1.0073
Neutron
None
(neutral)
1.0087
Electron
Negative
(1-)
5.486*10-4

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

22. 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

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

24. Charged atoms and is not Included for neutral atoms
THERE ARE TWO WAYS TO REPRESENT THE
DIFFERENT TYPES OF ISOTOPES
Mass number
Or H-1
Atomic number
Mass number
Charge
(is included for ions
Charged atoms and is not
Included for neutral atoms

25. An atom becomes an ion when it loses or gains electrons
An atom becomes an ion when it loses or gains electrons. Now the electrons and protons are note balanced.
-3e-
13p
13p
13p
13e
10e

26. 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; atoms of different elements differ in size, mass, and other properties
Modern theory states:
Atoms of an element have a characteristic average mass which is unique to that element.

27. Average Atomic Mass (Atomic Weight)
Average atomic mass is the average of all the naturally isotopes of that element
Atomic mass= ∑ (mass number ∙ % abundance)

28. Composition of the nucleus
Example
Find the average atomic mass of 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%
AAM= (.9889*12)+ (.0111*13)+ (.0001*14)=
AAM (Carbon)=

29. 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!

30. Mass spectrometer
The MS works by ionizing chemical compounds to generate charged molecules or molecule fragments and measurement of their mass to charge ratios