1. To describe the Democritus model of the atom
Linked 2
Linked 1
To describe the Democritus model of the atom
Democritus ((& Leucippus)) ~ 400 BC
Early philosophers thought that the material world must be made up of tiny indivisible particles called atomos, (meaning indivisible or uncuttable) that move through space
Later Democritus could not answer questions from Aristotle regarding his notion of empty space and the “atomic” view of matter faded away for centuries
What is matter ?
Pg 87-91

2. To describe the Dalton model of the atom
Scientists studying gases…you can feel the wind, thus air must be composed of invisible particles of air
John Dalton - Devised an
atomic theory based on the
Greek idea of “atomos”
Atom-smallest particles of an
element that retain the
chemical identity of the element
To describe the Dalton model of the atom
Pg38-39
Linked

3. Dalton’s Atomic Theory
Each element is composed of extremely small indivisible particles called atoms
All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.
Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.
Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kinds of atoms.

4. total massbefore = total massafter
Dalton’s Theory Explained several simple laws of chemical combination known at the time
Law of constant composition (definite proportions)– in a compound the relative numbers and kinds of atoms are constant (pure H2O is 11% H and 89% O by mass)
Law of conservation of matter – matter is neither created nor destroyed
total massbefore = total massafter

5. Law of Multiple Proportions
if 2 elements A&B combine to form more than one compound, different masses of B combine with the same mass of A in the ratio of small whole numbers
There is twice as much oxygen by mass in H2O2 as there is in H2O
Pg38-39

6. A problem with Dalton’s Theory
Are atoms “really” the smallest particles we know?
Are atoms “really” indivisible?
What are atoms made up of?
Are atoms of the same elements “really” identical?

7. Cathode Rays an Electrons
To describe the Thomson model of the atom
To state relative charge & mass of electron and proton
J J Thompson

8. Cathode Rays
Pg 92-94
Partially evacuated tubes produced radiation under high voltage
Called cathode rays = initiated from cathode side
Rays could not be seen but made materials fluoresce

9. Cathode Rays
Experiments showed cathode rays are deflected by electric or magnetic fields
Thomson found that cathode rays are the same
regardless of what material the cathode is made of
How do these findings help us answer the question…are cathode rays radiation (energy) or particles?
Link
Link 1

10. Electrons
Cathode rays deflected by “field” = must be electrical particle not pure energy
Deflected TOWARD positive field = must be negative particle
Rays were the same no matter what substance cathode was made of = must be a fundamental particle (all the same)

11. “So how do all these particles that make up matter go together?
Pg42
JJ Thomson’s “Plum Pudding” – Atomic Model
Small negatively charged electrons embedded in a positively charged atom
Like seeds (negative electrons) embedded in a watermelon (positive atom)

12. Finding the Center “Ernest Rutherford and Gold Foil”
To describe the Rutherford model of the atom
To state relative charge & mass of electron, proton and neutron
“Ernest Rutherford and Gold Foil”

13. Radioactivity & Rutherford
Pg41-42
Revealed three types of radiation:
alpha (α) -
beta (β) -
gamma (γ) –
How does each type respond to electric/magnetic field? And what does that imply about it?

14. Rutherford’s Gold Foil Experiment
Pg42
Rutherford’s Gold Foil Experiment
almost all α particles passed directly through the foil without deflection
a small % slightly deflected, on the order of 1 degree, consistent with Thomson’s plum-pudding model
                                                                                                     

15. Rutherford’s Gold Foil Experiment
Pg42
BUT eventually … a small amount of scattering was observed at large angles & some particles were even scattered back in the direction from which they had come
What implications
does this finding
have about the atom?
Linked

16. Rutherford’s Atom LiNkeD
Pg42
most of the mass of each atom and all of its positive charge reside in a very small, extremely dense region
Rutherford called this region the nucleus
LiNkeD

17. Pg42-43
Protons
LiNkeD
Subsequent experimental studies led to the discovery of positive particles (called protons) in the nucleus.
Protons were discovered in 1919 by Rutherford.
HOWEVER, when the nuclear mass was computed based on the number of protons, the mass value was much less than the actual mass…
What does this then imply about the atom???

18. The Neutron
Pg42-43
…and subsequent experimental studies led to neutral particles (called neutrons) in the nucleus.
Neutrons were discovered in 1932 by the British scientist James Chadwick (1891–1972).

19. “Planetary” Model
Pg43
Every atom has an equal number of electrons and protons, and so atoms have no net electrical charge.
The electrons are attracted to the protons in the nucleus by the force that exists between particles of opposite electrical charge.
The vast majority of an atom’s volume is the space in which the electrons reside.
LinKed

20. Rutherford’s Paradox Pg
You cannot simply explain electron as “orbiting” the nucleus…because (as per classical physics)
the e- will loose energy and be more attracted toward nucleus

21. Properties of Particles
Pg43-44
Link
charge of electron is −1.602 × 10−19 C and that of a proton is × 10 −19 C (charges of particles are expressed as multiples of this charge)
mass of atoms very small so instead of grams we use amu (“atomic mass units”) (1 amu = × 10−24 g )
Atoms are extremely small (SI unit angstrom; Å is used) (1 Å = m)

22. Atomic Notation To draw a diagram of an atom given its atomic notation
Pg45
To draw a diagram of an atom given its atomic notation
To explain & illustrate the concept of isotopes

23. Practice p+: p+: p+: n0: n0: n0: e-: e-: e-:
Pg45
How many protons , neutrons and electrons are there in each?
p+: p+: p+:
n0: n0: n0:
e-: e-: e-:

24. Another Problem with Dalton’s Theory
Pg45-46
Most elements are uniform mixtures of 2 or more unique substances called isotopes.
Isotopes of an element have very similar chemical properties but their atoms have slightly different masses
What about the different atoms, of the same element, could make their masses different???
Link 1
Link 2
Link 3

25. Atomic Mass Scale
Pg46-48
Masses of atoms are measured in atomic mass units (amu)
1 amu = x10-24g
1g = x1023amu
Defined by assigning a mass of exactly 12amu to an atom of the 12C isotope of carbon.

26. Average Atomic Weight
Pg47-48
Every sample of an element has the same isotopic composition (the average mass per atom is the same from sample to sample)
So…we can use the masses of each isotope and its relative abundance to determine the average atomic weight for the element!

27. Pg47-48
Average Atomic Weight
To calculate the atomic weight for an element given the mass and abundance the naturally occurring isotopes.
Naturally occurring chlorine is 75.78% 35Cl which has an atomic mass of amu, and 24.22% 37Cl, which has an atomic mass of amu. Calculate the average atomic mass (that is, the atomic weight) of chlorine.
Solution The average atomic mass is found by multiplying the abundance of each isotope by its atomic mass and summing these products. Because 75.78% = and 24.22% = we have
Average atomic mass = (0.7578)( amu) + (0.2422)( amu)
= amu amu
= amu
This answer makes sense: The average atomic mass of Cl is between the masses of the two isotopes and is closer to the value of 35Cl which is the more abundant isotope

28. Practice
Example: Copper occurs naturally as two isotopes Cu-63 and Cu-65. Given the atomic mass and the % natural abundance of each isotope below, calculate atomic weight of Cu (as on the periodic table).
Isotope mass natural abundance
Cu amu %
Cu amu %

29. To state the properties of alpha, beta and gamma radiation
Radioactivity – spontaneous emission of radiation (high energy & particles)
Pg41-42
To state the properties of alpha, beta and gamma radiation
In a nuclear reaction, protons and neutrons are rearranged
In a chemical reaction, electrons rearranged
Often, at least one isotope is unstable – the strong force of the nucleus is overcome by positive-positive repulsion of the+ protons

30. Radioactive Decay - breakdown of atom
Nucleus becomes unstable
Radiation is emitted until nucleus becomes stable
Stable, non-radioactive atom is formed
Can occur naturally, or be caused by bombarding the atom with energy
LinKeD

31. 3 Main Types of Radiation
Particles
Charge
Mass
Stopped By
Alpha
2 p+
2 no
no e- +2 4
Paper or light clothing
Beta
1 e-
no p+
no no -1
Heavy Clothing, Sunscreen or Lead (1cm thick)
Gamma
None
Concrete or Lead (10cm thick)

32. Penetrating Ability

33. HALF-LIFE is the is the time that it takes for 1/2 a sample to decompose
The rate of a nuclear transformation depends only on the “reactant” (original compound) concentration.
For each duration (half-life), For each duration (half-life), one half of the substance decomposes
To relate the amount of radioactive sample, or its radioactivity, to a given half-life.

34. For example
Ra-234 has a half-life of 3.6 days; if you start with 50 grams of Ra-234, how much do you have …
After 3.6 days :
After 7.2 days :
After 10.8 days :