1. How do you discover and study something you can’t see?
Atomic Structure
How do you discover and study something you can’t see?

2. What is a theory?
A hypothesis is a proposed explanation made as a starting point for further investigation (It’s bright outside because the sun is probably out)
A theory is an explanation confirmed through repeated observation and experimentation (When the sun is out, it tends to be bright outside)
A law is a fact of nature that is observed so often it is accepted as truth.
Theories EXPLAIN laws.
Theory vs. Law- Ted Ed

3. Atomic Theory What is it??
The idea that matter is made up of atoms, the smallest pieces of matter.

4. The atom is the smallest particle of an element that retains the properties of that element.
A scanning tunneling microscope allows individual atoms to be seen.
Imagine you could increase the size of an atom to make it as big as an orange. At this new scale, an orange would be as big as Earth.
The Atom

5. History of the Atom
As a class we will be creating a timeline of the discovery of the atom…
Each group will be responsible for one of the following scientists: Democritus Rutherford Bohr JJ Thomson Schrodinger/Heisenberg (quantum mechanics) Chadwick Dalton
Your group will research the following information and present it to the class:
Date – What was the time or year of the scientist’s work concerning the atom?  
Observations/Experiments/Evidence – Describe or explain what observation, evidence, or experiment the scientist used to devise his theory of the atom.
Discoveries/Conclusions – Explain what discovery or conclusion the scientist came to about the atom. What was his atomic theory?
Contribution – Explain how the scientist helped in the development of atomic theory. How did his work contribute to the theory? What did his experiment/discovery/conclusion do for the theory? Think about the larger picture and how the theory progressed as a result of his work.
Be able to draw a diagram of the scientist’s model of the atom

6. History of Atomic Theory
Greek philosophers believed all matter was made up of four basic elements: fire, earth, water, and air.
And that matter could be endlessly divided into smaller and smaller pieces

7. History of Atomic Theory
Democritus: 460 – 370 B.C.
Proposed the idea that matter was not infinitely divisible, but made of individual particles called “atomos”.
Aristotle ( B.C.) disagreed because he did not believe empty space could exist. His views
went unchallenged for 2000 years.

8. History of Atomic Theory
John Dalton revived the idea of the atom in the early 1800’s with his “Atomic Theory of Matter”
Which parts are still considered true? Which are not considered true?

9. Law of conservation of mass
John Dalton also easily explained the law of conservation of mass (which was proposed by Lavoisier) in a chemical reaction as the result of the combination, separation or rearrangement of atoms.
Law of conservation of mass

10. Law of Constant composition
Dalton’s theory also helps explain a law proposed by
Proust in the early 1800’s. The law of constant
composition states that the elemental composition
of a compound is always the same.
ex: water is always 11% hydrogen and 89%
oxygen (by mass)
Law of Constant composition

11. Discovery of Atomic Structure
JJ Thomson (1897)
Cathode Ray Tube experiment _ +
Thomson released cathode rays
from one side of the tube and they
were attracted to the positive plate.
Discovered first subatomic particle: the Electron.
What attracts to a positive charge?

12. JJ Thomson Video

13. Discovery of Atomic Structure
Rutherford (1895)
Gold foil experiment
Most particles pass
through, but some are
bounced back towards the
source.
Most particles pass through, but some are bounced back towards the source.
Discovered a positively charged nucleus and that atoms are mostly empty space.

14. Plum pudding atom
Nuclear atom
Alpha particles

15. Discovery of Atomic Structure
Later Rutherford (1919) refined his model to include the positively charged particles in the nucleus called protons.
Chadwick (1932) noticed the dense nucleus had a particle that was not charged!
Discovered the neutron

16. Rutherford and Chadwick Video

17. Bohr (early 1900s)
proposed the “electron cloud” in which electrons orbit at a given distance from the nucleus.
Small orbits = low energy
Big orbits = high energy

18. Modern Atomic Theory (quantum mechanical model)

19. Structure of the Atom Nucleus: dense, central part of the atom.
Protons and neutrons are found in the nucleus.
Electron cloud: large area outside of the nucleus.
Electrons occupy the electron cloud in orbitals.

20. Parts of the Atom Subatomic Particle Location Charge Relative Mass
Proton
Nucleus 1+
X C 1
1.67x10-24 g
Neutron
Electron
e- cloud 1-
X C
1/1840
9.11x10-28 g

21. What do all the numbers mean on the PT?
Atomic Number
- # of protons in an atom. It is also equal to the number of electrons in a neutral atom.
Atomic Mass – the decimal number on the periodic table. The average mass of all isotopes of that element.
The atomic number determines an element’s identity!

22. What about Neutrons? The mass of the atom comes from the nucleus.
Each proton has a mass of 1 amu (atomic mass unit)
Each neutron has a mass of 1 amu.
Add the number of protons and neutrons to find the mass number of the atom!

23. Isotopes
What are they?
atoms of the same element that have different mass numbers.
This means the number of ________ is the same, and the number of _________ is different.

24. 6 Neutrons 8 Neutrons ISOTOPES OF CARBON Carbon-12 Mass Number = 12
Atomic Number = 6
Neutrons = Mass – Protons
Carbon-14
Mass Number = 14
Atomic Number = 6
Neutrons = Mass - Protons
6 Neutrons
8 Neutrons
Protons
Neutrons

26. 2 ways to depict isotopes
Hyphen Notation: C-14
Super/Sub Script:

27. Element
Atomic #
Mass #
# p+
# n0
# e-
Super/sub method
Hyphen method
Ruthenium 78
117 32 41
Scandium

28. Practice:
1. What is the atomic number of zinc? _______
2. How many electrons does silver have? _______
3. How many protons does iodine have? _______
4. How many neutrons does Fe-53 have? _______
5. What is the mass number of Chlorine-37? _______
6. How many electrons does Chlorine-37 have? _______
7. What is the mass number of tritium? _______
8. How many neutrons does neon-20 have? _______
9. How many neutrons does deuterium have? _________

29. ATOMS ISOTOPES IONS Differ by number of protons
Differ by number of neutrons
IONS
Differ by number of electrons
 Differ by number of electrons
 ISOTOPES
 Differ by number of neutrons

30. How is the mass of atoms measured?
Atoms can’t easily be measured in grams because they are so small.
Scientists devised “atomic mass units” (a carbon-12 isotope is amu’s)
What is the mass, in amu’s, of Hydrogen-1?
You would think it was … right?!
It’s actually a little different.

31. Average atomic mass is a different kind of average – a “weighted” average. – think about your six weeks average.
This means that we take into account the abundance of each isotope found in nature.
Saying that Boron’s average mass is 10.5 would be misleading since there’s so much more Boron-11.

32. Average Atomic Mass
The average of the masses of the naturally occurring isotopes weighted for their abundance in nature. (weighted average)
Using this formula will ensure the correct number of sig figs…
(mass)(abundance/100)=
(mass)(abundance/100)= +
Is the AAM on the Periodic Table?

33. Calculating Average Atomic Mass
1. Argon has three isotopes with the following percent abundances: Ar-36 with a mass of amu and an abundance of %. Ar-38 with a mass of amu and an abundance of %. Ar-40 with a mass of amu and an abundance of %. Calculate the average atomic mass, but first write down what you expect the average atomic mass to be close to!

34. 3. The atomic weight of gallium is 69. 72 amu
3. The atomic weight of gallium is amu. The masses of naturally occurring isotopes are amu for Ga-69 and amu for Ga-71. Calculate the percent abundance of each isotope.

35. The atomic weight of gallium is 69. 72 amu
The atomic weight of gallium is amu. The masses of naturally occurring isotopes are amu for Ga-69 and amu for Ga-71. Calculate the percent abundance of each isotope.
 [(68.92)(Ga-69 abundance) + (70.92)(Ga-71 abundance)]/100 = 69.72
 We have 2 unknowns – we need another equation.
 Can you think of a mathematical relationship between the two
abundances????
 HINT – What should the abundances add up to?

36. Answer – Abundance of Ga-71 is 40% and abundance of Ga-69 is 60%.