1. The Structure of the Atom
The Atom: Early Theories of Matter
Subatomic Particles and the Nuclear Atom
How atoms differ
Unstable Nuclei and Radioactive Decay

2. In general, scientists in the 1700s believed:
Elements could not be broken down by ordinary means
Compounds formed from elements have different properties than the elements that formed them
Chemical reactions resulted in the transformation of a substance(s) into a new substance

3. John Dalton
In the early 1800s, performed a number of experiments that eventually led to the acceptance of the idea of atoms.

4. Dalton’s Atomic Theory (1808)
Matter is made of atoms
Atoms of one element are identical; atoms of different elements differ in size, mass, etc.
Atoms cannot be subdivided, created or destroyed
Atoms of different elements combine in whole number ratios to form compounds
Atoms are combined, separated, or rearranged during chemical reactions

5. J.J. Thomson (1897)
Deduced that cathode rays were made of negative charged particles
Called them electrons

6. Discovery of the Electron
Discovered with cathode-rays
Objects inside the cathode-ray tube cast shadows
A paddle wheel inside the cathode-ray tube spun and moved from cathode to anode
Cathode rays were deflected away from a negative charge by magnetic field

7. Ernest Rutherford
In 1908, he was hard at work on an experiment that seemed to have little to do with unraveling the mysteries of the atomic structure.

8. Ernest Rutherford (1911) Discovery of the Atomic Nucleus
Gold foil bombarded with alpha particles experiment proved that a powerful force had to exist inside the atom
The force had to occupy a very small space, was very dense, and positively charged
Called it the nucleus
Suggested that electrons
“orbited” the nucleus

9. Rutherford’s Gold Foil Experiment

10. Niels Bohr
In 1913, proposed an improvement. In his model, he placed each electron in a specific energy level.

11. Composition of the Nucleus
Two types of particles
protons (p+) x kg
neutrons (n) x kg
Same number of protons and electrons in a neutral atom
Number of protons differ for each element
Strong nuclear forces hold the atom together

12. Pre-AP 9/13 Pick up the papers from the side table
Turn in your lab report
Take out your periodic table and a clean piece of paper
Today we will learn about isotopes and how to write them in nuclear symbol and hyphen notation. We will also learn how to calculate mass number of an isotope.

13. Atomic Structure

14. Parts of an Atom
Smallest particle of an element that retains its chemical properties
Composed of two regions
Nucleus--dense core containing protons and neutrons
Electron cloud—large area containing the electrons
Three primary subatomic particles are protons (p+), neutrons (n) and electrons (e-)

15. Atomic Number
All atoms made of same basic particles: Protons, electrons, neutrons
Differ in number of each particle
Atomic number-number of protons in the nucleus of an atom
Determines the identity of the atom
Where do you find atomic number?
Above the symbol for the element on the periodic table
If an atom is neutral, what else does atomic # tell you?
Number of electrons

16. So….
Atomic number=number of protons=number electrons (in a neutral atom)
ALL THE SAME NUMBER!!!

17. Isotopes
All atoms of the same element must have same number of protons (atomic #) and electrons (if neutral)
Can have different number of neutrons.
This gives them different masses.
Called isotopes of that element (or nuclides)
Example: hydrogen

18. Hydrogen How many protons? How many electrons?1
How many electrons?
3 different isotopes of hydrogen:
Protium (0 neutrons)
Deuterium (1 neutron)
Tritium (2 neutrons)

19. Mass number=Mass of NUCLEUS
Sum of the protons and neutrons in the nucleus of an atom (masss #=p + n). NOT ON PERIODIC TABLE! Can round atomic mass to estimate.
Used to identify an isotope
Example
An isotope of what element has an atomic number of 17 and a mass number of 40?
Chlorine
How many of each subatomic particle is found in this atom?
17 protons, 17 electrons, 23 neutrons (40-17)

20. Using the periodic table, answer the following questions:
What information does the atomic number tell you about an atom of a specific element?
How many protons & electrons would a neutral phosphorus atoms have?
What information does the mass number of an atom tell you?
If a nitrogen isotope has a mass number of 14, how many of each subatomic particle would the atom have?

21. Designating Isotopes: Hyphen Notation
Write the mass number after the name of the element.
Protium=hydrogen-1 (H-1)
What is the hyphen notation for the following elements?
Lithium that has 4 neutrons
Lithium-7 (Li-7)
A neutral element with 6 electrons and 8 neutrons.
Carbon-14 (C-14)

22. Designating Isotopes: Nuclear Symbol
Uses both the atomic number and the mass number with the symbol of the element
Place the mass number as a superscript and the atomic number as a subscript to the left of the symbol.
Protium:
Hint: When you subtract the top from the bottom, you get the # of neutrons. Should always be positive!

23. Pre-AP 9/15
Today you will need the papers from the side table, a calculator, and the paper you received you on Tuesday.
WE will practice more isotope problems today and then learn to calculate average atomic mass of an isotope.
We will also perform a lab if time permits.
HW: I will pass this out at the end of the period. It is due tomorrow.

24. Answer the following on a clean piece of paper:
1. Write the nuclear symbol notation and the hyphen notation for the following:
a. Nickel isotope that has a mass number of 50.
b. Calcium isotope that has 10 neutrons.
c. Isotope of an element with 4 protons, 4 electrons, and 2 neutrons.
2. Which of Daltons postulates are incorrect? Why?
3. What is the mass number? What particles contribute to this number?
4.Describe Bohrs model.
5.How do the isotopes of carbon-12 and carbon-14 differ?
6. What type of radiation was used in Rutherfords experiment??

25. Using the periodic table, answer the following questions:
If a nitrogen isotope has a mass number of 14, how many of each subatomic particle would the atom have?
Write the hyphen and nuclear symbol notation for this nitrogen isotope.

26. You have 100 marbles, 25% have a mass of 2 g and 75% have a mass of 3g
Average Atomic Mass
Weighted average of atomic
masses of naturally occurring
isotopes
EXAMPLE:
You have 100 marbles, 25% have a mass of 2 g and 75% have a mass of 3g
CALCULATE:
(.25 x 2g) + (.75 x 3g) = 2.75g, which is the average mass of all the marbles

27. Example: Grades are weighted averages…
You are in a level chemistry class and you currently have a 85 for your summative average (60%) and a 54 for your formative average (40%). What is your grade?

28. Example: Grades are weighted averages…
You are in a pre-AP chemistry class and you currently have a 85 for your summative average (70%) and a 54 for your formative average (30%). What is your grade?

29. Average Atomic Mass Example
Oxygen has 3 naturally occurring isotopes. Oxygen-16 (15.99 amu) accounts for %, oxygen-17 (17.00 amu) for 0.038%, and oxygen-18 (18.00 amu) for %. Calculate the average atomic mass for oxygen.
Avg. AM=(15.99 amu)( ) + (17.00 amu)( ) + (18.00 amu)( )= amu
Round to 3 places past decimal  amu

30. Example…
Look at number one on your worksheet and try to calculate average atomic mass for the isotopes on your own.