1. Holt McDougal Chapter 4 Section 2
Atoms

2. The Development of Atomic Theory

3. NGSSS: SC912P8.4
Explore the scientific theory of atoms (also known as atomic theory) by describing the structure of atoms in terms of protons, neutrons and electrons, and differentiate among these particles in terms of their mass, electrical charges and locations within the atom

4. Questions you should be able to answer by the end:
What scientists helped to develop atomic theory?
What part of atoms did Thomson discover?
What part of atoms did Rutherford discover?

5. Origins of Atomic Theory
Democritus
Greek philosopher in 4th century BCE
Tiny, indivisible units and void
Atomos  unable to be cut or divided
No evidence; all conjecture and theory

6. Dalton’s Contribution
1808, British schoolteacher John Dalton performed experiments
Atoms DO exist!
Atoms could form compounds
Law of Definite Proportions

7. What’s the Answer?
How was Dalton’s theory different from that of Democritus?
Carbon makes up 27% of the mass of CO2. How many grams of C are there in 88g of CO2?

8. Thomson Contributes to the Development of Atomic Theory
1897, British scientist J.J. Thomson working with cathode rays
Discovered electrons, negatively charged particles
Plum
Pudding by another Thomson called Lord Calvin

9. Can you Explain? What was the Greek perspective of atoms?
How did Dalton support and detract from the Greek perspective of atoms?
How did JJ Thomson discover electrons?

10. Rutherford Breaks Away from the Pack!
Was a student of Thomson
Tested the idea that the mass of an atom was spread evenly over its volume
Gold Foil experiment
Bounce back indicates
a concentrated center
How would test that idea?

11. Gestation of The Planetary Model Begins . . .
Rutherford concluded that atom’s positive charge is concentrated at the center
Nucleus  dense positively charged core of atom
So tiny that like a marble in a football stadium
Negative electrons orbit the nucleus

12. Rutherford’s Atomic Model
No specific orbits
Only established the existence of the positive nucleus

13. Who did what?
Scientist
Contribution
First proposed that the universe is made up of tiny, indivisible units called atoms
Carried out scientific experiments that showed that atoms exist
Showed that atoms contain smaller particles called electrons
Showed that the positive charge in an atom is concentrated in a nucleus at its center

14. The Structure of Atoms

15. NGSSS: SC912P8.4
Explore the scientific theory of atoms (also known as atomic theory) by describing the structure of atoms in terms of protons, neutrons and electrons, and differentiate among these particles in terms of their ass, electrical charges and locations within the atom

16. By the end of the lesson, you should be able to answer:
What do atoms of the same element have in common?
What are isotopes?
How is an element’s average atomic mass calculated?

17. Who did what? Rutherford Dalton Thomson Democritus
Scientist
Contribution
First proposed that the universe is made up of tiny, indivisible units called atoms
Carried out scientific experiments that showed that atoms exist
Showed that atoms contain smaller particles called electrons
Showed that the positive charge in an atom is concentrated in a nucleus at its center
Democritus
Dalton
Thomson
Rutherford

18. Bohr and Chadwick: The next steps in Atomic Theory Development
1913, Danish physicist Neils Bohr
Fixed orbits for the electrons
James Chadwick, Bohr’s student
Found the neutron in the nucleus
Led to discovery of fission and the atomic bomb

19. Electron Cloud Model Established by 1926
Area where electrons are most likely to be found
100,000 times larger than the diameter of the nucleus
Impossible to accurately describe path and location at any given time
Orbital  a region in which an electron is most likely to be

20. What is Inside an Atom Particle Charge Mass (kg) Location Proton +1
1.67 x
Inside the nucleus
Neutron
Electron -1
9.11 x
Outside the nucleus

21. What Do Atoms of the Same Element Have in Common?
The number of protons is the same!!!
# of protons identify the atom and element

22. Balancing Charge with Electrons
Protons  positive
Electrons  negative
1 proton + 1 electron = neutral charge
Gain/lose electron = ion (ionized atom)
Ions are NOT neutral

23. The Electric Force Opposites attract
Protons and electrons attract each other
Farther apart = less attraction

24. What is an Atomic Number?
Atoms of same element = same # protons
Atomic # = # of protons
Neutral atoms means the atomic # = protons = electrons (i.e., net charge = zero)

25. Practice Atomic Number
Find the atomic number and name for the following elements: H Sc Ta Bh F Sn U
Hydrogen  1
Scandium 21
Tatalum  73
Bohrium  107
Flourine  9
Tin  50
Uranium  92
How many electrons does each of the listed elements have?

26. Atomic Mass Number Most atoms have protons and neutrons
Atomic Mass # = protons + neutrons
Atomic Mass = Atomic Number + neutrons
How can atoms of the same element have different mass #’s when they all MUST have the same atomic #?

27. Reminding Yourself
Which two types of subatomic particles are located in the nucleus?
How does the mass of an electron compare with the mass of a proton?
Why are atoms neutral?
What force holds atoms together?

28. Isotopes Atoms of an element with different mass #’s
Remember, the AN (# of protons) is ALWAYS the same
# of neutrons is different from one isotope to another

29. Let’s Peer at H Isotopes . . .
Protium
Deuterium
Tritium
What is the atomic number, atomic mass, and electrical charge for each?

30. How Common Are Different Isotopes?
Some more than others (e.g., O has AM of 16 usually; about 0.2% have mass of 18)
Some isotopes are unstable and decay
How can decayed isotopes form new/different isotopes of the same element (even though they are breaking apart)?

31. How Can You Show an Atom’s Atomic and Mass NumbersCl
Chemical symbol represents the element
AMN is written on top
AN is written on bottom
How can you compute the number of neutrons?
35 17

32. More About Atomic Mass
Too tiny  1 atom of F has mass (kg) of less than one trillionth of one billionth of one gram
What do you think that number looks like? Write it down and compare with your neighbor 1
1,000,000,000,000,000 X
1,000,000,000,000

33. Unified Atomic Mass Unit
Describes the masses of atoms
SI standard of measure for atoms
A AMU = 1/12 mass of C-12 atom
Mass number = protons + neutrons (i.e., whole number)
Atomic mass = mass of single atom of element (i.e., usually a decimal)

34. How’s Your Memory? What is the AN of most atoms of O?
What is the AMN of most atoms of O?
How many protons are in the nucleus of an atom of Cl?
What unit do scientist use to measure the masses of atoms?

35. The Behavior of Electrons