1. Atomos: Not to Be Cut or “indivisible”
The History of Atomic Theory
Atomos: Not to Be Cut or “indivisible”

2. Objectives Define “atom” Explain Democritus’ model of the atom
List the postulates of Dalton’s atomic theory
Explain what studies of cathode rays revealed about atoms
Explain the results of Rutherford’s gold foil experiment
Evaluate the atomic theories presented by Democritus, Dalton, Thomson, and Rutherford
Construct a model based on each different theory

3. Who are these men?
In this lesson, we’ll learn about the men whose quests for knowledge about the fundamental nature of the universe helped define our views.

4. Democritus
400 BC
Greek philosopher
lived from B.C.

5. Atomos
His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained.
This piece would be indivisible.
He named the smallest piece of matter “atomos,” meaning “not to be cut.”

6. Democritus’ Ideas Atoms are small hard particles.
Made of a single material that’s formed into different shapes and sizes.
They are always moving
They form different materials by joining together.

7. This theory was ignored and forgotten for more than 2000 years!

8. Why?
The eminent philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately wrong) theory.
Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

10. What is an Atom?
PIECES OF CARBON
An atom is the smallest particle that an element can be divided and still be that element.
For example the smallest particle of carbon is a single atom of carbon. If you divide it is no longer carbon anymore.
CARBON ATOM
Carbon atoms as seen through a scanning tunneling microscope

11. Dalton’s Model
In the early 1800s, the English Chemist and Schoolteacher John Dalton proposed a new atomic theory
His theory was based on experiments!

12. Dalton’s Theory 1. All elements are composed of atoms.
2. Atoms are indivisible and indestructible particles.
3. Atoms of the same element are exactly alike.
4. Atoms of different elements are different.
5. Compounds are formed by the joining of atoms of two or more elements.

13. Atoms and Electricity
In 1839, English chemicst Michael Faraday suggested that the structure of atoms was related to electricity
Experiments proved that he was right! Atoms contain particles that have electrical charge.

14. Atoms and Electricity
The word electricity comes from the Greek word “elektron” which meant “amber.”
The greeks knew that if you rubbed amber with a cloth, it would attract dust particles
Today, this is known as static electricity
Static electricity comes from electrical charges that are not in motion

15. Atoms and Electricity
Benjamin Franklin derived several facts about electricity
An object could have one of two kinds of charge, which he called (+) and (-).
Like charges repel and opposite charges attract
Some objects pick up excess negative charges and these charges may jump in a discharge of static electricity.
(walk across a carpet and touch a doorknob)
Lightning is static electricity on a larger scale
Where do these charges come from?

16. Thomson’s Plum Pudding Model
In 1897, the English scientist J.J. Thomson conducted a series of experiments with cathode ray tubes.
Thomson discovered the electron

17. Thomson’s Cathode Ray Experiment Conclusions
Cathode ray is composed of negative particles and these particles come from the cathode
This meant that atoms were not indivisible, but had negative particles which he called electrons
He determined the ratio of an electron’s charge to its mass

18. Thomson Model He proposed the “Plum Pudding” model.
Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

19. Rutherford’s Gold Foil Experiment
1908
Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)

20. Most of the positively charged “bullets” passed right through the gold foil
Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.

22. Rutherford’s Conclusions
The gold atoms in the sheet were mostly empty space. Atoms were not a pudding filled with a positively charged material.
Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged “bullets.”
He called the center of the atom the “nucleus”
The nucleus is tiny compared to the atom as a whole.

23. Rutherford’s Model
All of an atom’s positively charged particles were inside the nucleus.
The negatively charged particles were scattered outside the nucleus around the atom’s edge.

24. Subatomic Particles Particle Symbol Charge Relative Mass
Electron e
Proton p
Neutron n

25. Atomic Number
Counts the number of
protons
in an atom

26. Atomic Number on the Periodic Table11 Na
Atomic Number
Symbol

27. All atoms of an element have the same number of protons11 Na
11 protons
Sodium

28. Number of Electrons An atom is neutral The net charge is zero
Number of protons = Number of electrons
Atomic number = Number of electrons

29. Mass Number Counts the number of protons and neutrons in an atom
*Mass number is not on the periodic table

30. Atomic Symbols atomic number 11 Show the mass number and atomic number
Give the symbol of the element
mass number
23 Na sodium-23
atomic number 11

31. 16 31 65 8 15 30 More Atomic Symbols O P Zn 8 p+ 15 p+ 30 p+
O P Zn
8 p p+ 30 p+
8 n 16 n 35 n
8 e e e-

32. Isotopes
Atoms with the same number of protons, but different numbers of neutrons.
Atoms of the same element (same atomic number) with different mass numbers
Isotopes of chlorine
35Cl 37Cl
chlorine chlorine - 37

33. Learning Check AT 5 16 14 14 An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) ) ) 34
B. Its mass number is
C. The element is
1) Si 2) Ca 3) Se
D. Another isotope of this element is
1) 34X 2) 34X 3) 36X

34. Masses of Atoms
A scale designed for atoms gives their small atomic masses in atomic mass units (amu)
An atom of 12C was assigned an exact mass of amu
Relative masses of all other atoms was determined by comparing each to the mass of 12C
An atom twice as heavy has a mass of amu. An atom half as heavy is 6.00 amu.

35. Calculating Average Atomic Mass
You need to know…
Percent(%) abundance of isotopes
Mass of each isotope
Weighted average =
(mass of isotope 1)(% abundance/100) +(mass of isotope 2)(% abundance/100) + …

36. Atomic Mass of Magnesium
Isotopes Mass of Isotope Abundance
24Mg = 24.0 amu %
25Mg = amu %
26Mg = amu %
(.7870 x 24.0) + (.1013 x 25.0) + (.1117 x 26.0)
Atomic mass (average mass) Mg = 24.3 amu Mg
24.3

37. Learning Check AT7
Gallium is a metallic element found in small lasers used in compact disc players. In a sample of gallium, there is 60.2% of gallium-69 (68.9 amu) atoms and 39.8% of gallium-71 (70.9 amu) atoms. What is the average atomic mass of gallium?

38. Solution AT7 Atomic mass Ga = 69.7 amu
68.9 amu x = amu for 69Ga
100
Ga-71 (%/100)
70.9 amu x = amu for 71Ga
Atomic mass Ga = amu

39. Bohr Model
1913
Niels Bohr placed each electron in a specific energy level.

40. Bohr Model
electrons move in definite orbits around the nucleus, much like planets circle the sun.
These orbits, or energy levels, are located at certain distances from the nucleus.

41. Schrodinger’s Wave Model

42. The Wave Model
In today’s model, electrons do not move in a definite path, like the planets around the sun.

43. The Wave Model
It is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has.
According to the modern atomic model, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.

44. Electron Cloud: A space in which electrons are likely to be found.
Electrons whirl about the nucleus billions of times in one second

45. Electron Cloud:
Electrons with the lowest energy are found in the energy level closest to the nucleus
Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.

46. Greek X Dalton Thomson Rutherford Bohr Wave Indivisible Electron
Nucleus
Orbit
Electron Cloud
Greek X
Dalton
Thomson
Rutherford
Bohr
Wave

47. Current Atomic Theory Atoms are building blocks of elements
Similar atoms in each element
Different from atoms of other elements
Two or more different atoms bond in simple ratios to form compounds