1. Chapter 4: The Atom - - - + + + + - - + + - - -

2. Do Theories in Science Stay the Same?
Ideas and theories in science change as new information is gathered.
Our theory about the atom has changed over time as new studies are done. Even though no one has ever seen an atom up close we are still able to make new discoveries – just like we have made new discoveries about dinosaurs.

3. What do Dinosaurs and Atoms have in Common?
No one has seen an atom or a dinosaur directly. We know of their existence only by indirect evidence. Our theories of both dinosaurs and atoms has changed over time based on this indirect evidence.

4. Who was Democritus?
Democritus was an ancient Greek philosopher who lived from B.C.
What did Democritus conclude about cutting matter in half? There was a limit to how far you could divide matter. You would eventually end up with a piece of matter that could not be cut.
He thought matter is like motion. It cannot be divided in half forever. The tortoise and hare would never finish the race if you could keep dividing the distance to the finish line in half forever.

5. What does the Greek word atomos mean?
The Greek word “atomos” means not able to be divided or “indivisible.”

6. What did Democritus propose about the atom?
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.
(Which of these statements do we now know are correct? Was Democritus pretty smart for someone who lived thousands of years ago?)

7. Why weren’t Democritus’s ideas accepted?
Aristotle was a very famous Greek philosopher who believed that matter could be divided into smaller and smaller pieces forever. He held a very strong influence on popular belief and his views on this were accepted for two thousand years.

8. 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

9. John Dalton
Two thousand years later a British chemist and schoolteacher brings back Democritus’s idea of the atom
He performed many experiments to study how elements join together to form new substances
He found that they combine in specific ratios and he supposed it was because the elements are made of atoms.

10. What 3 new ideas did John Dalton propose about the atom?
All substances are made up of atoms which are small particles that cannot be created, divided, or destroyed.
Atoms of the same element are exactly alike and atoms of different elements are different.
Atoms join with other atoms to form different substances

11. J.J. Thomson
What particle did Thomson discover? J.J. Thomson discovered that atoms are made of smaller negatively-charged particles called electrons.
Thomson’s discovery was the result of doing experiments with “cathode ray tubes”

12. Thomson’s Cathode Ray Experiment
Stream of electrons is attracted to positively charged plate here.
"What are these particles? are they atoms, or molecules, or matter in a still finer state of subdivision?“ quote by Thomson

13. The Plum Pudding Model
Thomson did not know how the electrons in an atom were arranged. He believed they were mixed throughout an atom.
He proposed that the atom was a sphere of positively charged material. Spread throughout the atom were the negatively charged electrons similar to plums in a pudding or chocolate chips in ice cream.

14. Ernest Rutherford ( )
Awarded the Nobel Prize in Chemistry for his discovery of alpha particles, positively charged particles emitted from radioactive elements
Was a student of J.J. Thomson but disagreed with the “Plum Pudding Model”
Devised an experiment to investigate the structure of positive and negative charges in the atom.

16. An Interactive Model of Rutherford’s Gold Foil Experiment

17. What did most of the particles shot at the gold foil do?
Most of the particles traveled straight through the gold foil
What was the surprising behavior of a few of the particles?
A few of the particles were deflected and some even bounced back

18. Rutherford’s Revised Atomic Theory (1911)
Result: Most of the positively charged particles went straight through the gold foil.
Atomic Theory: Most of the matter of the atom is found in a very small part of the atom. This is called the nucleus of the atom. It is very tiny and extremely dense.
Result: Some of the positively charged particles were deflected or even
bounced back.
Atomic Theory: Like charges repel so the nucleus must have a positive charge. If electrons have a negative charge they could not be in a positively charged nucleus. Electrons must surround the nucleus at a distance.
Result: The diameter of the nucleus is 100,000 times smaller than the diameter of the entire gold atom.
Atomic Theory: Atoms are mostly empty space with a tiny, massive nucleus at the center .

19. Why is the head of a pin compared to the diameter of a stadium like an atom?
The diameter of a pinhead is 100,000 times smaller than the diameter of a stadium. Likewise the diameter of the nucleus of an atom is 100,000 times smaller than the diameter of an atom

20. The Bohr Model of the Atom What did Bohr learn about electron movement?
Bohr proposed that electrons move in paths at certain distances around the nucleus.
Electrons can jump from a path on one level to a path on another level.

21. The Modern Theory of the Atom
Electrons travel in regions called “electron clouds”
You cannot predict exactly where an electron will be found

22. Energy Levels
The energy that an electron has is based on its location around the nucleus. (Electrons that are closer to the nucleus have less energy than those that are farther away from the nucleus)

23. How can bookshelves help you understand the movement of electrons?
Each shelf represents an energy level
Each book represents an electron
You can move a book to a higher or lower shelf with the correct amount of energy.
A book cannot be between shelves
(An electron can move by gaining or losing energy but can never be between energy levels)

24. How small are atoms?
THERE ARE 2 X 1022 ATOMS IN A PENNY. If all the atoms in a penny were blown up to the size of a grain of sand they would cover the entire state of California

25. What can a scanning tunneling electron microscope show us?
These images do not show an actual picture of an atom. They show a color-enhanced image of the surface of a material at the atomic level.

26. Matter Anything that has mass and takes up space (volume) Examples:
A brick has mass and takes up space
A desk has mass and takes up space
A pencil has mass and takes up space
Air has mass and takes up space
All of the above examples are considered matter because they have mass and take up space. Can you think of anything that would not be considered matter?

27. Atoms
Smallest possible unit into which matter can be divided, while still maintaining its properties.
Made up of:
protons
neutrons
electrons
The solar system is commonly used as an analogy to describe the structure of an atom - +
For example, what is the
smallest possible unit
into which a long essay can be
divided and still have some meaning? - + + - + + - -

28. Atoms are so small that…
it would take a stack of about 50,000 aluminum atoms to equal the thickness of a sheet of aluminum foil from your kitchen.
if you could enlarge a penny until it was as wide as the US, each of its atoms would be only about 3 cm in diameter – about the size of a ping-pong ball
a human hair is about 1 million carbon atoms wide.
a typical human cell contains roughly 1 trillion atoms.
a speck of dust might contain 3x1012 (3 trillion) atoms.
it would take you around 500 years to count the number of atoms in a grain of salt.
C-C-C-C-C-… + 999,995 more
1 trillion atoms  .
Is made of approximately 3 trillion atoms
Just one of these grains

29. Let’s Experiment
In order to try to gain an idea of how small an atom really is, you will complete the following activity.
Cut a strip of 11 in. paper in half.
Discard one half.
Cut the remaining piece in half.
Continue cutting and discarding the strips as many times as you can.
Make all cuts parallel to the first one. When the width gets longer than the length, you may cut off the excess, but that does not count as a cut.

30. Results How many cuts were you able to make?
Do you think you could keep cutting the paper forever? Why or why not?
You would have to cut the paper in half around thirty-one (31) times to get to the size of any atom.

31. + Protons (+) Positively charged particles
Help make up the nucleus of the atom
Help identify the atom (could be considered an atom’s DNA)
Equal to the atomic number of the atom
Contribute to the atomic mass
Equal to the number of electrons + - +

32. Neutrons Neutral particles; have no electric charge+ -
Neutral particles; have no electric charge
Help make up the nucleus of the atom
Contribute to the atomic mass

33. - Electrons (-) Negatively charged particles
Found outside the nucleus of the atom, in the electron orbits/levels; each orbit/level can hold a maximum number of electrons ( 1st = 2, 2nd = 8, 3rd = 8 or 18, etc…)
Move so rapidly around the nucleus that they create an electron cloud
Mass is insignificant when compared to protons and neutrons
Equal to the number of protons
Involved in the formation of chemical bonds + - -

34. Hydrogen (H) Atom - Notice the one electron in the first orbital = 1 +
= 0 -
How many
more electrons
can fit in the 1st
orbital/ level? +
Even though there are no neutrons present,
Hydrogen is still considered an atom

35. Oxygen (O) Atom
Notice the two electrons in the first orbital/level and the six in the second + -
= 8
How many
more electrons
can fit in the 2nd
orbital/ level? - - - + + + + - - + + - - -

36. Sodium (Na) Atom
Notice the two electrons in the first orbital/level, eight in the second, and one in the third + -
= 11
= 12 - - -
How many
more electrons
can fit in the 3rd
orbital/ level? - + + + + - - - + + - - - -

37. + The Atom’s “Center” - - -
Protons and neutrons are grouped together to form the “center” or nucleus of an atom.
Notice that the electrons are not apart of the nucleus + - - -

38. + QUARKS Particles that make up protons and neutrons
Notice the smaller particles that make up this neutron after you take a closer look.
Notice the smaller particles that make up this proton after you take a closer look. +
What do you notice about the number
of quarks in the neutron and proton?

39. Sub-Atomic Particles Weight Comparison (protons, neutrons, electrons)
Neutron = x10-27 kg Proton = x10-27 kg Electron = x10-31 kg - - +
1839 electrons = 1 neutron
1836 electrons = 1 proton +
How do you think the mass of a neutron
compares to that of a proton?
1 neutron ≈ 1 proton

40. Sub-atomic Particles Size Comparison (protons, neutrons, electrons, & quarks)
Size in atoms
Size in meters (m)
Atom 1
10-10
Nucleus
__1__
10,000
10-14
Proton or Neutron
___1___
100,000
10-15
Electron or Quark
_____1____
100,000,000
10-18
(at largest) - - - + + + + - - + + - - -

41. What would be the atomic number of this atom?
The number of protons in the nucleus of an atom -
What would be the atomic number of this atom? + - -

42. + Mass Number + What would be the mass number of this atom? - - -
The total number of protons and neutrons in an atom’s nucleus
Expressed in Atomic Mass Units (amu)
Each proton or neutron has a mass of 1 amu
What would be the mass number of this atom? - +
 3
 4 + -
3 protons + 4 neutrons = a mass number of 7 amu
Why did we not account for the electrons when calculating the mass number? -

43. Building Atoms Atoms Protons Neutrons Electrons Carbon 6 Beryllium 4 5
Using the whiteboard and the proton, neutron, and electron pieces, build the following atoms, and determine their atomic and mass numbers.
Atoms
Protons
Neutrons
Electrons
Carbon 6
Beryllium 4 5
Oxygen 8
Lithium 3
Sodium 11 12

44. Electromagnetic Force
FORCES IN THE ATOM
Gravitational Force
Electromagnetic Force
Strong Force
Weak Force

45. Gravitational Force
The force of attraction of objects due to their masses
The amount of gravity between objects depends on their masses and the distance between them
Do you think this force plays a significant
role in holding the atom together?

46. Electromagnetic Force
The force that results from the repulsion of like charges and the attraction of opposites
The force that holds the electrons around the nucleus + + + - - -
Notice how the particles with the same charge move apart and the particles with different charges move together.
Why are neutrons not pictured above?

47. Would an atom have a nucleus if the strong force did not exist?
The force that holds the atomic nucleus together
The force that counteracts the electromagnetic force
Notice how the electromagnetic force causes the protons to repel each other but, the strong force holds them together. + + + +
Would an atom have a nucleus if the strong force did not exist?

48. Notice how the original particle changes to something new.
Weak Force
This force plays a key role in the possible change of sub-atomic particles.
For example, a neutron can change into a proton(+) and an electron(-)
The force responsible for radioactive decay.
Radioactive decay  process in which the nucleus of a radioactive (unstable) atom releases nuclear radiation.
Notice how the original particle changes to something new. + n -

49. Isotopes
Atoms that have the same number of protons, but have different numbers of neutrons
Examples
Notice that each of these atoms have one proton; therefore they are all types of hydrogen. They just have a different mass number (# of neutrons). - - + + + -
Hydrogen (Protium)
Hydrogen (Deuterium)
Hydrogen (Tritium)

50. Atomic Mass
The weighted average of the masses of all the naturally occurring isotopes of an element
The average considers the percent abundance of each isotope in nature
Found on the periodic table of elements
Example
What would be the atomic mass (≈) of Hydrogen if these three isotopes
were found in the following percentages (99.9, 0.015, 0) respectively? - + + + - -
Hydrogen (Protium)
Mass # = 1 amu
Hydrogen (Deuterium)
Mass # = 2 amu
Hydrogen (Tritium)
Mass # = 3 amu
If you simply average the three, 2 amu (1 amu + 2 amu + 3 amu/3) would be the atomic mass, but since 99.9% of the Hydrogen is Protium, the atomic mass is around 1 amu (.999 x 1 amu)

51. Ion
Charged particle that typically results from a loss or gain of electrons
Two types:
Anion = negatively charged particle
Cation = positively charged particle - - - + -
= 8 + + + + - - 9 6 + + - -
Now that three electrons were lost, the number of electrons (6) and protons (8) is still unbalanced; therefore, it is still an ion, but now it is specifically referred to as a cation.
Now that this atom of oxygen just gained an electron, it is no longer neutral or an atom. It is now considered an ion (anion). This ion has more electrons (9) than protons (8).
Currently, this atom of oxygen is neutral because it has an equal number of electrons (8) and protons (8). - -
Symbol = O1-
Symbol = O2+
Symbol = O

52. I. Bohr’s Model / Energy Levels
A. Def – a specific area where an electron is likely to be. e- e- e- e- e-
2 electrons e- e- e- e- e- e-
18 electrons e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e-
32 electrons e- e-
8 electrons e- e- e- e- e- e- e- e- e- e-
nucleus e- e- e- e- e-

53. Electrons in Energy Levels
Maximum # of Electrons 1 2 8 2 3 18 4 32

54. Valence Electron
Electrons in the outer energy level of an atom.

55. Building Ions
Using the whiteboard and the proton, neutron, and electron pieces, build the following ions, and determine their atomic and mass numbers.
Ions
Protons
Neutrons
Electrons
Carbon (C³¯) 6 9
Hydrogen (H¹+) 1
Oxygen (O²¯) 8 10
Lithium (Li³+) 3 4
Sodium (Na¹¯) 11 12
Be aware that the atomic and mass numbers are not
impacted by the loss or gain of electrons.