1. Tiny Particles of Matter
Atoms
Tiny Particles of Matter

2. Start of the atom Dalton 1808 atomic theory
Moseley 1st used atomic number
1913 Bohr used planetary model of atom

3. Atoms- Early Models
Democritus- first suggested that there was tiny particles- atoms
Dalton- provide scientific basis for atoms, and their chemical reactivity (1808

4. Atomic Theory
Play atomic theory video clip

5. Dalton’s Atomic Theory
1) All matter is made of atoms. Atoms are indivisible and indestructible.
2) All atoms of a given element are identical
in mass and properties
3) Compounds are formed by a combination of two or more different kinds of atoms.
4) Chemical reaction occurs when atoms separate, or rearrange. Atoms never change into atoms of another element as a result of a chemical reaction

6. Ernest Rutherford’s Gold Foil Experiment - 1911
Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil
Particles that hit on the detecting screen (film) are recorded

7. Rutherford’s problem:
In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?
Target #2
Target #1

8. The Answers:
Target #1
Target #2

9. Rutherford’s Findings
Most of the particles passed right through
A few particles were deflected
VERY FEW were greatly deflected
“Like howitzer shells bouncing off of tissue paper!”
Conclusions:
The nucleus is small
The nucleus is dense
The nucleus is positively charged

10. The Rutherford Atomic Model
Based on his experimental evidence:
The atom is mostly empty space
All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus”
The nucleus is composed of protons and neutrons (they make the nucleus!)
The electrons distributed around the nucleus, and occupy most of the volume
His model was called a “nuclear model”

11. Empty Space? Activity: Jump up and down
Why don’t we fall through the floor?

12. Bohr Model
Niels Bohr in 1913 came up with the Bohr model to explain how electrons are arranged around the nucleus of an atom.
He showed that electrons move around the nucleus of an atom in an orbit
Like planets around the sun

13. Bohr Model

14. Timeline In 1700 there were 13 elements In 1869 there were 26 elements
Now there are 118

15. Mass of Atom The mass of 1 amu is about 1.67 x 10-24 grams.
The proton is amu and the neutron is amu, which is essentially equal in mass.
The mass of the electron is u, or about 1/2000 the mass of a proton.

16. Size of Atoms
If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long
A scanning tunneling microscope allows scientist to see atoms
These are nickel atoms from STM

17. Subatomic particles Electrons, protons and neutrons
The nucleus of the atom contains protons and neutrons
Electrons revolve around the nucleus

18. Protons and Neutrons 1886 Protons found Mass = 1 amu Positive charge
Neutrons found in 1932
Neutral charge cathode ray video

19. Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron

20. Modern Cathode Ray Tubes
Television
Computer Monitor
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

21. Mass of the Electron
Mass of the electron is
9.11 x g
The oil drop apparatus
1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge

22. Electrons Electrons are negatively charged Discovered in 1897
Found by passing electrical current through gas
Size 1/2000 the size of Hydrogen atom
Today:
size 1/1840

23. # protons in an atom = # electrons
Atomic Number
Atoms are composed of identical protons, neutrons, and electrons
How then are atoms of one element different from another element?
Elements are different because they contain different numbers of PROTONS
The “atomic number” of an element is the number of protons in the nucleus
# protons in an atom = # electrons

24. Atomic Number
Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.
Element
# of protons
Atomic # (Z)
Carbon 6
Phosphorus 15
Gold 79

25. Mass Number
Mass number is the number of protons and neutrons in the nucleus of an isotope:
Mass # = p+ + n0
Nuclide p+ n0 e-
Mass #
Oxygen - 10 - 33 42
- 31 15 18 8 8 18
Arsenic 75 33 75
Phosphorus 16 15 31

26. Complete Symbols
Contain the symbol of the element, the mass number and the atomic number.
Mass
number X
Superscript →
Atomic
number
Subscript →

27. Br Symbols 80 35 Find each of these: number of protons
number of neutrons
number of electrons
Atomic number
Mass Number 80 Br 35

28. Symbols
If an element has an atomic number of 34 and a mass number of 78, what is the:
number of protons
number of neutrons
number of electrons
complete symbol

29. Symbols
If an element has 91 protons and 140 neutrons what is the
Atomic number
Mass number
number of electrons
complete symbol

30. Symbols
If an element has 78 electrons and 117 neutrons what is the
Atomic number
Mass number
number of protons
complete symbol

31. Isotopes Elements have different versions
Each version has a different number of neutrons so different mass
Same element different # neutrons
Keeps same chemical properties
Play elements and Isotope video

32. Isotopes
Dalton was wrong about all elements of the same type being identical
Atoms of the same element can have different numbers of neutrons.
Thus, different mass numbers.
These are called isotopes.

33. Isotopes
Frederick Soddy ( ) proposed the idea of isotopes in 1912
Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.

34. We can also put the mass number after the name of the element:
Naming Isotopes
We can also put the mass number after the name of the element:
carbon-12
carbon-14
uranium-235

35. Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium) 1
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium) 2

36. Isotopes Elements occur in nature as mixtures of isotopes.
Isotopes are atoms of the same element that differ in the number of neutrons.

37. Atomic Mass How heavy is an atom of oxygen?
It depends, because there are different kinds of oxygen atoms.
We are more concerned with the average atomic mass.
This is based on the abundance (percentage) of each variety of that element in nature.
We don’t use grams for this mass because the numbers would be too small.

38. Measuring Atomic Mass
Instead of grams, the unit we use is the Atomic Mass Unit (amu)
It is defined as one-twelfth the mass of a carbon-12 atom.
Carbon-12 chosen because of its isotope purity.
Each isotope has its own atomic mass, thus we determine the average from percent abundance.

39. To calculate the average:
Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results.
If not told otherwise, the mass of the isotope is expressed in atomic mass units (amu)

40. Composition of the nucleus
Atomic Masses
Atomic mass is the average of all the naturally occurring isotopes of that element.
Isotope
Symbol
Composition of the nucleus
% in nature
Carbon-12
12C
6 protons
6 neutrons
98.89%
Carbon-13
13C
7 neutrons
1.11%
Carbon-14
14C
8 neutrons
<0.01%
Carbon =

41. Electrons
Electrons determine the physical and chemical properties of an element
P.T. classifies elements by electron configuration (4 groupings)
Noble gases
Representative elements
Transition elements
Inner transition elements
Look at bottom of pg. 395

42. Electrons
Play electron video clip

43. Energy Levels Electrons in a particular path have a fixed energy
Electrons don’t lose energy so they don’t fall into the nucleus
The energy level is the region around the nucleus where electrons are likely to be found

44. Energy Levels Like the rungs of a ladder
Electrons close to the nucleus have less energy
For electrons to move from energy level to the next it must gain or lose the right amount of energy
The farther away the less force the nucleus has on the electron, so it is easier for the electron to leave the atom

45. Energy Levels The P.T. can help determine electron levels
Valence electrons are outer electrons- affect reactions
Oxidation numbers follow Group numbers

46. Energy Levels Like the rungs of a ladder
Electrons close to the nucleus have less energy
For electrons to move from energy level to the next it must gain or lose the right amount of energy
The farther away the less force the nucleus has on the electron, so it is easier for the electron to leave the atom

47. Quantum Mechanical Model
Just like the Bohr model but
Electrons don’t have set orbits
It uses probability to show where an electron could be located.
Math based
Electron cloud
Areas of high probability

48. Electron Cloud

49. Energy Levels Principal energy level= major levels
Sublevels, each principal level has a set # of sublevels that coincides with the number of principal level
1 = 1 sublevel
2 = 2 sublevels

50. Energy Levels
Orbitals are areas where electrons are likely to be found
Letters denote the orbitals
S= spherical
p= dumbbell shape
d and f

51. Energy levels
The number of electrons in a principal energy level is based on 2n2
p.level
sub

52. Orbitals
s= 1 orbitals
P= 3 orbitals
D= 5 orbitals
F= 7 orbitals

53. Electron Configuration
3 rules pg
Electrons enter orbitals of lowest energy 1st
An atomic orbital describes at most two electrons
When electrons occupy orbitals of equal energy, one electron enters each orbital until they are all full.