1. IB Chemistry 1: Topic 2
Atomic Structure: atomic particles, atomic number, mass # , isotopes and atomic mass

2. Defining Atoms & Electrons in Atoms

3. Originated idea of the atom
Democritus ( BC)
“Atomos” indivisible and indestructible
No evidence, no experiments for support
Plato and Aristotle argued against him
“the properties of matter that we see are explained by the properties and behavior of atoms that we cannot see”
Originated idea of the atom

4. For the next 2000 years… Alchemy Good or bad? Contributions?
Pseudoscience
Trying to turn metals into gold
Discovered several elements in the process

5. Isaac Newton – 1704 AD
“It seems probable to me that God in the beginning, formed matter in solid massy, hard, impenetrable movable particles of such sizes and figures and with such other properties, in such proportions to space, as most conduced to the end for which he formed them.”

6. 1803 Dalton’s Atomic Theory
John Dalton ( )
Father of Atomic theory, not bad considering he had very little evidence. Controlled experiments, ratios determine
Studied the ratios in which elements combine in chemical reactions (used experimental methods)
Proposed an atomic theory based on results
Lavoisier –
Wrote the first chemistry book
He was able to explain the true nature of combustion (oxygen) with candles and bell jars.
Discovered that mass was conserved in a chemical reaction. (Law of Conservation of Mass)
1803 Dalton’s Atomic Theory

7. Proust
Showed that the proportions by mass of the elements in a given compound are always the same. (Law of Definite Proportions)
Conducted quantitative experiments
Showed that regardless of how copper carbonate was prepared in the lab, or how it was isolated in nature, it always contained the same proportions of copper, oxygen, and carbon – 5:4:1 by mass

8. At the time, current thinking…
George Stahl said that fire went out in a closed container because of phlogiston given off by the fire
Joseph Priestly said a substance with heavy combustion is low in phlogiston. Oxygen was dephogisicated air

9. Modern Atomic Theory Dalton said:
Several changes have been made to Dalton’s theory.
Dalton said:
Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties
Modern theory states:
Atoms of an element have a characteristic average mass which is unique to that element.

10. Modern Atomic Theory
Dalton said: Atoms cannot be subdivided, created, or destroyed Modern theory states: Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

11. J.J. Thomson (1856-1940) 1897 Discovered the electron
(“plum pudding” model)

12. Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.
Showed cathode ray tube consists of a stream of negatively charged particles he called corpuscles (later called electrons)
Applied a magnet tube – electrons moved toward the positive magnet.
Concluded negative particles
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

13. Cathode Ray Tube
Tube
Cathode
Alligator Clip
Anode
Stand

14. Cathode Ray Tube in Action

15. Mass of the Electron
1909 – Robert Millikan determines the mass of the electron.
The oil drop apparatus
Mass of the electron is
9.109 x kg

16. Conclusions from the Study of Electrons
Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.
Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons.
Electrons have so little mass that atoms must contain other particles that account for most of the mass

17. Thompson’s Plum Pudding Model

18. Rutherford (1871-1937) 1911 Discovered the nucleus
(gold foil experiment)

19. Rutherford’s Gold Foil Experiment
Alpha particles are helium nuclei
Particles were fired at a thin sheet of gold foil
Particle hits on the detecting screen (film) are recorded

20. Try it Yourself
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?

21. The Answers
Target #1
Target #2

22. 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

23. Since opposite charges attract each other, why don’t the electrons fall into the nucleus?

24. 1913 proposed Planetary model
Niels Bohr ( )
1913 proposed Planetary model

25. The Bohr Model of the Atom
I pictured electrons orbiting the nucleus much like planets orbiting the sun.
But I was wrong! They’re more like bees around a hive.
WRONG!!!
Neils Bohr

26. The Quantum-Mechanical Model
1926
The Quantum-Mechanical Model
Based upon the work of several men, a new mathematical model was developed to describe the structure of the atom.
Louis de Broglie ( )
Werner Heisenberg ( )
Erwin Schrodinger ( )

27. Previous models were physical models based upon the motion of objects
Previous models were physical models based upon the motion of objects. This model is primarily a mathematical model and had never been proven....

28. …until recently!
The New York Times, 9/7/99

29. Now we know for sure!

32. Atomic Number 6 6 15 15 79 79 In a neutral atom:
Atomic number of an element is the number of protons in the nucleus of each atom of that element.
Element
Atomic #
# of protons
# of electrons
Carbon 6
Phosphorus 15
Gold 79 6 6 15 15 79 79
In a neutral atom:
# electrons = # protons

33. The Atomic Scale
Most of the mass of the atom is in the nucleus (protons and neutrons)
Electrons are found outside of the nucleus (the electron cloud)
Most of the volume of the atom is the electron cloud.

34. Atomic Particles Particle Charge Mass (g) Location Electron e- -1
9.109 x 10-28
(1/1840 amu)
Electron cloud
Proton p+ +1
1.673 x 10-24
(1 amu)
Nucleus
Neutron n
1.675 x 10-24

35. The Atomic Scale
Most of the mass of the atom is in the nucleus (protons and neutrons)
Electrons are found outside of the nucleus (the electron cloud)
Most of the volume of the atom is empty space
“q” is a particle called a “quark”

36. About Quarks… Protons and neutrons are NOT fundamental particles.
Protons are made of two “up” quarks and one “down” quark.
Neutrons are made of one “up” quark and two “down” quarks.
Quarks are held together
by “gluons”

37. Reading the Periodic Table3 Li
6.941
Lithium
Atomic Number
# p+
# e- (in a neutral atom)
Element Symbol
Element Name
Atomic Mass
# p+ + # n0
# n0 = Atomic Mass - #p+

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

39. Mass Number Mass # = p+ + n0
Mass number is the number of protons and neutrons in the nucleus of an isotope.
Mass # = p+ + n0

40. C C Cl Symbols of Isotopes 6 Mass # 12 Carbon-12 Atomic Number 14
Chlorine-35
Mass # Cl 35 17

41. Mass Number Mass # = p+ + n0 O As P Isotope p+ n0 e- Mass # Oxygen -10 - 33 42
-31 15 18 O 18 8 8 8 18 As 75 33
Arsenic 75 33 75 P 31 15
Phosphorus 16 15 31

42. Isotopes…Again (must be on the test)
Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.
Isotope
Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium) 1
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium) 2

43. Composition of the nucleus
Atomic Masses
Atomic mass is the weighted average of all the naturally occuring isotopes of that element.
Multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.
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 =

44. Find the missing percentage.
Silver has two naturally occurring isotopes:
Isotope name
Isotope mass (amu)
percentage
Silver-107
51.86
Silver-109
remainder
Find the missing percentage.
Find the average atomic mass of an atom of silver.

45. Isotope name Isotope mass (amu) Relative Abundance
Silicon has three naturally occurring isotopes:
Isotope name
Isotope mass (amu)
Relative Abundance
Silicon-28
27.98
92.21
Silicon-29
28.98
4.70
Silicon-30
29.97
3.09
Look over the data before you begin the problem. Estimate the value of the answer before you begin the calculation. Will the weighted average be closer to 28,  29, or 30?
Find the average atomic mass of silicon.

46. Isotope name Isotope abundance Isotope mass (amu)
Iron has four naturally occurring isotopes:
Isotope name
Isotope abundance
Isotope mass (amu)
Iron-54
5.90%
53.94
Iron-56
91.72%
55.93
Iron-57
2.10%
56.94
Iron-58
0.280%
57.93
Estimate the average mass.
Find the average atomic mass of iron.