1. Chemistry Chapter 3
Atoms:
The Building
Blocks of Matter

2. Atom who?
Atom
The smallest particle of an element that retains the chemical properties of that element

3. Law of Conservation of Mass
Mass is neither created nor destroyed during chemical or physical reactions.
Total mass of reactants =
Total mass of products
Antoine Lavoisier

4. Dalton’s Atomic Theory (1808)
All matter is composed of extremely small particles called atoms
Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties
John Dalton
Atoms cannot be subdivided, created, or destroyed
Atoms of different elements combine in simple whole-number ratios to form chemical compounds
In chemical reactions, atoms are combined, separated, or rearranged

5. Modern Atomic Theory
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.

6. 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!

7. Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

8. Some Modern Cathode Ray Tubes

9. Mass of the Electron
1909 – Robert Millikan determines the mass of the electron.
Mass of the electron is
9.109 x kg and the charge is 1.67 x C
An experiment performed by Robert Millikan in 1909 determined the size of the charge on an electron. He also determined that there was a smallest 'unit' charge, or that charge is 'quantized'. He received the Nobel Prize for his work. We're going to explain that experiment here, and show how Millikan was able to determine the size of a charge on a single electron.
What Millikan did was to put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling. Since he was able to work out the mass of the oil drop, and he could calculate the force of gravity on one drop, he could then determine the electric charge that the drop must have. By varying the charge on different drops, he noticed that the charge was always a multiple of -1.6 x C, the charge on a single electron. This meant that it was electrons carrying this unit charge. Here's how it worked. Have a look at the apparatus he used:
An atomizer sprayed a fine mist of oil droplets into the chamber. Some of these tiny droplets fell through a hole in the upper floor. Millikan first let them fall until they reached terminal velocity. Using the microscope, he measured their terminal velocity, and by use of a formula, calculated the mass of each oil drop. Next, Millikan applied a charge to the falling drops by illuminating the bottom chamber with x-rays. This caused the air to become ionized, and electrons to attach themselves to the oil drops. By attaching a battery to the plates above and below this bottom chamber, he was able to apply an electric voltage. The electric field produced in the bottom chamber by this voltage would act on the charged oil drops; if the voltage was just right, the electromagnetic force would just balance the force of gravity on a drop, and the drop would hang suspended in mid-air. Now you try it. Click here to open a simulation of Millikan's chamber. First, allow the drops to fall. Notice how they accelerate at first, due to gravity. But quickly, air resistance causes them to reach terminal velocity. Now focus on a single falling drop, and adjust the electric field upwards until the drop remains suspended in mid-air. At that instant, for that drop, the electric force on it exactly equals the force of gravity on it. Some drops have more electrons than others, so will require a higher force to stop. When you've finished playing with the apparatus, close the window and we'll continue. O.K., let's look at the calculation Millikan was now able to do. When a drop is suspended, its weight  m · g is exactly equal to the electric force applied  q · E
The values of  E, the applied electric field,  m the mass of a drop, and  g, tha acceleration due to gravity, are all known values. So you can solve for  q, the charge on the drop:
Millikan determined the charge on a drop. Then he redid the experiment numerous times, each time varying the strength of the x-rays ionizing the air, so that differing numbers of electrons would jump onto the oil molecules each time. He obtained various values for  q. The charge  q on a drop was always a multiple of -1.6 x C,  the charge on a single electron. This number was the one Millikan was looking for, and it also showed that the value was quantized; the smallest unit of charge was this amount, and it was the charge on a single electron.
The oil drop apparatus

10. Millikan's Oil Drop Experiment

12. Conclusions from the Study of the Electron
Electrons are negative.
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

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

14. Gold Foil Experiment

15. Rutherford’s Findings
Most of the particles passed right through
A few particles were deflected
VERY FEW were greatly deflected
Conclusions:
The nucleus is small
The nucleus is dense
The nucleus is positively charged

16. The Structure of the Atom
Atoms consist of two regions
Nucleus
Very small region in the center.
Contains protons & neutrons.
Electrons Cloud
Mainly empty space.
Very large compared to the nucleus.
Contains electrons.
Subatomic particles
Protons, neutrons, and electrons

17. Atomic Particles Particle Charge Mass (kg) Location Electron -1
9.109 x 10-31
Electron cloud
Proton +1
1.673 x 10-27
Nucleus
Neutron
1.675 x 10-27

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

19. Mass Number Mass # = p+ + n0
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

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

21. Isotopes…Again (must be on the test) Hydrogen-2 (deuterium)
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

22. Atomic Masses Carbon = 12.0125 amu
Atomic mass is the average of all the naturally isotopes of that element.
On Periodic Table
Carbon = amu
Isotope
Symbol
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%

23. Writing Nuclear Symbols3 He
Mass #
(proton + neutrons)
Atomic Symbol
Atomic #
(proton) 2
How many protons, electrons, and neutrons?
2 protons, 2 electrons, 1 neutron
Mass # - Atomic # = # Neutrons

24. Writing Isotopes Using Hyphen Notation
Uranium-235, Helium-3, or Carbon-14
How many proton, electrons, neutrons?
Name
of atom
Mass #
92 protons, 143 neutrons, 92 electrons

25. Convert these hyphen notation to nuclear symbols.
Isotope problems
Convert these hyphen notation to nuclear symbols.
Uranium-235, Helium-3, or Carbon-14
235 U 3 He 14 C 92 2 6

26. Chapter 22 – Nuclear Chemistry

27. The Nucleus Contains nucleons Nuclear forces Protons & Neutrons
Short-range proton-neutron, proton-proton, and neutron-neutron forces hold the nuclear particles together.

28. Nuclear Stability Kinetic Stability
Describes the probability that a nucleus will decompose (radioactive decay)

29. Nuclear Stability
Decay will occur in such a way as to return a nucleus to the band (line) of stability.

30. Number of Stable Nuclides Related to Numbers of Protons and Neurons

31. Types of Radioactive Decay4 2+
alpha production (a): helium nucleus
beta production (b): He 2 e - 1

32. Alpha Radiation
Limited to VERY large nucleii.

33. Beta Radiation
Converts a neutron into a proton.

34. Types of Radioactive Decay
gamma ray production (g):
positron production :
electron capture: (inner-orbital electron is captured by the nucleus) e 1

35. Types of Radiation

36. Half-Life of Nuclear Decay

37. The Decay of a 10.0-g Sample of Strontium-90 Over Time

38. QUESTION

39. ANSWER 2) Sc Section 18.1 Nuclear Stability and Radioactive46 21 Sc
Section 18.1 Nuclear Stability
and Radioactive
Decay (p. 841)
Make sure to memorize the abbreviations for the
subatomic particles.
HMCLASS PREP: Table 18.2

40. QUESTION

41. ANSWER 2) Th Section 18.1 Nuclear Sta bility and Radioactive
234 90 Th
Section 18.1 Nuclear Sta
bility and Radioactive
Decay (p. 841)
Just as chemical equations need the same
number of each type of atom on each side,
nuclear equations need the same number of
each type of nucleon on each side.

42. QUESTION

43. ANSWER 2) b Section 18.1 Nuclear Stability and Radioactive+ 2) b
Section 18.1 Nuclear Stability and Radioactive
Decay (p. 841)
According to the band of stability graph
(Figure
18.1) this nuclide is neutron
poor, so it
must do something to decrease the number of
protons or increase the number of neutrons. -

44. QUESTION

45. ANSWER 5) b Section 18. 1 Nuclear Stability and Radioactive– 5) b
Section 18.
1 Nuclear Stability and Radioactive
Decay (p. 841)
This process is the opposite of positron emission
and allows the change of a neutron into a
proton.

46. QUESTION

47. ANSWER 3) 4 Section 18.2 The Kinetics of Radioactive Decay (p. 846) 1
1/8
1/16
Each arrow indicates a half
life of 1.0 hour. . -