Download Chapter 4 The Structure of the Atom

Chapter 4
The Structure of the Atom
Early Theories of Matter
Ideas of the Philosophers
ë Democritus (460 - 370 B.C.) - matter is composed of tiny particles called
atomos
ë Aristotle (384 - 322 B.C.) - disagreed with the idea of atoms
John Dalton (1766 - 1844)
ü marks the beginning of modern atomic theory
ü based statements on scientific research
Dalton’s Atomic Theory
1. All matter is composed of tiny, indivisible particles called atoms.
2. Atoms of the same element are identical in size, mass and chemical
properties.
3. Atoms of different elements are different.
4. Atoms of different elements can physically mix or chemically
combine.
5. Chemical reactions occur when atoms are separated, joined or
rearranged.
Neutrons - neutral (uncharged) subatomic particles
• Discovered by Chadwick in 1932
• Found in the nucleus
• Relative electrical charge ZERO
• Relative mass 1
• Symbol
n0
Subatomic Particles
• the building blocks of atoms
Protons - positively charged subatomic particles
• Discovered by Goldstein in 1886
• Found in the nucleus
• Relative electrical charge 1+
• Relative mass 1
• Symbol
p+
Electrons - negatively charged subatomic particles
• Discovered by Thomson in 1897
• He used a cathode ray tube
• Mass 1/1840th of a proton
• Relative electrical charge 1• Relative mass…ZERO à it is negligible compared to the proton
• Symbol
e-
rutherford-scattering.jar
Rutherford’s Gold Foil Experiment
ü an experiment designed to verify Thomson's model
ü the results led to the definition of the nucleus
rutherford animation
Nucleus - central core of the atom (1/100,000th diameter)
• composed of p+ and n0
• contains all of the atom’s positive charge
• contains 99.97 % of the atom’s mass
Atomic Number
• the number of protons in the nucleus
• H. Moseley discovered that each element contains a unique
positive charge
• determines the place for each element on the periodic table
Mass Number
• total number of protons and neutrons found in the nucleus
• Remember: All of the mass of the atom is found in the nucleus.
# n0 = mass # - atomic #
Isotopes
M atoms with the same number of protons and different number of
neutrons
M each isotope has essentially the same chemical properties because the
protons and electrons are the same
Example: Fill in the table
Isotope
atomic #
mass #
#p+
#n0
47
60
C-14
#e-
Nuclear Chemistry
Radioactivity
Nuclear reactions
ë changes in the nucleus
ë identity of the atom changes
radiation
ë rays or particles emitted by a radioactive substance
ë emitted because nucleus was unstable
ë radioactive decay - process that the unstable nucleus undergoes
as it emits radiation
ë decay continues until a stable state is reached
radioisotopes
ë isotopes of atoms with unstable nuclei
Radioactive Decay
Nuclear stability
ë the protons have electrostatic repulsion pushing them apart
ë the neutrons and protons have a strong nuclear force holding
them together
ë the stability is based on the neutron-to-proton ratio
`
Summary of the Band of Stability as it relates to Radioactive Decay
Radioactive Decay Series
> a series of decay steps
> an unstable nucleus decays until a stable nucleus is formed
Decay Rates à Half-life
***Time required for ½ of the radioisotope’s nuclei to decay into its
products.
sample remaining can be calculated:
amount remaining = (initial amount)(½)n
Radon-222 has a half life of 3.80 days. How much of a 1125 g sample remains
after 155 hours?
amount remaining = (initial amount) (0.5)n
Radiochemical dating
> process of determining the age of an object by measuring the amount
remaining of a certain isotope
Carbon dating
ë all living things contain some radioisotope carbon-14
ë when things die, C-14 uptake stops
ë C-14 à half-life = 5730 years
ë
by measuring the remaining amount of C-14, the age can be
determined up to 24,000 years
Uranium dating
> longer half-life: U-238 à 4.5 billion years
> useful for dating of rocks
Nuclear Reactors
ë generate large amounts of energy by controlled nuclear fission
ë energy released by 1 kg U-235 = 4,000,000,000 kg coal
ë fuel rods contain UO2 (uranium (IV) oxide)
ë control rods contain Cd or B to absorb neutrons
ë waste must be processed and stored away from nature
Nuclear Fission
> splitting of a large nucleus into fragments
> atoms with a mass number > 60
> releases large amounts of energy
nuclear-fission.jar
chain reaction
M self-sustaining process in which one reaction initiates the
next reaction
M uncontrolled chain rxn - nuclear bomb explosion
M critical mass - the minimum mass of fissionable material
that can sustain a chain rxn
Nuclear Fusion
> combination of small nuclei
> fusion of H powers the Sun
Fusion for electricity generation?
Advantages
> more energy released
> products are not radioactive
> abundant fuelà lightweight elements
Disadvantages
> extremely high energy to initiate the process
> can't contain the temperature: 40,000,000 K
Detecting Radiation
> film badge
> Geiger counter
roentgen (R)
• measures exposure (ionization) of air by gamma-rays
rad (radiation absorbed dose)
• different materials with the same exposure may not absorb the
same amount of energy
• measures energy deposited in any material by all types
rem (Roentgen Equivalent to Man)
• estimates biological damage or health risk due
to
absorption of all types of radiation
• For gamma rays and beta particles, 1 rad =1 rem of dose.
> Scintillation counter
Acute Radiation Exposure
Effects of Large, Whole-Body Radiation Doses
Effect
No observable effect
Dose (rems)
0-25
Slight blood changes
25-100
Significant reduction in blood
platelets and white blood cells
(temporary)
100-200
Severe blood damage, nausea,
hair loss, hemorrhage,
death in many cases
200-500
Death in less than
two months for over 80%
>600
Biological Effects of Radiation
Uses of Radiation
> radiotracer - used to track reaction mechanisms
> diagnostic medicine - PET & CT scans
radiation therapy - destroy cancer cells