Limitations of radiometric dating
Rock Type…
Can’t use radiometric dating on sedimentary rocks
WHY?
Sedimentary rocks are made two different ways:
Small fragments of rock pieces lithifying
Precipitation from a liquid
How does this material form?
Weathering of rocks
Two types
Mechanical: physically breaking rock into smaller pieces
Chemical: changes rocks chemical components into one or more new components
Mechanical weathering
Does not change the chemical constituents of the rock
increases surface area of the rock (allows for more rapid weathering)
Mechanisms of Mechanical Weathering
Frost wedging
Unloading or sheeting
Less dominant forms of mechanical weathering:
thermal expansion
primarily occurs in desert regions
biological activity
plant roots and animals expand fractures and cracks
abrasion
caused by wind or water
Chemical Weathering
complex processes that breaks down mineral structures
remember rocks are made of minerals
Primary agent is water
three primary reactions for chemical weathering
dissolution, oxidation and hydrolysis
To understand these three reactions fully we first must further look at basic chemistry
Atomic bonding review
Nucleus of Atom
protons and neutrons
radioactivity occurs when nucleus is unstable
Electrons
important for chemical bonds and reactions
reside in shells or orbitals
orbitals fill in specific order
Octet rule
Oribitals fill from inside out
1st shell up to 2 electrons
2nd shell up to 8 electrons
3rd shell up to 8 electrons
4-7th shells varies
Valence electron are mobile
Move from one orbital to another
higher to lower orbital--> radiates energy
lower to higher orbital--> absorbs energy
Move from one atom to another to stabilize valence shell
atom gains an electron--> anion
atom loses an electron--> cation
Cations form when an element gives up electrons and then will have more
protons than electrons.
Anions form when atoms take on electrons and then have more electrons than
protons
Types of Bonding
Cations and anions join together due to opposite charges
Known as ionic bonding
Creates neutral/stable compounds
Covalent bonding
Atoms can share valence electrons with other atoms
Creates molecules
Certain elements are most stable as molecules (O2 & H2)
Lewis structures
A short hand notation for depicting valence electrons in an atom
Dots reflect each electron in valence shell
Hydrogen molecule
Fluorine molecule
Sodium Chloride
Chemical reactions
A chemical reaction is basically when atoms bond or when bonds break between
atoms
Chemical reactions occur all the time
You need to be able to distinguish between chemical changes and physical changes.
(more in lab)
Formulas, formula weights and chemical equations
To understand chemical reactions we must first become comfortable with the
notations used in chemistry
Formulas
What does the formula of a compound or molecule tell us?
ex// NaCl or HCl
Which elements are part of the compound/molecule
How many atoms of each element are part of the compound/molecule
From the formula we can calculate the formula weight
Formula weight
Atomic mass of all the elements in the compound
If oxygen has an atomic mass of 16 amu what is the formula weight for O2?
Ex// HCl:
hydrogen (H) = 1amu chlorine (Cl) = 35.4 amu
what is the formula weight for HCl?
Ex// H2O: hydrogen (H) = 1amu oxygen (O) = 16 amu
what is the formula weight for H2O (water)?
Chemical equations
Formulas also allow us to express how elements combine together
Shows how much of each element is needed for reactions
Equations must be balanced
Tips for balancing equations
Count number of atoms for each element on each side
Add coefficients
You cannot add subscripts--> why?
Must always be in lowest common denominator
Exercise
Use the periodic table and calculate the formula weight for :
HClO4
H3PO4
HC2H3O2
Balance the following equations
Mg + HCl --> MgCl2 + H2
TiCl4 + H2O --> TiO2 +HCl
H2 + O2 --> H2O
Fe + O2 --> Fe2O3
Mg + B2O3 --> B + MgO
Other measurements
1 atom of carbon weighs 12 amu
1 atom of iron weighs 56 amu
What is an amu? How do you measure 1 atom
How about measuring the atoms in grams?
How many carbon atoms would be in 12 grams of carbon?
How many iron atoms would be in 56 grams of iron?
# atoms should be equal for both 12 grams of carbon and 56 grams of iron
This # is known as a mole of atoms
1 mole of atoms is 6.02 x 1023 atoms
1 mole of ANYTHING is 6.02 x 1023 of that thing
Example
1 penny weighs 10 grams
1 nickel weighs 20 grams
how much does 10 pennies weigh?
How much does 10 nickels weigh?
Different weights but same number of coins ( 10 of each)
Moles?
1 mole is 6.02 x 1023 atoms--> always the same number of atoms but different
weights because dealing with different atoms
1 mole of carbon = 12 grams of carbon = 6.02 x 1023 atoms of carbon
1 mole of iron =56 grams of carbon = 6.02 x 1023 atoms of iron
6.02 x 1023 is known as Avogadro’s number
the number of atoms in one mole of a substance
Exercise:
How many atoms are in one mole of hydrogen?
If Titanium (Ti) weighs 47.88amu how much does one mole of Ti weigh?
How much would one mole of CO2 weigh if Carbon weighs 12amu and Oxygen
weighs 16amu?
If carbon has a mass of 12amu, calculate the mass of 75 carbon atoms
how many carbons atoms are in a mass of 9888amu?
How many atoms are in a mass of carbon weighing 24 grams?