Things you need to review and may
need help with when we return in
the fall.
You will find notes and examples within this
PowerPoint as well as useful video links.
History and Experiments
• Dalton’s Atomic Theory – two parts are incorrect
• all atoms are not identical
• atoms can be divided
• JJ Thomson – neutral atom, p+ = e• discovered electrons with cathode tube experiment
• Lord Kelvin – Plum Pudding model
• Protons and electrons are mixed together in equal quantities
• Millikan – oil drop exp
• quantitative charge and mass of e-
• Rutherford - gold foil exp
• small dense positive nucleus
• atom mostly empty space
• Bohr – with Rutherford
• planetary model, only works for H
Atoms
More about atoms
• Are there any other particles in the atom?
– Scientists have found the subatomic particles can be subdivided into smaller
parts, for example quarks. Quarks are the elementary particles that protons
and neutrons are made up of. (1968)
• What is an atomic number? Where do we find it? How can we use
it?
– Atomic # = # of protons
– Found on the periodic table
– Used to identify an element, an element only has one atomic number
• In an atom, what is the relationship between protons and
electrons?
– An atom is neutral, so that means the protons=the electrons
We never change or manipulate protons, which would mean
we are changing the element and we aren’t nuclear chemists.
Isotopes (discovered by Chadwick)
Definition – atoms of an element with the same number of protons,
but different numbers of neutrons
Impact on atom - changes the mass number and the mass of the atom
Discredited part of Dalton’s atomic theory
Name of an isotope – name of element – mass number,
example carbon – 12
Symbol of an Isotope
A
Z
X = element symbol
A = mass number = (# of p+) + (# of n0)
Z = atomic number = # of p+
Calculating Average Atomic Mass
Average atomic mass
– the weighted average of the mass of all the isotopes that
exist for an element,
– given the unit amu,
– found on the periodic table
AAM = S[(isotope mass) x (decimal abundance of isotope)]
For each isotope you multiply its mass by its decimal abundance.
Then you add these products together.
Example #1 – Calculate Average Atomic Mass
Naturally occurring chlorine is 75.53% 35Cl, which has an atomic mass
of 34.969 amu, and 24.47% 37Cl, which has a mass of 36.966 amu.
Calculate the average atomic mass of chlorine.
Mass Spectrometry and Isotopes
https://youtu.be/tOGM2gOHKPc
This is an excellent video that using a simple
analogy to introduce the concept and then
applies that knowledge to the actual working of
a mass spectrometer.
https://youtu.be/NuIH9-6Fm6U
Excellent video on mass spectrometry. Gives
great insight into how data is collected.
@ STP, 1mol = 22.4L
Liters
Used to convert gases only!
MOLES
Grams
Particles
Percent Composition
Steps for Calculation of Percent Composition
①Calculate the molar mass of the compound.
②Divide the mass of the element in 1 mol of
compound by the mass of 1 mol of the compound.
③Multiply by 100.
④Check the sum of the mass percents; they should
equal 100.
Examples #1 and #2:
CCl4
NaOH
Calculating Empirical Formula
Steps for Calculating Empirical Formula
① Obtain the mass of each element, generally given, but may involve a
subtraction step.
② Convert grams to moles.
③ Divide the number of moles of each element by the smallest number
of moles to convert the smallest number to one. If all calculated
numbers are whole numbers, these are the subscripts in the empirical
formula, OTHERWISE go to step four.
④ Multiply all the numbers from step three by the smallest whole
number that will convert all of them to whole numbers.
Example #1:Calculate the empirical formula of a compound using a sample
that contains 4.151 g Al and 3.692 g of O.
Calculating Molecular Formula
Calculation of Molecular Formulas
To calculate the molecular formula, three things are needed:
① the empirical formula
② the empirical formula molar mass
③ the molecular molar mass (always given)
To find the molecular formula, the empirical formula molar mass if compared to the
molecular molar mass.
Steps for Calculating Molecular Formulas
① Calculate the empirical formula, if necessary.
② Calculate the empirical formula molar mass.
③ Divide the molecular molar mass by the empirical formula molar mass.
④ Multiply the empirical formula be the result of step 3.
Example #1: Calculate the molecular formula of a compound that has a molecular molar
mass of 283.88 g and an empirical formula of P2O5.
Naming and Formula Writing
Please see the notes on the wiki page.