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Atoms – Building Blocks of Matter Notes - Chapter 3
I. The Atom: From Idea to Theory
A. 400 BC Democritus VS Aristotle
__________________________, an ancient Greek and student of Aristotle, proposed the 1st atomic theory he said that the
world is composed of 2 things: void (empty space) and matter. No one supported him and he had NO experimental evidence to
support his idea.
__________________________ proposed that matter was composed of one continually flowing substance called hyle.
This idea was widely supported and accepted until the late 1700’s and he too had NO experimental evidence to support his idea.
B. Late 1700’s Isaac ____________and Robert ______________ – It was not until the late 1700’s that anyone dared to
question Aristotle’s wisdom. They suggested that Aristotle was incorrect but did not have their own theory to submit.
At this time chemist did believe, based on experiments, that there were different elements and that an element was a substance that
could not be broken down by chemical means. Chemist knew that some substances could transform into different or new
substances, they called this a chemical reaction.
C. 1790’s - Basic laws that were established:
Chemist also new, via improved _____________, that when a chemical reaction occurred in a closed space that the mass of
the material before the change equaled the mass of the marital after the change. Now known as the Law of Conservation of Mass.
Another realization was that substances always contained their elements in the same ___________________ by mass. For
example: for any sample of sodium chloride, the mass of the sample is always 39.34% Na and 60.66% Cl. Now known as the Law
of Definite Proportions.
It was also known that elements combined to form more than one compound. Example: carbon monoxide and carbon
dioxide. This is the Law of Multiple Proportions.
D. 1803 John Dalton – British chemist who was the first to have a theory about matter being composed of atoms and how
atoms might look and behave. Dalton proposed an explanation for the Law of Conservation of Mass, Law of Definite Proportions,
and Law of Multiple Proportions. He reasoned that elements were composed of atoms and that only whole numbers of atoms can
combine to form compounds. He conceived on the atom as a solid billiard ball. Here is a summary of his theory:
1. All matter is composed of _______________.
2. Atoms of the same elements are exactly the same and atoms of different elements are different.
3. Atoms cannot be ____________________________, destroyed, or subdivided.
4. Atoms of different elements combine in whole number rations to form _____________________________.
5. In chemical reactions, atoms are combined, separated, or _________________________________.
Democritus’s idea, because Dalton was able to relate atoms to the measurable property of mass, turned into a scientific theory!!
The only aspect of Daltons’ Theory that is now known to be incorrect is the fact that atoms can be subdivided (into p+, e-, n). And
that atoms of the same element can have deterrent ____________________________ (these are called isotopes).
II. The Structure of Atoms
Atom – smallest particle of an element that retains the chemical properties of that element. All atoms consist of 2 regions
– the ________________________________ (p+ & n) and surrounding the nucleus is the electron cloud – a region occupied by the
negatively charged particles called electrons. How do we know this?!
1. Discovery of _________________________ 1897 (by J.J. Thompson)
1st subatomic particle to be discovered – Thompson was working with ________________________ and magnetic fields. He was
taking various gases and sending an electric current through the gas. When he did this he noticed that a glow was emitted. (What
he was doing, he believed, was separating the electron from the nucleus of the gas atoms – this caused the glow!) Thompson went
on to prove that the glow was actually a stream of negatively charged particles – called electrons. Symbol e-, charge –1, and mass
of 0.00055amu (atomic mass unit, 1amu = 1.66 X10 –27 kg)
Plum Pudding Model – Thompson proposed that the atom had negative electrons scattered throughout a positively
charged area (proton area).
2. ________________________ 1919 (discovered by Rutherford/J.J. Thompson)
Both Rutherford and Thompson knew that positively charged particles (protons) must exist (because an atom is
_________________, if there is a negative charged electron then there has to be a positively charged proton to make it neutral.)
They worked together to prove they existed. Proton symbol: +p, charge +1, mass 1.008 amu.
3. Discovery of the _______________________________________ 1911
Discovered by Rutherford during his famous _________________________________________ and realized that the main part of
the atom’s mass is in the nucleus, and that it is positively charged. Summary of his experiment:
Subatomic Mass
-Bombarded a thin piece of gold foil with positive alpha particles
Particle
-Most went through as though nothing was there
Electron
0.00055amu
-Few (1 in 8000) ricochet back toward the source
Proton
1.008 amu.
-Few were deflected off to the side
Neutron
1.008 amu.
Rutherford’s Conclusion: the positive alpha particles had to have hit something else that was ______________________ charged to
cause the ricochet effect. The “something” was very small and dense because only a few hit it, therefore the atom must have a small
positively charged nucleus, surrounded by mostly empty space (because most particles went through the gold foil.) New model of
atom:
4. _______________________ 1932 (Proved by Chadwick)
New something else existed in an atom because of the _________________ of the atom. Neutron is an electrically neutral particle,
symbol n, mass equal that of protons.
5. ______________ – the +p and n stay close to each other due to these short-range forces that hold the +p and n together.
Current Model of Atom:
III. Counting Atoms
Reading the periodic table
11
atomic number
Na
symbol
22.990
average atomic mass (in amu’s)
Sodium
name of element
23
mass number (the average atomic mass rounded to the nearest whole number)
1. Atomic Number – the number of protons in the nucleus. The atomic number identifies the element!!!!!!!!! Because
atoms are neutral they contain the same number of electrons as protons. (Therefore the atomic number is the number of electrons as
well.)
2. Atomic Mass – mass of 1 atom (measured in amu’s)
3. Mass Number – the average atomic mass rounded to the nearest whole number, therefore it is the total number of
protons and neutrons in an atom’s nucleus.
Practice: How many protons are in each of the following? neutrons? electrons?
Symbol
Be
Ne
Atomic Number Mass Number
4
9
10
20
Na
11
Protons (p+)
Neutrons (n) Electrons (e-)
Symbol
23
4. Isotopes – atoms with the same number of protons (atomic number is the same) but different numbers of neutrons
(mass number is different). Usually isotopes are referred to by their name (of symbol) and their mass number. Every element on the
chart has at least 2 isotopes and some elements have as many as 25 isotopes.
Example: The isotopes of hydrogen have separate names rather than being called hydrogen-1, hydrogen-2, etc. Their names are
protium (H-1), deuterium (H-2), and tritium (H-3). Diagram of protium, deuterium, and tritium:
Name in Hyphen
Notation*
Protium (H– 1)
Number of
protons
1
Number of
electrons
1
Number of
neutrons
0
Mass Number
Deuterium (H – 2)
1
1
1
2
Tritium (H – 3)
1
1
2
3
(*Hyphen notation – element is written first and then the mass number)
1
Symbol
Practice: carbon-14, carbon-13, carbon-12
Name in Hyphen
Number of
Number of
Notation*
protons
electrons
Number of
neutrons
Mass Number
Symbol
Most elements occur naturally as __________________________ of isotopes, as indicated in Table 3-4 of textbook. The
percentage of each isotope in the naturally occurring element on Earth is nearly always the same, no matter where the element is
found. The percentage at which each of an element’s isotopes occurs in nature is taken into account when calculating the element’s
average atomic mass (which appears on the periodic table).
nuclide-a general term for any isotope of any element
5. Relative atomic masses - a.m.u.- atomic mass unit; One amu is exactly 1/12 the mass of a carbon-12 atom. So the
atomic mass of any nuclide is determined by comparing it with the mass of the carbon-12 atom. The hydrogen-1 atom has an atomic
mass of about 1/12 that of the carbon-12 atom, or 1 amu. 1 amu = 1.66X10-27kg
6. Average atomic mass: It is the weighted average of the masses of all the isotopes of that element. A weighted average
reflects both the mass and the abundance of the isotopes as they occur in nature.
Ex:ample:
isotope
Atomic mass
abundance (%)
protium
deuterium
tritium
1.0078
2.0141
3.0160
99.985
0.015
negligible
The average atomic mass of hydrogen is 1.0079. Multiply each mass number by the percent abundance and add them up.
Practice: Element Z has 2 natural isotopes. The isotope with a mass number of 15 has a relative abundance of 30%. The isotope
with a mass number of 16 has a relative abundance of 70%. Estimate the average atomic mass for this element.
IV. Relating Mass to Numbers of Atoms
1. The Mole (can be abbreviated mol, but NOT m, which is the abbreviation for meter!) - the SI unit for amount of substance. A
mole is the amount of a substance that contains as many particles as there are atoms in exactly 12 grams of ______________________.
2. Avogadro’s Number-the number of particles in exactly one mole of a pure substance. This number was determined
experimentally and its value is 6.022 X 1023, which means that 12 g of carbon-12 contains 6.022 x 1023 carbon-12 atoms.
3. Using the Mole and Avogadro’s Number – A mole can be thought of as a counting unit just like a dozen (12), gross
(144), pair (2), ream (500), mole (6.022X10 23 ).
A. How many is a mole? If every person living on Earth (6 billion people) worked to count out one mole of
oranges (or anything else), and if each person counted continually at a rate of one orange per second, it would take about 4 million
years for all the oranges to be counted!
If we had a mole of sand it would cover the earth 7 times over! If you had a mole of dollar bills, you could spend a million dollars
every minute of your life and never spend it all!
Since the mole is so large, we use it to count very tiny things – like atoms. Because the mole is so large, (and we now know that we
cannot count out a mole of anything), how do we know when we have a mole of anything?
We determine the mass and relate that to the number of atoms present. (Aluminum cans example.)
4. Molar Mass – The mass of one mole of a pure substance. The pure substance can be an element or a compound.
The atomic mass is the mass of 1 atom of that element measured in amu’s.
The atomic mass is also equal to 1 mole of atoms measured in grams it is called the molar mass!!!! What a coincidence!!!!
Mass of 1 atom of Pb =
Mass of 1 atom of N =
Mass of 1 atom of Ba =
Mass of 1 atom of Al =
amu
amu
amu
amu
Mass of 1 mole of Pb atoms =
Mass of 1 mole of N atoms =
Mass of 1 mole of Ba atoms =
Mass of 1 mole of Al atoms =
g
g
g
g
Let’s prove it: Determine the mass, in grams, of 6.022X10 23 atoms of aluminum. Use 1amu = 1.66X10-27kg.
With this information we can write some new conversion ratio’s!!
1 mole = 6.022X1023 atoms OR molecules OR formula units
1 mole Al = 26.98 grams
1 atom Al = 26.98 amu
V. Mole Problems – When in doubt go to the mole!
The MOLE has been defined as 6.022 x l023 atoms of a pure element or the molar mass of a substance expressed in grams. It can also be
defined as 6.022 x l023 molecules of a compound or diatomic molecule (O2, N2, H2, etc)
THE ONLY THING HARD ABOUT UNDERSTANDING THE DEFINITION OF A MOLE IS THAT YOU UNDERSTAND THAT
THE VALUE OF A MOLE IS DIFFERENT FOR EVERY DIFFERENT ELEMENT AND COMPOUND.
1. Gram/Mole conversions-how to convert moles to grams or grams to moles.
Example: 120 g Ca x 1 mole Ca = 3 mole Ca
40 g Ca
Practice:
How many grams of sodium are in 5.00 moles of sodium?
How many grams of magnesium are in 0.250 moles of magnesium?
How many moles of lead, Pb, are in 210. g of lead?
How many moles of nitrogen are in 44.0 g of nitrogen?
2. Conversions with Avogadro’s Number
Example: How many atoms of silver, Ag, are in 4.25 moles of Ag?
4.25 moles Ag X 6.022 x 1023 atoms Ag =
1 mole Ag
Practice:
1. How many atoms of Pb are in 3.80 moles of Pb?
2.
How many moles of Na are in 8.24 x 1024 atoms of Na?
Two-step conversions:
Ex: How many atoms of gold, Na, are in 5.25 g of Na?
5.25 g Na x 1 mole Na
x 6.022 x 1023 atoms Na =
23 g Na
1 mole Na
Practice:
1.
How many atoms of potassium, K, are in 3.99 g of K?
2.
How many g of He are in 3.03 x 1021 atoms of He?
3. How many atoms of Li are in 0.755 g of Li?
3. Molar Mass for compounds
How to find the molar mass:
1. Write a CORRECT formula for the compound (we’ll do this later)
2. Look up the atomic mass of each element in the compound
3. Multiply the atomic mass by the subscripts, if any. Add all masses of elements together and use the unit, g/mol
Example: find the molar mass of NaCl.
1. Na=22.99 g/mol
2. Cl=35.45 g/mol
58.44 g/mol
Example: find the molar mass of calcium phosphate, Ca3(PO4)2.
Ca = 40.08 x 3 = 120.24
P = 30.97 x 2 = 61.94
0 = 16.00 x 8 = 128.00
310.18 g/mol
Practice:
l. Find the molar mass of ammonium sulfate, (NH4)2SO4
2.
Find the molar mass of Cl2O7
3.
Find the molar mass of NaCl.
Hydrates - Some compounds trap water inside their crystal structure and are known as hydrates. You will not be able to predict
which compounds will form hydrates. All you have to do is to be able to name them and find their molar masses (including the water).
CuSO4 · 5H20 is an example of a hydrate. This says that one formula unit of cupric sulfate will trap 5 molecules of water inside its
crystal.
Hydrates are named by naming the ionic compound by the regular rules and then adding (as a second word) a prefix indicating the
number of water molecules. You will use the word “hydrate” to indicate water. The above compound would be called cupric sulfate
pentahydrate.
To find the formula mass of a hydrate, simply find the mass of the ionic compound by itself and then ADD the mass of water
molecule(s) to that mass.
Practice: What is the formula mass of barium chloride dehydrate, BaCl2 · 2H2O?
2.
What is the formula mass of aluminum sulfate octahydrate, Al2(SO4)3 · 8H2O?
VI: Real World Connections – Travels with Carbon (book page 68)
There is basically is no such thing as “new” air. When a living thing inhales, molecules are taken in, and when it exhales,
molecules are released back into the atmosphere to be reused later by some other living organism. Thus, at least in principle,
the molecules of “air” (nitrogen and oxygen, mostly) that Caesar exhaled from his last breath have since that time been
redistributed throughout Earth’s entire atmosphere. When you breathe, it is entirely possible that you will inhale one or more
of these molecules.
Mixed Practice Problems:
1. How many moles of water in 72.0 grams?
2.
How many moles are in 100. grams of nitrogen gas, N2?
3.
How many moles are there in 28.7 grams of lithium nitrate, LiNO 3?
4.
How many moles are there in 1.75 tons of magnesium chloride, MgCl2?
5.
How much would 38.0 moles of oxygen gas, O2, weigh in pounds? (454 g = 1 lb)
6.
How many atoms are there in 75.0 grams of pure iron, Fe?
7.
How many molecules are in 5.00 moles of water?
8.
How many atoms are in 3.50 moles of nitrogen gas, a diatomic molecule?
9.
How much would 7.12 x 1023 molecules of magnesium phosphate, Mg3(PO4)2 weigh in grams?
10. How many barium atoms are there in 5.89 grams of barium, Ba?
11. What is the formula mass of cupric sulfate petahydrate, CuSO4 · 5H2O?
A good thing to remember is this flow chart:
Grams ↔ mole ↔ # of atoms
OR # of molecules
OR # of formula units