Atoms: The Building
Blocks of Matter
3 Foundational Laws of
Chemistry
Law of conservation of
mass
States that mass is
neither created nor
destroyed during
ordinary chemical
reactions or physical
changes
3 Foundational Laws of
Chemistry
Law of definite
proportions
States that a
chemical
compound contains
the same elements
in exactly the same
proportions by
mass regardless of
the size of the
sample or source of
the compound
3 Foundational Laws of
Chemistry
Law of multiple proportions
States that if two or more
different compounds are
composed of the same
two elements, then the
ratio of the masses of the
second element combined
with a certain mass of the
first element is always a
ration of small whole
numbers.
The Size of an Atom
Just How Small Is An Atom
Democritus
Evolution of the
Atomic Model
• Greek philosopher
• First to propose that matter is made up of tiny particles, which
he termed atomos
• Believed atoms could not be created or destroyed
Dalton
• Ideas led to the development of the modern atomic theory
• His theory has since been revised based on current research
and understanding of the atom
Evolution of the
Atomic Model
Thomson
•
His experiments with cathode-ray tubes led to the discovery of the
first subatomic particle—the electron
•
Proposed a new model of the atom, known as the Plum Pudding
Model, that suggested electrons resided in area of positive charge
Rutherford
•
His experiment with alpha particles and gold foil led to the
discovery of the positively charged atomic nucleus
•
Proposed a new model of the atom where the atom is composed
of a dense positively charged nucleus surrounded by electrons
Evolution of the
Atomic Model
Bohr
• Developed a new atomic model debunking Rutherford, which
proposed that electrons were arranged in concentric circles
around the nucleus and that these paths had fixed energies
(energy levels)
Millikan
• His experiment with an oil drop apparatus led to
discovery/determination of the quantity of charge carried by
an electron
Evolution of the
Atomic Model
Chadwick
• His experiments led to the discovery of the neutron
Schrödinger
• His wave equation provided a theoretical explanation of
electron in atoms—which estimates the probality of finding an
electron in a certain position (unlike Bohr which defines
electrons to exact pathways)
Dalton’s Atomic Theory
All matter is composed of extremely small particles
Atoms of a given element are identical in size, mass, and
other properties; atoms of different elements differ in
size, mass, and other properties
Atoms cannot be subdivided, created or destroyed
Atoms of different elements combine in simple wholenumber ratios to form chemical compounds.
In chemical reaction, atoms are combined, separated or
rearranged
Discovery of the Electron
Discovery of the Electron
Discovery of the Atomic Nucleus
Discovery of the Atomic Nucleus
Properties of
Subatomic Particles
Particle
Symbols
Relative
Electric
Charge
Mass
Number
Relative
Mass
Actual mass
Electron
e-
-1
0
0.000 5486
9.109 X 10-31
Proton
p+
+1
1
1.007 276
1.673 X 10-27
Neutron
no
0
1
1.008 665
1.675 X 10-27
Counting Atoms
Atomic number—
Of an element tells the number of protons of each
atom of the element and is represented by Z
Composition of the
Atomic Nucleus
All atomic nuclei are made of
two kinds of particles:
• protons
•neutrons
Atomic number
The number of protons in the nucleus of each
atom of that element. The protons NEVER
change and ALWAYS identify the element
Look at the periodic table
Ag silver atoms contain 47 protons
In a neutral atom:
number of electrons = number of protons
Quick Review
Atoms are made up of three particles:
Protons
Neutrons
Electrons
Question: Which of the three particles identifies what
element an atom is?
The PROTON! (very important)
Different Forms of the
Same Element
In any element, the # of protons is always constant.
Unlike the number of protons, the number of electrons
and neutrons can vary within an element without
changing the identity of the element.
Ex. Carbon (C) ALWAYS has 6 protons, but it can have
anywhere from 6-8 neutrons and 2-10 electrons
Isotopes
An ISOTOPE is a form of an element that has a different
number of neutrons than “normal”
Carbon has three isotopes
Notice how the # of protons does
NOT change!
Other Isotopes
Most atoms have naturally occurring isotopes including:
•
•
•
Radon
Potassium
Uranium
When an element is “radioactive” it means it has an
unstable number of neutrons (an unstable ISOTOPE)
Designating Isotopes
Hyphen notation
name of element – mass number
Hydrogen – 3
Nuclear symbol
mass number
atomic number
20
IONS
An atom usually has a neutral charge. That means it has
the same number of protons as electrons
• Remember, a proton has a positive charge and an
electron has a negative charge
ION – an atom that has lost or gained one or more
electrons and has become charged either positively or
negatively
Positive Ions
When an atom LOSES electrons, it becomes more
POSITIVE
• Why?
If you are getting rid of negative particles (electrons)
but your number of positive particles (protons) are
staying the same.
• In other words, you are subtracting negative
numbers
Examples
What would the charge be if:
The neutral form of Gold (Au) lost 4 of its 79 electrons.
It now has 79 protons and 75 electrons
The neutral form of Mg lost 2 of its 12 electrons. It now
has 12 protons and 10 electrons.
Negative Ions
When an atom GAINS electrons it becomes more
NEGATIVE
Why?
Electrons have a negative charge, so the more you
have, the more negative you become
Representing Ions
Ions are represented by placing a “superscript”
charge number next to the atomic symbol.
Ex.
•
•
•
•
O-2 = oxygen with a negative 2 charge
K+ = potassium with a positive 1 charge
N-3 = nitrogen with a negative 3 charge
And so on
Periodic Table
The elements, as they are found on the periodic table,
are neutral atoms and their mass is an average of all
isotopes
• Remember the atomic mass is the average of
ALL isotopes, but when we round it for
calculating the number of neutrons, we always
get the most abundant isotope.
Examples
Examples
How many P’s, N’s, and E’s does each of the following have?
Label each as an isotope, ion, or the version found on the P.T.
HONORS CHEM ONLY
Relative Atomic Mass
One atom, carbon-12, is set as a
standard
All masses are expressed in
relation to this standard
1 atomic mass unit = 1/12 the
mass of a carbon-12 atom
Chapter 3 Section 3 Counting Atoms pages 7787
29
HONORS CHEM ONLY
Relative Atomic Mass
Examples
• Hydrogen – 1 = 1.007825 amu
• Oxygen – 16 = 15.994915 amu
• Magnesium – 24 = 23.985042 amu
p+ = 1.007276 amu, n0 = 1.008665
amu, e- = 0.0005486 amu
Relative mass and mass number are
close in value but not the same
30
HONORS CHEM ONLY
Average Atomic Mass
The weighted average of the atomic
masses of the naturally occurring isotopes
of an element
Example
• Copper
Cu-63: .6915 x 62.93 amu = 43.52
Cu-65: .3085 x 64.93 amu = 20.03
63.55 amu
Chapter 3 Section 3 Counting Atoms pages 7787
31
HONORS CHEM ONLY
Average Atomic Mass
The weighted average of the atomic masses of the
naturally occurring isotopes of an element—
expressed in amu (atomic mass units)
Example
HONORS CHEM ONLY
Average Atomic Mass
Isotope
Atomic Mass (amu)
Carbon-12
12 (by definition)
Percentage natural
abundance
98.93
Carbon-13
13.003355
1.07
Calculate carbon’s average atomic mass
HONORS CHEM ONLY
Average Atomic Mass
Isotope
Atomic Mass
Percentage Natural Abundance
Oxygen-16
15.994915
99.757
Oxygen-17
16.999132
0.038
Oxygen-18
17.99160
0.205
Calculate oxygen’s average atomic mass
The Mole
A counting unit
Similar to a dozen, except instead of 12, it’s 602
billion trillion 602,000,000,000,000,000,000,000
• 6.02 X 1023 (in scientific notation)
• This number is named in honor of Amedeo
Avogadro (1776 – 1856), who studied
quantities of gases and discovered that no
matter what the gas was, there were the same
number of molecules present
Just How Big is a Mole?
Enough soft drink cans to cover the
surface of the earth to a depth of over
200 miles.
If you had Avogadro's number of
unpopped popcorn kernels, and spread
them across the United States of
America, the country would be covered
in popcorn to a depth of over 9 miles.
If we were able to count atoms at the
rate of 10 million per second, it would
take about 2 billion years to count the
atoms in one mole.
Everybody Has Avogadro’s Number!
But Where Did it Come From?
It was NOT just picked! It
was MEASURED.
One of the better methods
of measuring this number
was the Millikan Oil Drop
Experiment
Since then we have found
even better ways of
measuring using x-ray
technology
Learning Check
Suppose we invented a new collection unit
called a rapp. One rapp contains 8 objects.
1. How many paper clips in 1 rapp?
a) 1
b) 4
c) 8
2. How many oranges in 2.0 rapp?
a) 4
b) 8
c) 16
3. How many rapps contain 40 gummy bears?
a) 5
b) 10
c) 20
The Mole
• 1 dozen cookies = 12 cookies
• 1 mole of cookies = 6.02 X 1023 cookies
• 1 dozen cars = 12 cars
• 1 mole of cars = 6.02 X 1023 cars
• 1 dozen Al atoms = 12 Al atoms
• 1 mole of Al atoms = 6.02 X 1023 atoms
Note that the NUMBER is always the same, but the
MASS is very different!
Mole is abbreviated mol (gee, that’s a lot quicker to
write, huh?)
A Mole of Particles
Contains 6.02 x 1023 particles
1 mole C
= 6.02 x 1023 C atoms
1 mole H2O
= 6.02 x 1023 H2O molecules
1 mole NaCl = 6.02 x 1023 NaCl “molecules”
(technically, ionics are compounds not
molecules so they are called formula units)
6.02 x 1023 Na+ ions and
6.02 x 1023 Cl– ions
Avogadro’s Number as
Conversion Factor
6.02 x 1023 particles
1 mole
or
1 mole
6.02 x 1023 particles
Note that a particle could be an atom OR a molecule!
Learning Check
1.
Number of atoms in 0.500 mole of Al
a) 500 Al atoms
b) 6.02 x 1023 Al atoms
c) 3.01 x 1023 Al atoms
2.Number of moles of S in 1.8 x 1024 S atoms
a) 1.0 mole S atoms
b) 3.0 mole S atoms
c) 1.1 x 1048 mole S atoms
Molar Mass
• The Mass of 1 mole (in grams)
• Equal to the numerical value of the average
atomic mass (get from periodic table)
1 mole of C atoms
=
12.0 g
1 mole of Mg atoms
=
24.3 g
1 mole of Cu atoms
=
63.5 g
Other Names Related to Molar
Mass
• Molecular Mass/Molecular Weight: If you have a single
molecule, mass is measured in amu’s instead of grams.
But, the molecular mass/weight is the same numerical
value as 1 mole of molecules. Only the units are
different. (This is the beauty of Avogadro’s Number!)
• Formula Mass/Formula Weight: Same goes for
compounds. But again, the numerical value is the
same. Only the units are different.
• THE POINT: You may hear all of these terms
which mean the SAME NUMBER… just different units
Learning Check!
Find the molar mass
(usually we round to the tenths place)
A. 1 mole of Br atoms = 79.9 g/mole
B. 1 mole of Sn atoms = 118.7 g/mole
Molar Mass of Molecules and
Compounds
Mass in grams of 1 mole equal numerically to
the sum of the atomic masses
1 mole of CaCl2
= 111.1 g/mol
1 mole Ca x 40.1 g/mol
+ 2 moles Cl x 35.5 g/mol
= 111.1 g/mol CaCl2
Learning Check!
A. Molar Mass of K2O = ? Grams/mole
A. Molar Mass of K2O = ? Grams/mole
= 39 (2) + 16 = 94 g/mol
Molar Mass of Rolaids antacid
Al(OH)3 = ? Grams/mole
Molar Mass of Rolaids
Al(OH)3 = ? Grams/mole
27 + (17) x 3 = 78 g/mol
Molar Conversions
Calculations with Molar Mass
molar mass
Grams
Moles
Converting Moles and Grams
Aluminum is often used for the
structure of light-weight bicycle
frames. How many grams of Al are in
3.00 moles of Al?
3.00 moles Al
? g Al
1. Molar mass of Al 1 mole Al = 27.0 g Al
2. Conversion factors for Al
27.0g Al
or
1 mol Al
1 mol Al
27.0 g Al
3. Setup 3.00 moles Al
x
27.0 g Al
1 mole Al
Answer
= 81.0 g Al
Learning Check!
The artificial sweetener aspartame (NutraSweet) formula C14H18N2O5 is used to
sweeten diet foods, coffee and soft drinks.
How many moles of aspartame are present
in 225 g of aspartame?
(12x14)+(1x18)+(14x2)+(16x5) = 312 g/mole
225 g Asp. X 1 mole = .721 moles Aspertame
312 g
Atoms/Molecules and Grams
Since 6.02 X 1023 particles = 1 mole
AND
1 mole = molar mass (grams)
You can convert atoms/molecules to moles and then
moles to grams! (Two step process)
You can’t go directly from atoms to grams!!!! You
MUST go thru MOLES.
That’s like asking 2 dozen cookies weigh how many
ounces if 1 cookie weighs 4 oz? You have to
convert to dozen first!
Calculations
molar mass
Grams
Avogadro’s number
Moles
particles
Everything must go through
Moles!!!
Atoms/Molecules and Grams
How many atoms of Cu are present in 35.4 g
of Cu?
35.4 g Cu
1 mol Cu
63.5 g Cu
6.02 X 1023 atoms Cu
1 mol Cu
= 3.4 X 1023 atoms Cu
Learning Check!
How many atoms of K are present in 78.4 g of K?
Conversions Image
p. 84
Chapter 3 Section 3 Counting Atoms pages 7787
60