CHAPTER 2: ATOMS
AND MOLECULES
Chemistry 140
HCC/TCHS
LEARNING
OBJECTIVES/ASSESSMENT
• 1. Use symbols for chemical elements to write formulas for
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chemical compounds.
2. Identify the characteristics of protons, neutrons, and electrons.
3. Use the concepts of atomic number and mass number to
determine the number of subatomic particles in isotopes and to
write correct symbols for isotopes.
4. Use atomic weights of the elements to calculate molecular
weights of compounds.
5. Use isotope percent abundances and masses to calculate atomic
weights of the elements.
6. Use the mole concept to obtain relationships between number of
moles, number of grams, and number of atoms for elements, and
use those relationships to obtain factors for use in factor‐unit
calculations.
7. Use the mole concept and molecular formulas to obtain
relationships between number of moles, number of grams, and
number of atoms or molecules for compounds, and use those
relationships to obtain factors for use in factor‐unit calculations.
2.1 Symbols and Formulas
• All elements in the Periodic Table have
symbols that are recognized world wide.
• It does not matter which country you are in or
the language you speak.
• Example: The element Iron is always
identified by “Fe” and Oxygen is “O”.
• The names are not always the same but the
symbols are. “Fe” is iron in Canada, fer in
France and fier in Romania.
Chemical Symbols
• 1. The first letter of a symbol is always capitalized.
• 2. If there are two or three letters in a symbol, they
are never capitalized.
• 3. Some symbols were derived from other languages.
• Examples: sodium – Na, tungsten – W, lead – Pb
• Sodium comes from the word sodanum, a headache remedy,
and it’s symbol (Na) comes from the Latin word Natrium.
• There are 36 common elements you should know the
symbol for. A quiz will be given soon!
Chemical Formulas
A chemical formula is the combination of symbols that
represent a particular compound.
The chemical formula indicates which elements are
present in the compound and in what proportions.
Ex. 1: Water molecule
H2O: 2 atoms of hydrogen, 1 atom of oxygen
Ex. 2: Iron Oxide molecule
Fe2O3: 2 atoms of iron, 3 atom of oxygen
Chemical Formulas - Examples
• Calcium Carbonate (chalk) – CaCO3
• Sodium Chloride (salt) – NaCl
• Acetylsalicylic acid (aspirin) – C9H4O8
• Acetic acid (vinegar) – C2H4O2
Chemical Formulas
• NOTES:
- Each symbol in a formula represents an
element.
- If only one atom of an element is present in
the compound, no subscript is used.
- If more than one atom of an element is used,
then the symbol is followed by a number
indicating how many atoms are used. This is
called the subscript.
Chemical Formula Quantification
• Subscripts
2.2 Inside the Atom
• Particle
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Symbol
Charge
Location
Size(relative)
electron
eelectron shell
S
proton
p+
+
nucleus
M
neutron
n0
0
nucleus
L
Protons and neutrons are made up of smaller particles called
quarks.
So far, scientists have confirmed the existence of six uniquely
different quarks.
2.3 Isotopes
• isotope - atoms of the same
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element having the same atomic
number but different mass
numbers.
atomic number - the number of
protons in an atom.
mass number - the number of
protons and neutrons in an atom.
Models of two isotopes of boron
are shown. Because the numbers
of neutrons in the isotopes are
different, the mass numbers are
also different.
You use the name of the element
followed by the mass number of
the isotope to identify each
isotope: boron-10 and boron-11.
2.4 Relative Masses of Atoms
and Molecules
The formula mass of sucrose, C12H22O11, is
equivalent to the molar mass or the mass of
one mole of the compound in grams.
• Element
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# atoms
mass
total
C
12
x
12
=
144
H
22
x
1
=
22
O
11
x
16
=
176
------------------------------------------------------------------------formula mass.....342g/mol
2.5 Isotopes and Atomic Weights
• The average atomic mass of an element is the weighted-
average mass of the mixture of its isotopes.
• For example, four out of five atoms of boron are boron-11, and
one out of five is boron-10.
• To find the weighted-average or the average atomic mass of
boron, you would solve the following equation:
Atomic Mass
• The actual mass of an atom is extremely
small.
• For example the mass of the oxygen atom is
2.65 X 10-23 g.
• The mass of atomic particles is expressed in
atomic mass units, amu. One amu is equal
to 1.6605655 X 10-24 g. This is 1/12 the mass
of the carbon 12 isotope. The atomic masses
of all atoms is based on this concept.
• atomic mass - the actual mass of an atom
expressed in amu
Isotopes of Hydrogen
• Example: hydrogen has three isotopes;
protium, deuterium, and tritium. Each of
these three isotopes has a single electron
located in the first orbit or shell. These shells
or orbits are also referred to as energy levels.
• The shells are labeled with the letters
K,L,M,N,O,P, and Q.
2.6 Avogadro’s Number: The Mole
• mole - that amount of a substance containing
Avagadro's number(6.02 X 1023) of chemical
units
• If a substance is elemental, such as iron, one
mole of iron contains 6.02 X 1023 iron atoms.
• If a substance is a compound, such as water,
one mole of water contains 6.02 X 1023 water
molecules.
2.7 The Mole and Chemical
Formulas
• Atomic weight or molar mass is the
average weight of all the available
isotopes of a particular element, which
occur in nature, in amu. We will mainly
be concerned with the gram-atomic
weight of the elements.
• Molar mass – the mass of one mole of a
substance expressed in grams
Molar Mass
• The mass of one mole of hydrogen is ____ g.
• The mass of one mole of C is 12.011 or 12 g.
• What is the mass of 0.5 moles of carbon?
• mass = # moles x molar mass
• How many moles in 24g of carbon?
Sample Problems
• Sample Problem 1: Find the number of moles
in 96 grams of oxygen.
• Sample Problem 2: Find the mass of 2.5
moles of magnesium.
• Sample Problem 3: Find the number of moles
in 50 grams of calcium carbonate, CaCO3.
• Sample Problem 4: Find the mass of 0.25
moles of CaO.