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Chapter #2
Atoms and Molecules
Chapter Overview
Atoms and Molecules
 Symbols and Formulas
 Inside the Atom
 Isotopes
 Relative masses of atoms and molecules
 Isotopes and atomic weights
 Avogadro’s number: the mole
 The mole and chemical formula’s
Elements
•
Elements are pure substances made up of identical
atoms.
•
There are 115 known kinds of atoms. Each has its own
symbol.
•
The first character in an atoms symbol is an uppercase
letter that corresponds the first letter of the elements name.
•
Most elements have a second character in their symbol
which is is a lowercase letter usually the second letter of
the elements name.
Element Names
Elements discovered long ago have names and symbols with
Latin or other origins, such as Au for gold (from aurum, meaning
“bright dawn”), or Pb for lead (plumbum). Tungsten has a
symbol of W, which is the first letter of its German name,
wolfram.
Symbol and names for the known atoms are given in Table 2.1.
The symbol and names
for the known atoms are
given in Table 2.1.
Periodic Chart
You can also find these symbols and names in the periodic
table inside the front cover of your text although this chart does
not have names provided.
Subatomic Particles
What are the atoms made of?
Atoms are composed of subatomic particles most of which
exist outside the stable structure of atoms for very short periods of
time.
Three subatomic particles are of interest to chemists:
1. The proton (p)
2. The neutron (n)
3. The electron (e-)
Subatomic Particles
1 amu = 1.67 x 10-24 g
Neutrons and protons are about 2,000 times larger than
electrons.
Atoms have a dense heavy, positively charged nucleus
containing the neutrons and protons.
Outside the nucleus are the small, negatively charged
electrons.
Combining Atoms
What happens when we combine atoms?
When two or more different types of atoms combine compounds
are formed.
In a compound formula we write the symbol for each atom type
present in the compound, if more than one atom of a given type
then a subscript is used to indicate the number of each atom.
e.g. NH3
H2O
SO2
CH4
Atomic Number
Atoms have no overall charge so contain the same number of
protons (+) as electrons (-).
The number of protons in the nucleus of an atom is called the
atomic number It is given the symbol Z.
All atoms of a given type or element have the same atomic
number (Z).
The periodic table arranges atoms in order of atomic number.
Mass Number
Neutrons have no electrical charge. For a given atom type the
number of neutrons may vary.
The sum of the number of neutrons and protons in an atom is
called the mass number. It is given the symbol A.
Atoms with the same number of protons but different numbers of
neutrons in their nucleus are called isotopes.
BOHR MODELS OF HYDROGEN
(The three isotopes of hydrogen)
E
E
P
Hydrogen-1
P N
Hydrogen-2
E
NP
N
NP
N
Hydrogen-3
BOHR MODELS OF HYDROGEN
E
P
Hydrogen-1
1
H
E
E
P
N
Hydrogen-2
2
H
P
NN N
N
P
N
Hydrogen-3
3
H
BOHR MODELS OF HYDROGEN
E
P
P
1
H
2
1
H
P
NN N
N
P
N
N
Hydrogen-2
Hydrogen-1
1
E
E
Hydrogen-3
3
1
H
BOHR MODELS OF HYDROGEN
E
P
Hydrogen-1
1
1
E
E
P
N
Hydrogen-2
P
NN N
N
P
N
Hydrogen-3
2
3
1
1
Atomic Number (number of protons); found on the periodic chart
H
H
H
BOHR MODELS OF HYDROGEN
E
P
Hydrogen-1
1
1
P
Hydrogen-2
H
P
NN N
N
P
N
N
2
1
Atomic Number (number of protons)
H
E
E
Hydrogen-3
3
1
H
Mass number (sum of protons and neutrons); Not found on periodic chart
BOHR MODELS OF HYDROGEN
E
P
Hydrogen-1
1
1
E
E
P
N
Hydrogen-2
P
NN N
N
P
N
Hydrogen-3
2
3
1
1
Atomic Number (number of protons)
Mass number (sum of protons and neutrons); Not found on periodic chart
H
H
Oxidation number (Protons – electrons)
H
Atomic Symbols
We can distinguish isotopes with the following notation:
Where:
A
ZE
E is the atom’s symbol
 A is the mass number
 Z is the atomic number
e.g. 612C, 613C, 614C, 11H, 12H and 13H
As all atoms with a given symbol have the same atomic number
sometimes we skip writing Z.
e.g. 612C (carbon-12), 613C (carbon-13) and 614C (carbon-14)
Relative Atomic Mass
If we assign a mass of 12 atomic mass units (u or sometimes
amu) to a carbon-12 atom then we can compute the relative
atomic weight for any other atom.
Atomic weights or atomic masses are given in your periodic
table.
Relative Atomic Mass
Some atoms naturally occur as a mixture of isotopes.
e.g. 11H, 12H and 13H
The atomic weights given in the periodic table take this into
account.
They are the average atomic weight taking into account the
amounts of each isotope present.
Relative Atomic Mass of Hydrogen
When computing the average atomic mass of any element
the radioactive (unstable) isotopes are excluded since there
relative abundances are slowly decreasing.
Hydrogen has three isotopes. The first two protium and
deuterium are stable isotopes and the third tritium is unstable,
thus excluded in the calculation.
The relative abundance of isotopes of a particular element
is constant here on our planet Earth. Mass spectroscopy
gives information of relative abundances and relative
masses of isotopes.
Relative Atomic Mass of Hydrogen
Mass spectroscopy gives the following information for the two
stable isotopes of hydrogen.
isotope name
Hydrogen-1
(protium)
Hydrogen-2
(deuterium)
relative abundance
99.985 %
0.015 %
Mass (amu)
1.007825
2.01355
To calculate a weighted average, convert the percent to
a decimal by moving the decimal two places to the left.
Then multiply this number by the mass and add the two
masses.
Relative Atomic Mass of Hydrogen
0.99985 X 1.007825 = 1.007673
0.0015 X 2.01355 = 0.003021
1.010694 amu
1.0107 amu
Relative Atomic Mass
we defined the relative atomic weight of each kind of atom as
follows:
“assign a mass of 12 atomic mass units to a carbon-12 atom
then compute the relative atomic weight for all other atom
types.”
Atomic mass units are very small.
1 amu = 1.67 x 10-24 g
Not very practical !!!
Relative Atomic Mass
Consider the following ozone has the formula O3. Two molecules
of ozone contains 6 oxygen atoms. Oxygen has the formula O2.
What is the mass of three molecules of O2?
Three molecules of O2 has the
same mass as two molecules of
O3 as they each contain a total of
six oxygen atoms.
In chemistry we are often more interested in the number of
particles but we can only measure the mass easily.
The Mole
So we have two problems:
1. It is difficult to count particles as small as molecules, atoms,
ions, protons, neutrons, or electrons.
2. Atomic mass units are not very practical.
We need a system that allows us to switch between mass (in
units which we can measure) and number of atoms or molecules.
The Mole
We can define a constant called Avogadro’s constant (NA):
“NA is the number of atoms needed to give a carbon-12 a
mass of 12 grams”
An Avogadro’s number of hydrogen atoms would weigh
1.008g, or of any other element on the Periodic Chart their
atomic weight.
The Mole
One Avogadro’s number of something is called a mole (mol).
NA = 6.022 x 1023
A mole is often compared to a “dozen”. Just as a dozen of
something means there are 12 of them, a mole of something
means there are 6.022 x 1023 of them. Or, how about a ream?
How many pieces of paper are in a ream? Dozens and reams
are convenient counting units for objects, but not for particles.
Moles of Objects
Mole is Latin for heap
Suppose we order a mole of marshmallows for a chemistry
party. How much space here at Central would be required
to store the marshmallows?
Moles of Objects
Mole is Latin for heap
Suppose we order a mole of marshmallows for a chemistry
party. How much space here at Central would be required
to store the marshmallows?
Would cover the entire 50 states 60 miles deep
Moles of Objects
Mole is Latin for heap
Suppose we order a mole of marshmallows for a chemistry
party. How much space here at Central would be required
to store the marshmallows?
Would cover the entire 50 states 60 miles deep
How about a mole of computer paper instead of a
ream of computer paper, how far would that stretch?
Moles of Objects
Mole is Latin for heap
Suppose we order a mole of marshmallows for a chemistry
party. How much space here at Central would be required
to store the marshmallows?
Would cover the entire 50 states 60 miles deep
How about a mole of computer paper instead of a
ream of computer paper, how far would that stretch?
Way past the planet Pluto!
Formula Weight Calculation
To calculate the molar mass of a compound we sum together the
atomic weights of the atoms that make up the formula of the
compound. This is called the formula weight (MW, M).
Formula weights are the sum of atomic weights of atoms
Making up the formula.
The following outlines how to find the formula weight of water
symbol
H
O
number
weight
= 2.02
1.01 X
2
1
= 16.0
X
16.0
18.0 g/mole
Percent Composition
Find the formula weight and the percent composition of
glucose (C6H12O6)
symbol
C
H
O
%C =
weight
number
12.0 x 6 = 72.0
1.01 x 12 = 12.12
16.0 x 6 = 96.0
180.1 g/mole
72.0
X = 40.0 %C
180.1
%H = 12.12
180.1
96.0
%O =
180.1
X = 6.73 %H
X =
53.3 %O
Formula Weight
We can define the molar mass or molecular weight of a
substance can as:
“the mass of one mole of that substance.”
We give molar mass the symbol M and it has units g/mol-
For an atom the molar mass is equal to the atomic weight we
find on the period table.
Its All About Moles
For a molecule the molecular weight is equal to the sum of the
atomic weights of the atoms that make it up.
The mathematical relationship between mass and moles is:
m = nM
This can be summarized by the following diagram:
moles
Mole Concepts
The key equation to remember for mole calculations is:
m = nM
Where:
M = molecular weight (gmol-1)
n = number of moles (mol)
m = mass of sample (g)
Mole Concepts
For a mole of a molecule the number of moles of each atom is
determined by how many of that atom are in each molecule.
e.g. One mole of H2O contains:
One mole of oxygen atoms
Two moles of hydrogen atoms
In 5 moles of H2SO4 how many moles of oxygen atoms is there?
20 moles of O atoms.
Mole Conversions
In 50.0g of H2SO4 how many moles of oxygen atoms are there?
50.0g of H2SO4
Mole Conversions
In 50.0g of H2SO4 how many moles of oxygen atoms are there?
50.0g of H2SO4 mole H2SO4
98.0g of H2SO4
=
Mole Conversions
In 50.0g of H2SO4 how many moles of oxygen atoms are there?
50.0g of H2SO4 mole H2SO4
4mole O
98.0g of H2SO4 mole H2SO4
= 2.04 mole O
Mole Conversions
In 5 moles of H2SO4 how many atoms of oxygen are present?
Mole Conversions
In 5 moles of H2SO4 how many atoms of oxygen are present?
5 moles H2SO4
=
Mole Conversions
In 5 moles of H2SO4 how many atoms of oxygen are present?
5 moles H2SO4 4 mole O
mole H2SO4
Mole Conversions
In 5 moles of H2SO4 how many atoms of oxygen are present?
5 moles H2SO4 4 mole O
6.02 x 1023 atoms O
=
mole
O
mole H2SO4
1.20 x 1025 atoms
The End