Chapter 2 Atoms and Molecules
2-1 Elements and their symbols
Most of the chemicals you find in everyday life can be broken down into simper
substances
Key Concepts:
A substance that cannot be broken down into simpler substances is called an
element
A substance that can be broken down into two or more elements is called a
compound
The above definition of elements is good, but here is a better one:
An element is a substance that consists only of atoms with the same nuclear
charge (more on this in a minute)
There are currently about 120 known elements
but only 40 of these make up 99.99% of the materials we know
(So 80 elements are very rare)
only 10 make up 99% of the total mass of the earth
Table 2.2
Key Concept:
Elements can be divided into two broad classes: metals and nonmetals
Metals
Properties of solid metals
luster
cast into shapes
good conductors of heat and electricity
malleable - (can be rolled or hammered into sheets)
Ductile - (can be drawn into a wire)
Table 2.3 lists some of the common metals
Key Concept:
Chemical Symbols are one or two letter abbreviations used to designate
elements
Chemical symbols usually consist of the first couple of letters in an elements
name. The problem is that sometimes the name dates back to the original latin!
See interchapterA (online) if you are interested in where these names came
from.
2
You need to start memorizing the symbols of the most common elements!
They are your basic vocabulary for the rest of this year.
Nonmetals
vary greatly in appearance
over ½ are gases at RT
some are solid
one (bromine) is a liquid
Poor conductors of heat or electricity
not ductile or maleable
do not have luster
Common nonmetals shown in table 2.5
Note that several nonmetal elements have a subscript of 2
This means that the elemental unit consists of two atoms that are joined
together
Key concept:
A molecule is when two are more atoms are joined together to form a single unit
A molecule consisting of just two atoms is called a diatomic molecule.
When we refer to a naturally occurring diatomic element by name, we are
referring to the diatomic molecule. IE oxygen means the O2 molecule. If
we want to refer to a single atom of that element we will say ‘the oxygen
atom’.
3
2-2 States of Matter
Not only are we concerned with the elements that compose a chemical, we are
also concerned with the physical state of the chemical.
The most common states are:
KEY TERMS:
Solid - When a chemical is in the solid form we write the chemical formula
followed by (s) as in NaCl (s). Solids have a fixed shape because they
are composed of a rigis, well define matrix locking the atoms into place.
Liquids - Denoted by (l) in a chemical formula. Here there are
intramolecular forces holding the individual molecules in contact with each
other so there is a fixed volume, but the forces do not fix the molecules in
a rigid geometry so they are free to move around. Thus we have a fixed
volume, but the shape can change.
Gases - Denoted by (g). Here there are no forces holding the molecules
together, so the molecules move around and expand to fill the space they
are in. That is why there is no fixed volume or shape.
Aqueous - Denoted by (aq). Here the chemical is dissolved in water to
make an aqueous solution. (Next section, but I might as will throw it in
now!)
2-3 Separation of mixtures
In nature most substances occur in some kind of mixture.
A mixture is where component substances are mixed together, but they do not
combine chemically so there can be many different molecules all mixed together
Mixtures can be classed as
KEY TERMS:
Homogenous - the same from point to point
Or
Heterogeneous - different from point to point
The usual way you can tell a homogenous mixture from a heterogeneous
mixture is with your eyes. If you can see some visibly different
components within a mixture it is homogenous. If you can’t see different
‘chunks’ then it is homogeneous
KEY TERM: Another name for a homogeneous mixture is solution.
Note that you probably think of a solution as something dissolved in water.
4
And that is correct, because once it is dissolved in water, you can no
longer see the individual components of the mixture.
However the Chemical term solution applies to any uniform mixture, so it
also can be applied to such diverse mixtures as the air we breath, or fog
or a piece of solid brass
Many solutions are solids dissolved in liquids. In this case we have more
specialized terminology
KEY TERMS
Solvent - is the liquid that something is dissolved in
Solute - is the solid that is dissolved in the liquid
Since the components of a mixture are not chemically linked to each other we
can separate the components using various physical process. The book
mentions several, but I am going to take a rain check on them
2-4 Law of Constant Composition
If we start with any mixture, and then apply the various physical process to
separate the individual components, we will eventually come up with various
materials that have a constant composition.
Key Concept:
This has lead to the Law of Constant Composition for compounds:
The relative amount of each element in a compound is always the same
regardless of the source of the compound or how it was prepared.
One way to observe this law is by looking at the mass percentage of elements in
a compound.
Key Equation:
5
Example 1:
In a 20 g sample of water we find that there is 17.69 grams of O, what is the %
composition of oxygen in water?
17.69/20.00 x 100% = 88.45%
And this is true for any sample of water, it will always contain 88.45% by mass
oxygen, no mater where it comes from.
Note: the % compositions of all the elements in a compound should sum to
100% (Duh!). This can be a nice check on your calculations
Example 2:
I have a 20.15 gram sample that contains only sodium and chlorine and is
39.33% sodium. How many grams are chlorine are in this sample?
Approach 1 : grams sample 6grams sodium 6grams chlorine
39.33 % = X/20.15 x 100%
39.33%/100% = .3933 = X/20.15
.3933 x 20.15 = g sodium = 7.92 g Na
Mass Sample = Mass Sodium + Mass Chlorine
20.15 = 7.92 + Mass Cl
20.15 - 7.92 = mass Cl =
12.23 g Cl
Approach 2: % Na6% Cl6 mass Cl
100% = %Na + % Cl
100% = 39.33% + %Cl
100-39.33 = 60.67% Cl
60.67% Cl = Mass Cl/Total mass x 100%
60.67%/100% = .6067 = Mass of Cl / 20.15
.6067 x 20.15 = mass of Cl = 12.23 g Cl
The Law of constant composition lead to another law called the law of multiple
proportions. This law is a little harder to explain because you compare ratios of
composition in different compounds. It is one of the pieces that lead to Dalton’s
Atomic theory, but it is not something we use much today, so I will skip any
further details or calculations.
Clicker question: set up for a % composition problem
6
2-5 Dalton’s Atomic Theory
In the early 1800's Dalton proposed the atomic theory, on which much of our
current understanding of chemistry relies. His theory (updated with modern
terms and concepts) is:
Key Theory:
1. Matter is composed of small, indivisible particles called atoms
2. The atoms of a given element have the same mass and are identical in all
ways
3. The atoms of different elements differ in mass and in chemical behavior.
4. Chemical compounds are composed of two or more atoms of different
elements joined together. This particle of joined atoms is called a molecule.
5. In chemical reactions atoms are not created or destroyed, rather they are
rearranged to form different molecules
This theory works well and is consistent with all of the laws we have talked about
of to this point, the conservation of mass, the law of constant composition, and
the law of multiple proportions.
Let’s talk a bit about the mass of an atom
As we look at different compounds and use our law of constant composition and
law of multiple proportions, we can define a quantity called Atomic Mass Ratio
Atomic mass ratio = mass of atom A / mass of atom B
The beauty of this is that since one mass id divided by another mass, the units
cancel out, and the Atomic Mass ratio is a unitless number
Using these ratios we can make a mass system that relates the mass of all the
atoms on the periodic table to any other atom. All we have to do is to chose one
to base everything on. At different times we have used different atoms as our
key atom that we base the weights on, H one time, Oxygen another. Currently
our mass system is based on carbon (more on this later)
The term atomic mass ratio has since been shortened to simply atomic mass.
While the Atomic mass ratio is a unitless number, sometimes we assign in a unit
called the atomic mass unit or amu which has now been shortened to the
abbreviation u in the IUPAC (International Union of Pure and Applied Chemistry)
system. Unfortunately for you, biochemists don’t use IUPAC nomeclature, so
they use the term Dalton, (Da) for the atomic mass ratio.
Thus you will see the ‘atomic mass’ of C as being:
7
12.01, 12.01 amu, 12.01 u or 12.01 Da, depending on who is writing the test
you are reading!!
2-6 Molecules
Molecules are groups of atoms that are joined together in some way. As we saw
earlier some elements are actually diatomic molecules like O2 or H2. So some
molecules are elements. On the other hand, most molecules have one or more
different atoms in them. We call this kind of molecule a compound
There are lots of different ways to represent compounds. In the simplest case
we simply give a formula that expresses the number of each atom in a
compound like H2O for water or C6H12O6 for glucose. At another level we will
write structural formulas that indicate the bonds linking the atoms with dashes.
H-O-H, etc. If we get really tricky we can make three-dimensional models that
show the relative size and spacing between atoms to scale.
It is with these kinds of models that we can really see that last part of Dalton’s
theory. Look at the following three chemical reactions:
Figures 3,4,5
(2H2 + O26H2O)
(C + O26CO2)
(C + H2O 6H2 + CO)
Notice that in each chemical reaction no atoms are created or destroyed, we just
rearrange the molecular groupings.
Summary clicker question: identify various materials as Mixture, solution, pure
subastance, compound, element
2-7 Chemical Nomenclature
Chemical nomenclature refers to a system used to assign names to compounds.
In this book we will introduce the correct nomenclature with every new class of
compounds we study.
In ths chapter we will study the simplest nomeclature, one that applies to Binary
compounds (only 2 elements)
Binary compounds composed of a metal and a nonmetal
And that only occur in 1 fixed ratio
Key: In this case the naming system goes like this:
1. Name the metal first
2. Name the nonmetal second
3. Change the ending on the nonmetal to -ide
8
Try the following examples
K2O
AlBr3
Cadmium selenide
Magnesium hydride
Binary compounds composed of 2 nonmetals
These are more difficult because there can be many binary compounds
with the same element but different fixed ratios like N2O, NO, N2O3,N2O5
or NO2!
Since there are all sorts of different ratios, be have to say what the ratio is
explicitly in the name
For this we will use the Greek prefixes as shown in Table 2-7
1 mono- up to 10 decaSo you put the prefix in front of the element to tell how many of each
element there is. The one exception is when the first element is a one. In
that case you don’t bother with the monoExamples:
SO2
SO3
sulfur dioxide
sulfur trioxide
Problems to try
N2O
NO
N2O3,
Dinitrogen pentaoxide
Nitrogen dioxide
Dinitrogen monoxide
Nitrogen monoxide
Dinitrogen trioxide
N2O5
NO2
Notice there is some funny business when the Greek prefix ends with an a
or and o and the name it is combined with starts with a vowel
Dinitrogen monoxide instead of monooxide
Tricky Hydrogen
Hydrogen can act as either a metal or a nonmetal, so sometimes you use
one of the above systems, sometimes the other. How can you tell?
When Hydrogen is given first in binary formula, it is generally treated as a
metal so you use the first system
9
H2S hydrogen sulfide
When hydrogen is given second in a binary formula it is treated as a
nonmetal
NaH sodium hydride
The Two compounds that we never name properly
H2O is always called water
NH3 is always called ammonia
Clicker question on names of some simple binary compounds
2-8 Atomic and Molecular Mass
Now that we can identify molecules that are made up of combinations of atoms,
we will now introduce the idea of a molecular mass
Key Concept: The molecular mass is the sum of all the atomic masses of all the atoms
in a molecule.
Example 1 :
What is the molecular mass of water?
Water = H2O
=2(1.008) + 16.00
= 18.02
Example 2:
What is the molecular mass of cesium chloride?
Cesium chloride = CsCl
=132.09 + 35.45
=167.54
Clicker question on molecular mass of some compounds
2-9 The Nucleus
Nice historical background on the experiments that helped us to understand the
parts of a atom, but lets move directly to the results
2-10 Protons, Neutrons, and Electrons
Atom
Roughly 1x10-8 cm diameter
1x10-10 m, 0.1 nm
The bulk of this space is occupied but a cloud of electrons
electron
Charge -1
Mass .00054u
10
The nucleus is 10-13 cm, .00001 the size of the atom
has all the mass of the atom concentrated in it
Electron cloud or atom about the size of this room
nucleus has about .1 mm radius!
Proton
Neutron
Charge +1 (1.602x10-19 coulombs)Mass 1.007u
Charge 0
Mass 1.008u
Key Concepts:
1.Number of protons in an atom is called the atomic number (Z) of the element
2.In a neutral atom the number of electrons = number of protons
3.Difference between different elements is due to difference in atomic number
4.Total number of protons and neutrons is called the mass number (A)
2-11 Isotopes
If protons and neutrons have a mass of ~1 u, and an atom has in integral
number of protons and neutrons, Why do we have non-integral atomic masses?
(Cl 35.45 or Cu 63.546?)
Key concept:
Isotopes are atoms of one element that contain the same number of protons but
different numbers of neutrons.
For instance Chlorine has two major isotopes, both with atomic number (Z) of 17
so the nucleus has 17 protons, but one isotope has 18 neutrons so A = 35 and
the other isotope has A=20 so A = 37!
We differentiate between isotopes with the following convention
X = chemical symbol
, where A = mass number and Z = atomic number
So our two isotopes of Cl are:
Many times it is useful to know the number of neutrons in an isotope
The number of neutrons N = A-Z
Before we go on to problems I want you to notice something. This convention
that we use for writing isotopes IS NOT USED on the Periodic table! Notice on
the periodic table how the Atomic number is above and to the left, and a mass
number is below and to the left, Just the opposite of the above convention!
11
Problem:
Symbol
Z
20
N
35
H
A
40
65
3
Answer:
Symbol
40
20 Ca
65
30 Zn
3
1 H
Z
20
30
1
N
20
35
2
A
40
65
3
So now if you go back to Dalton’s atomic theory you can see that he had one
points wrong! Since an element may be composed of more than one isotope, all
atoms of the same element do NOT have the same mass! They gave the same
number of protons, but not the same mass!
So of a element is composed of isotopes of different masses, where does the
mass number that is given on the periodic table come from?
Key Concept:
Atomic Mass is the average of the masses of the individual isotopes weighted by
the natural abundance of each isotope
For instance our Chlorine isotopes
75.78 % of chlorine is the 1735Cl isotope, and 24.22% is the 1737Cl isotope
So the mass of the periodic table is
34.968 x (75.78/100) = 26.50
+
36.966 x (24.22/100) = 8.953
= 35.45
So now you know why the atomic masses on the periodic table are nive interger
numbers
Let’s go back and talk about our atomic mass units again. Remember how I said
that atomic masses are more appropriately called mass ratios because we
measure one mass against the other? And that the mass we used to set the
standard has changed from one time to another?
What is our current standard? The current standard that all the numbers are
currently based on is that the isotopic mass of 612C is exactly 12.
The discerning student may notice one additional problem. If C-12 has exactly 6
protons and 6 neutrons. Why isn’t the atomic mass of 1735Cl exactly 35 and
37
17 Cl exactly 37? Well we saw earlier that the mass of a proton and a neutron
are exactly 1, so that can explain some, but not all of the variation. The other
part of the explanation lies in nuclear physics. When we bind the neutrons and
12
protons into the nucleus some energy is released to hold the particle together.
Where does that energy come from? Good old E=mc2 some of the energy
comes from the mass of the particles themselves! If you want more on this you
need to talk to Kara Keeter!
2-12 Ions
In our atom we cannot change anything in that tiny dense nucleus of the atom,
so we can’t transmute lead into gold as the alchemists wanted to do. What we
can do is to change the number of electrons around the nucleus. And this is by
and large what chemistry is all about
Key Concepts:
1. When we remove electrons from an atom we give the atom a positive charge
and change it into a cation.
2. When we add extra electrons to an atom we give the atom a negative charge
and change it into an anion
Problems:
How many protons, electrons and neutrons are in:
(P=14,N=14, E=18)
(P=74, N=112,E=69)
More clicker questions on same topic