One of the most important tools a chemist has is a
periodic table. If you would like to see the periodic table at any time
during your mission, use the link below. We have provided you with both an
online version and a downloadable version.
Atoms contain three primary particles:
protons, neutrons, and electrons. These particles are referred to as
subatomic particles. Protons are positively charged, electrons are
negative, and neutrons are neutral. Structurally, an atom is set up with a
positively charged nucleus containing protons and neutrons that is surrounded by
a space where electrons move around the nucleus because of their attraction to
the positive charge. Although electrons orbit the nucleus, they do not do
so the same way planets orbit the sun. Electrons do not orbit the nucleus
in a definite path. Furthermore, electrons move so fast that it is
impossible to determine where an electron is at any given time.
diagram on the left shows how the nucleus lies on the center of the
atom with electrons moving in the space around it. However,
electrons do not move in a defined path. Therefore, chemists look
at electron orbitals as indistinct clouds as is shown by the picture on
The strength of a proton's positive (+)
charge is equal to the strength of an electron's negative (-) charge. All non-bonded elements
have the same number of protons as electrons. Therefore, all elements in
their pure form are electrically neutral. Yet, a proton has a far greater
mass than an electron. The table below summarizes the properties of each
of the subatomic particles.
||+1.602 x 10-19
||1.673 x 10-24
||1.0073 » 1
||1.675 x 10-24
||1.0087 » 1
||-1.602 x 10-19
||9.109 x 10-28
||.0006 » 0
Notice how the mass of each
subatomic particle is given in AMU's (Atomic Mass Units). The AMU unit was
created to provide a more simple way of working with the masses of elements and
compounds. As you will see, the atomic mass unit plays an important role
Although the exact location
of an electron can never be known for sure, there are regions within the
electron cloud where electrons most likely to be found. The ability to
determine probable locations of electrons is described by orbitals.
Orbitals describe the probability of finding electrons in certain regions of an atom.
All orbitals have distinctive shapes and sizes. Every orbital holds up to two
electrons. One electron in that orbital spins clockwise while the other
spins counterclockwise, creating a magnetic field.
There are many different
kinds of orbitals found in atoms, each having a different shape. The
different kinds of orbitals have been named with the letters s, p,
d, and f. All s orbitals, for example, have a
spherical shape. The p orbitals are dumbbell shaped. The shapes
become more complex with the d and f sublevels.
All atoms contain
principal energy levels (n). Each energy level can hold a
specific number of electrons. The innermost level is n
= 1; the next one out is n = 2,
and so on. The energy of the electron increases from as n increases
from 1 to 2 to 3, etc. Each energy level is divided into one or more
sublevels. There is an interesting and important pattern to these
sublevels: the quantum number n equals the number of sublevels in
that principal level (n=1 has one sublevel, n=2 has two sublevels, and so
forth). The diagram below shows the energy and sublevels of some principal
As you can see, the second
principal energy level (n = 2) contains two sublevels: 2s and 2p.
They are called 2s and 2p because they are the types of orbitals
found in that sublevel. So 2p contains p orbitals found in
the second principal energy level. This concept also applies to all other
sublevels. Just as energy increases form n = 1 to n = 2 to n
=3, the size of the sublevels increases from lower to higher energy
levels. Therefore, 1s is smaller than 2s which is smaller
The 2p energy level,
however, does not just have one p orbital, it has three. It has one
p orbital for each axis (x, y, and z). Look at the table below to
see how many orbitals are found in each principal energy level.
Principal Energy Level
n = 1
n = 2
n = 3
n = 4
3s, 3p, 3d
4s, 4p, 4d, 4f
2s (one) + 2p (three)
3s (one) + 3p (three) + 3d (five)
4s (one) + 4p (three) + 4d (five) + 4f (seven)
You may have noticed a
pattern in the number of orbitals in each sublevel. All s sublevels
have 1 orbital, all p sublevels have 3 orbitals, all d sublevels
have 5 orbitals, and all f sublevels have seven orbitals.
The Periodic Table
The modern periodic table has
109 elements. That means that everything around us is made up of only 109
different primary pieces! Each square on the periodic table represents one
distinctive element. There are several different periodic tables, each
presenting different information about a particular element, but there are some
topics that are common among periodic tables. First, all elements have an
abbreviated name called a symbol. Hydrogen's symbol is H; helium's symbol
is He, etc. Some elements, on the other hand, have symbols that are more
difficult to remember. That is because many of the symbols found on the
periodic table come from that element's Latin name. Iron's symbol is Fe
from its Latin name ferrum. Use the link at the top of this page to
see the periodic table and learn each element's symbol. Another topic that is commonplace
among periodic tables is an element's atomic number, which is a whole number
representing the number of protons in the nucleus of an atom. Next is the
atomic mass. An element's atomic mass is the sum of the masses of that
element's protons, neutrons, and electrons. However, elements sometimes have what
are called isotopes. An isotope is an atom that has the same number of protons as another
atom but that has a different number of neutrons.
Carbon, for example, usually has 6 neutrons but is sometimes found with 8.
Therefore, the atomic mass that you will find on the periodic table is the average of the masses of the existing isotopes of an element.
Too see a full periodic table,
click on the link at the top of the page.
You may be wondering why the
periodic table is shaped the way it is. The periodic table gets its shape
form periodic law. Periodic law is determined by elements' electron
configuration. Elements with similar bonding properties are found in the
same column, called a family or group. The rows are called periods.
Elements in a period are arranged in order of increasing atomic number.
Elements with the same bonding properties have the same number of valence
electrons (electrons in the outermost energy level). During a chemical
reaction, elements either lose or gain valence electrons. When they do so,
they become ions. Let's look at family
7A. All elements in that group have 7 valence electrons (notice 7A = 7
valence electrons). Remember the maximum number of electrons in each
energy level mentioned earlier? Those determine an atom's number of
valence electrons. Fluorine has an atomic number of 9, so it has 9 protons
and 9 electrons in its pure form. The first energy level (n = 1)
holds 2 electrons. That leaves 7 electrons for the second energy level (n
= 2), which can hold up to 8 electrons. Therefore, fluorine has 7 valence
electrons! Now, let's see if that works for chlorine, another element in
family 7A. Chlorine has an atomic number of 17, so it has 17 protons
and 17 electrons. The first energy level holds 2 of those electrons so we
are left with 15 electrons. The second energy level holds another 8
electrons. Now, we are left with 7 electrons in the third energy level,
which can hold up to 18. There are 7 valence electrons again!
will learn more about chemical bonding in Case 3.
Metals, Nonmetals, and
If you look at the periodic
table, you will notice three different sections: metals, nonmetals, and
metalloids. All elements in each category have a few of the same
Most elements on the periodic
table are metals. Metals are found on the left side of the periodic
table. Metals are elements that typically a have high melting point, are
ductile (able to be pulled into fine wires), malleable (able to be hammered into
thin sheets), shiny, and good conductors of heat and electricity. Metals
tend to lose electrons in a chemical reaction.
Nonmetals are found on the
right side of the periodic table. Nonmetals are elements that have a low melting point, dull surface,
break easily, are a poor conductors of heat and electricity, and tend to gain electrons in a chemical reaction.
Metalloids, or semimetals, form a diagonal line
on the periodic table between the metals and nonmetals. Semimetals, as
you may have guessed, share properties with both metals and nonmetals but do not
accurately fit into either category.
In addition to categories
such as metals, nonmetals, and metalloids, some families on the periodic table
have group names. The elements in group 1A are alkali metals; the ones in
group 2A are called alkaline earth metals. The elements in group 7A are
called halogens and those in 8A are noble gases. Noble gases have a full
outer energy level and tend not to form bonds with other elements.
or patterns, exist on the periodic table. One of these periodic trends is
atomic radius. Atomic radius measures the distance between the center of the nucleus of an atom and the outermost electrons.
Since the exact location of the outermost electrons is not known, this
measurement is not precise. The trend that it follows is that atomic
radius increases as you go more toward the bottom-left of the periodic
table. That is because as you move down the periodic table, more energy
levels are added, making the element larger. However, as you proceed to the right
side of the periodic table, the radius decreases even though more electrons are
added. The reason is that as you add more electrons to the same energy
level, there is a greater force of attraction between the negatively charged
electrons and the positively charged nucleus, drawing them closer
together. So fluorine would have a very small atomic radius while cesium would have a relatively large one.
Another periodic trend is
found in the elements' ionization energies. Ionization energy is the energy required to remove the most loosely held electron from an atom.
Metals, as you learned earlier, tend to lose electrons in a chemical
reaction. Thus, they have a far lower ionization energy than
nonmetals. Ionization energy increases as you proceed to the top-right of
the periodic table.
Next is electron
affinity--the energy change that occurs when an atom gains an electron.
The energy change that occurs becomes progressively more negative toward
the top-right of the periodic table, excluding the noble gases.
Finally, there is
electronegativity, which is the property of an element that indicates how strongly
an atom of that element attracts electrons to itself in a chemical bond.
Elements become gradually more electronegative toward the top-right of the
periodic table, excluding the noble gases, with fluorine being the most
Nomenclature is simply the
naming of compounds. This is done by looking at the atoms and their bonds
in a compound. Although you will learn more about bonding later, it is
good to know nomenclature in advance. You will also learn more about how
to identify ionic and covalent bonds in Case 3.
An ionic compound is a chemical bond resulting from the transfer of electrons from one bonding atom to another.
Most often, ionic bonds are found between a metal and a nonmetal or a metal and
a polyatomic ion. It is useful to have the common polyatomic ions
memorized for more rapid recognition.
pattern in the naming system:
per- . . . -ate greatest number of oxygen atoms
. . . -ate greater
. . . -ite smaller
hypo- . . . -ite smallest number of oxygen atoms
Ionic compounds are the
easiest to name. If it is a compound with only two different types of
atoms, you state the name of the first atom followed by the name of the second
atom; but the name of the second atom must have the suffix -ide.
Thus, the compound NaCl is named sodium chloride. KI is potassium
iodide. The number of atoms in the compound does not change the name for
ionic compounds. Hence, MgCl2 is still magnesium chloride even
though there are two chlorine atoms.
If you have a metal bonded to
a polyatomic ion, simply state the name of the metal followed by the name of the
polyatomic ion. AgNO3 is silver nitrate. You can even
have two polyatomic ions bonded to one another, as long as the sum of their
charges equals zero. For example, NH4C2H3O2
is ammonium acetate.
Some atoms, however, can lose
different numbers of electrons in a bond. Listed below are some common
examples of these atoms.
Since these atoms can lose
different amounts of electrons, you must indicate how many are being lost.
Chlorine will always want to gain one electron when forming an ionic bond.
Therefore, if you are given the formula CuCl, you would know that copper lost
one electron to chlorine. To show that copper lost one electron, the name
of the compound is copper (I) chloride. It has the same naming process as
all other ionic compounds except for the Roman numeral (I) to indicate the loss
of one electron. If you are given the formula CuCl2, you can
now determine that the name of that compound is copper (II) chloride since
copper is losing two electrons (one to each chlorine atom). Do not worry
if you are a bit perplexed at this point, you will obtain a better understanding
of nomenclature when you study Case 3
when we discuss bonding.
When naming molecular
compounds, you use some of the same rules as you do for ionic compounds.
The first atom in the compound has its name stated first and the second has its
name with the -ide suffix. Yet, unlike ionic compounds, molecular
compounds can have two types of atoms bonded in several ways. For example,
carbon and oxygen can bond to form CO (carbon monoxide) or CO2
(carbon dioxide). Prefixes are used to indicate the number of atoms in a
molecule. Listed below are the common prefixes used to name molecules.
When naming molecular
compounds, you use a prefix before each of the atoms in the compound. P2O5,
for instance, is called diphosphorus pentoxide. There is an exception to
this rule. If there is only one of the first atom in the formula, you do
not use the prefix mono-, you just write the name of the atom
itself. That is why CO2 is called carbon dioxide not monocarbon
dioxide. Also, if you have a molecule such as H2 or O2,
you do not use the di- prefix; you would simply call them hydrogen and
Acids also have their own set
of rules for naming. If you have a binary acid (an acid made up of two
atoms), such as HCl, you take the name of the second atom in the formula and add
the suffix -ic. Then, you add the prefix hydro- (from hydrogen) to
the beginning. Finally, you add the word "acid" to the
end. Accordingly, the name of HCl is hydrochloric acid.
If you have a ternary acid
(an acid that contains a polyatomic ion), you do not use the prefix hydro-.
Additionally, the suffix -ic is not used in all cases. What
you do is look at the name of the polyatomic ion--if the ion has the suffix -ate,
change it to -ic; if the ion has the suffix -ite, it becomes -ous.
Other than that, you keep the name of the ion the same and add the word
"acid" to the end. Here are a few examples to show you how this
|| Perchloric acid
|| Chloric acid
|| Chlorous acid
|| Hypochlorous acid
A mole (abbreviated mol) is
defined as the quantity of a substance that has a mass in grams numerically equal to its mass
in AMU's. Interestingly, there are always 6.02 x 1023 atoms or
molecules in one mole of that substance. Consequently, if you have 6.02 x
1023 atoms of carbon, it would have a mass of 12.011 grams because
the atomic mass of carbon is 12.011 AMU's. In addition, if you had one
mole of NaCl, it would have a mass of 58.443 grams because the combined atomic
masses of Na (22.990) and Cl (35.453) equal 58.443. The number 6.02 x 1023
is called Avogadro's number in honor of the Italian chemist and physicist Amadeo