Atomic Structure and the Periodic Table
Atomic Structure
All atoms are made up from particles
called protons, electrons and neutrons.
Neutrons are important in that they account
for the stability of the nucleus and are
neutral. Protons are the positive electrical
charges within the atom. The protons and
neutrons of an atom are packed in a dense
nucleus while negative charge of electrons
orbit around them. The neutrons have no
electrical charge, while each proton carries a
positive charge that is equal and opposite to
the negative charge of the electron. No one
knows what holds together the protons and
neutrons in the nucleus but this force must be
great indeed to hold several positive charges
in close proximity to one another; this force
has been named the strong nuclear force.
An atomic model of an atom is very
complex. It is based on mathematical theory
and they way the waves interact. In the new
model, it is impossible to predict exactly
where an electron is placed on an orbit.
Bohr’s Model of the Atom
In this model, electrons orbit a central
nucleus containing protons and neutrons.
The electrons are arranged in shells or
energy levels. Electrons in shells nearer the
nucleus have lower energy than those farther
away. In the helium atom shown below,
there are two electrons in the only shell. The
nucleus contains two protons and two
neutrons.
Bohr diagram of a helium atom
The first electron shell can only hold
two electrons. If the atom has more than two
electrons, some must go into outer shells.
The second shell can hold up to eight
electrons. The carbon atom shown below
has six electrons. Two of these electrons are
in the first shell, and the other four in the
second shell.
Bohr diagram of a carbon atom
The third shell can hold eighteen
electrons, but will usually only hold eight
before the fourth shell starts to fill up. Once
the first two
electrons have
gone into the
fourth shell,
the next ten
electrons go
into the
remaining
space in the
third shell. (In
the periodic
Atomic Structure and the Periodic Table pg. 1
table these elements are shown in the fourth
row, as transition elements.) Similarly, the
fourth shell holds 32 electrons, but is not
completely filled until the sixth shell has
been started: the first 8 of the 32 go into the
fourth shell; the fifth shell starts to fill; the
next 10 go into the fourth shell; more go into
the fifth shell; the sixth shell starts to fill;
and the final 14 go into the fourth shell. This
is due to the energy of the electrons.
When writing symbols there are several
rules to follow:
1. The symbol is always one or two letters.
(O- oxygen, He- Helium)
2. The first letter is always capitalized any
other letter must remain lower case. (Agsilver)
Nuclear Symbols
Elements
There are some substances that cannot
be decomposed or broken into more
elementary substances by ordinary chemical
means. These substances are called
elements. If we were to take salt for
example we would find that it is composed
of sodium and chloride, which can no longer
be broken into smaller parts. There are
roughly 90 natural elements meaning these
elements may be found in nature. There are
about 20 more that have been artificially
produced. Every substance on earth is made
up of these elements which means element
can be solids, liquids or gas.
Element Symbols
Scientists work in many different
countries all over the world. They all speak
different languages. So that scientist can
communicate they have created symbols for
every element. The symbol "O" will always
mean oxygen whether the chemist lives in
Kenya, China or Russia. These chemical
symbols are universal language all scientists
understand.
Many of the elements names come from
ancient languages such as Greek and Latin.
Bromine is a Greek word that means "bad
smell". Elements may also be named to
honor a place or an individual. The element
Berkelium was created at University of
California at Berkeley.
The nuclear symbol of an element
consists of three parts: the symbol of the
element, the atomic number of the element
and the mass number of the specific isotope.
Here is an example of a nuclear symbol
(ignore the box around it):
The element symbol Li is for lithium. The
three, subscripted left, is the atomic number
and the seven, superscripted left, is the mass
number.
Here's another:
The atomic number is the number of
protons in the nucleus of the atom.
The mass number is the number of
protons and neutrons in the nucleus of the
atom.
Reactivity of Atoms
The outermost electron shells of atoms
interact when two or more atoms get close
together. The interaction between the
outermost electrons of two atoms is known
Atomic Structure and the Periodic Table pg. 2
following superscript: Cl-1. The name of this
ion is changed from chlorine to chloride to
identify this ionic form of the atom. Most of
the non-metals of the periodic table (with the
exception of the noble gasses of Group 18)
form negatively charged ions.
Atoms can also become positively
charged ions. This is typical of the metallic
elements. Sodium (Na), for example has a
single electron in its outermost (second)
shell. In order to achieve a full outer shell,
sodium would have to either acquire seven
more electrons to make eight, OR it could
give away that outer electron, leaving the
next innermost shell (first energy level)
exposed. Since the first energy level is
as a chemical reaction. The protons and
neutrons in the nucleus have no effect on the
chemical behavior of an atom.
Atoms have an innate tendency to lose
or gain outer shell electrons so that they end
up with a full outer shell. Except for the
first shell, which holds a maximum of two
electrons, the second and third shells fill up
with 8 electrons each. The tendency to fill
up the outer shell with electrons is therefore
often referred to as the Octet Rule: atoms
will acquire or lose electrons in order to get
an octet in their outermost shells.
Ionization of Atoms
An electrically balanced atom of any
element has an equal number of positive
protons and negative electrons, yielding a net
overall charge of zero. One might conclude
that atoms should be "happy" when they have
no net charge, but this is not always so.
Atoms are the most stable when they
have a full outer shell of electrons. This
concept, known as the Octet Rule, means that
atoms will attempt to steal or give away
electrons to achieve a full outer shell. The
halogens of Group 17 are a collection of
elements (chlorine, fluorine, iodine) that each
have 7 electrons in their outermost shell. In
the case of chlorine (Cl), whose outermost
shell is the second energy level, a total of
eight electrons completes this shell.
Chlorine, then, needs one more electron to
form a more stable configuration. For this
reason, chlorine is a ferocious electron
stealer.
In swiping an electron to fill its
outermost shell, chlorine becomes
electrically imbalanced: it now has 9
negative electrons but only 8 positive
protons. Adding together these numbers (-9
+ +8) produces a net -1 charge. Any atom or
molecule that has a net charge is known as an
ion. The charge on the new chlorine atom is
written with the element symbol as a
Sodium (Na) becomes an ion with a +1 charge by
throwing away an electron to have a full outermost
shell. Calcium (Ca) throws away two electrons to
become an ion with a +2 charge. For chlorine (Cl), it
is easiest to gain an electron to reach the maximum of
8 for the second shell, producing an ion with a -1
charge.
already full with two electrons, the sodium
atom more easily becomes more stable by
Atomic Structure and the Periodic Table pg. 3
losing a single electron rather than stealing
seven electrons. Hence, sodium atoms lose
and electron, leaving them with 10 electrons
and 11 protons. Doing the math, this means
the total charge on sodium ion would be (-10
+ +11 = +1), and is written as Na+1.
The periodic table arranges these metals
into families. In each family, the metals have
similar properties. There are four different
types of metals: alkali, alkaline earth,
transition elements and alloys.
The Periodic Table
Early in the 1800’s chemists discovered
that some of the elements had similar
properties. For example sodium and
potassium are both shiny and have explosive
properties when dropped into water. Due to
this knowledge scientist attempted to
organize these elements. The arrangement
that is used today is called the periodic
table. When a property is repeated within a
regular interval, that property is said to be
periodic.
The periodic table consists of both rows
and columns. The rows across the table are
the periods. They a referred to as the first
period second period and so on. The
columns are the groups or families (IA, IIA,
IIIA, etc.). Element groups or families have
similar properties. Elements are listed in the
table in order of atomic number, which is
the number of protons in an atom.
Elements have several properties: metals,
nonmetals and metalloids. Their shininess or
luster distinguishes metals. With the
exception of mercury, all metals are solid at
room temperature. Metals are excellent
conductors of electricity. They permit
electrons to pass through them. Metals are
also great heat conductors. Copper, iron, and
aluminum are use to create pots and pans
used in cooking because of this property.
Metals are malleable. They can be hammered
into different shapes. Their primary color is
silver-gray with the exception of copper and
gold. Most metals have a react when exposed
to oxygen. When this happens, compounds
called oxides are formed.
Alkalis (or alkali metals) are the softest
of the three. The Alkalis are found under
Group IA - with the exception of hydrogenand are usually shiny and silver in color and
very reactive. Sodium (Na) is an example.
The high chemical reactivity of the alkali
metals is due to their single electron in their
outermost shells. Atoms of these elements
are "eager" to get rid of the single electron,
leaving them with a full outer shell (the next
one in). In losing an electron, alkali metals
become positively charged (+1) atoms
because they lose one of the negative charges
that neutralizes one of the positive proton
charges in the nucleus. Charged atoms are
called ions (e.g. Na+1)
Alkaline earth metals are harder and
denser than alkali metals. Magnesium (Mg)
is an example. They have a higher melting
point and are also chemically active.
(Periodic Group IIA). Alkaline earth metals
are highly reactive, like the alkali metals,
because they have only a couple electrons in
their outermost shells. The "eagerness" to
lose these two electrons and conform to the
Octet Rule makes them reactive. Thus, these
atoms lose two electrons to become
positively charged ions (e.g. Ca+2)
There are thirty elements between group
IIA and IIIA called transitional elements or
metals. They are hard, brittle and have high
melting points., and are also considered
metals.
Other substances that we call "metals" in
common speech are really alloys. An alloy is
a mixture of two or more elements having the
properties of a metal. Some alloys include
solder, which is made from tin and lead, and
Atomic Structure and the Periodic Table pg. 4
bronze, which is made from copper and tin.
Not all alloys are made up from elements that
are metals. Steel for example is a
combination of iron and carbon.
Group VIIA, which includes fluorine (F),
chlorine (Cl), and bromine (Br) are known as
the halogens. Halogen elements are often
gaseous and are extremely reactive. A look
at their electron configuration reveals seven
electrons in their outermost shells. These
atoms are eager to steal an electron from
other atoms to complete their outer shells,
and become negatively charged.
Given the alkali metals and alkali earth
metals with their desire to lose electrons and
the halogens' desire to gain electrons, it
comes as no surprise that mixing these two
types of elements together provides for some
exciting chemical reactions. For example, the
reactive alkali sodium, which catches on fire
when thrown in water, can react with the
very poisonous green gas called chlorine to
produce sodium chloride, a very safe
compound made of positive sodium atoms
and negative chlorine atoms that we refer to
as table salt.
The noble gases or inert gases make up
the last column of the Periodic Table, VIIIA.
These elements, which include helium,
neon, argon, and xenon, all have a full octet
of electrons in their outermost shells.
Consequently, these elements are
completely unreactive (inert), leading to
their name (as aloof aristocrats) of noble
gases.
Summary of Major Groups of Elements
and Their Properties
The Alkali Metals of Group 1A elements (Li,
Na, K, Rb, Cs, Fr) are a family of elements that
are:
 soft
 shiny
 metallic
 good conductors of heat
 good conductors of electricity
 Low melting points
 Very reactive in water
 Form white products as oxides
 (when reacted with oxygen)
The Alkaline Earth Metals, Group 2A elements
(Be, Mg, CA, Sr, Ba, Ra) are:
 solid
 silvery colored
 shiny
 metallic
 Not as reactive as alkali metals, but still
reactive
The Transition Elements (Metals), Groups 312, are:
 Shiny solids
 Ductile (can be pulled out into a wire)
 Malleable (can be hammered flat)
 Good conductors of heat
 Good conductors of electricity
 Solid at room temperature (except
mercury)
 Relatively high melting points (must heat
up to liquefy)
 Relatively high density
 Examples: Cu, Au, Ag, Fe, Sn
The Halogens, Group 7A, (F, Cl, Br, I) are
 from term meaning "salt generator"
 either solid (I, As), liquid (Br), gaseous
(Fl, Cl) at room temp
 non-metallic
 Very reactive
Living things are made mostly of C, H, O, and N
The Noble Gases are Group 8A (He, Ne, Ar, Kr,
Xe, Rn) and are:
 gaseous
 non-reactive
 from "nobility" or above it all
Atomic Structure and the Periodic Table pg. 5
Atomic Structure and the Periodic Table pg. 6
Atomic Structure and the Periodic Table pg. 7
Variable Neutron Number: Isotopes
We have already noted that atoms
belonging to an element do not all have the
same structure. While atoms of an element
always have a fixed number of protons, they
vary in the number of neutrons in the
nucleus. Variant forms of an atom are
known as isotopes. Most elements have two
or more isotopes that can be found in nature.
Usually one of the isotopes of an element is
much more abundant than the others.
Because isotopes vary in the number of
neutrons they have, they also vary in mass.
Neutrons (and protons) have arbitrarily been
assigned an atomic mass unit (amu) of 1.
Electrons are so tiny that they are not
considered when computing the total atomic
mass of an atom. The mass of an atom,
therefore, is the same as the mass number.
In our previous example where a tungsten
atom had a mass number of 184, this atom
has an individual mass of 184 amu. To
briefly differentiate between the tungsten
isotope with a mass of 184 from one with a
mass of 185, chemists use the hypenated
shorthand W-184 and W-185 (remember W
is the symbol for tungsten).
In thinking about the atoms of a
particular element, it is useful to have a
sense of the atomic mass of a natural sample
of this element taken from the earth. A
natural sample is a mixture of all the
different isotopes of that element. The
weighted average, based on abundance, of
the masses of all the isotopes of an element
is called the atomic mass or atomic weight.
The mass of each isotope is multiplied by its
percentage occurance in nature and these
products are then summed together.
In the example below, the element
lithium has three isotopes, Li-6, Li-7, and
Li-8. Because Li-7 is the most abundant, the
final atomic mass is biased toward this mass
number (6.94 amu). The atomic mass is
reported in the Periodic Table under the
symbol for each element. One can always
determine the mass number of the most
abundant isotope of an element by rounding
the atomic mass to the nearest whole
number.
The atomic mass is the weighted average of the masses of all the known isotopes for an element, based on relative
abundance in nature.
Atomic Structure and the Periodic Table pg. 8
Isotopes That Just Can't Hold It
Together
One of the reasons that certain isotopes
are more abundant than others is because some
isotopes spontaneously fall apart, or decay.
Isotopes that disintegrate to become more stable
atomic forms are called radioisotopes. Henry
Becquerel and Marie Curie were the first
scientists to discover radioisotopes. Becquerel is
staid to have placed a sample of uranium rock on
a photographic plate and later, upon developing
the plate, discovered a dark exposed area where
the rock had sat on it. Radioisotopes are
radioactive; they emit particles and radiation as
they disintegrate that often can be detected by
their interference with an electric circuit. A
Geiger counter is a device that senses
particulate radiation and registers it on a meter
and through clicking sounds in a speaker.
Radioisotopes are useful atoms to use in
research and medical science because they can
be substituted for non-radioactive forms in
molecules. For example, a molecule of water
(H2O), which usually contains H-1, can be
modified to carry H-3, commonly known as
tritium. If this water molecule drunk by an
animal, the radiation emitted by the decaying
tritium can be tracked as the water travels
through the body. In other words, radioisotopes
act as tags and labels on molecules which allow
them to be followed from place to place.
In medicine, positron-emitting
radioisotopes are hooked onto biological
molecules and fed to a patient. These labeled
molecules (like sugar) then move to areas of the
body where they are normally metabolized. A
machine called a positron emission tomography
(PET) scanner can take a picture of the body,
showing the areas where the radioactive
molecule is being used.
A handheld Geiger counter (above) is
used to detect energetic beta radiation
while a scintillation counter (below) is
employed in detecting weaker alpha
radiation.
A Positron Emission Tomography (PET) scan looks
for where radioactive fluorine-sugar is being used in
the brain (dark gray areas in these pictures). Note
that the Alzheimer patient shows much less sugar
metabolism and therefore less brain activity than
from a "normal" patient.
Atomic Structure and the Periodic Table pg. 9
Atomic Structure and the Periodic Table
Lab Report
Name
Nuclear Symbology
For the element whose nuclear symbol is:
What is the atomic number?
(Please note: This is not a box from the
Periodic Table)
What is the mass number?
How many neutrons does it have?
---------------------------------------------------------------------------------------------------------------------
Periodic Table Completion
Complete the 8 spaces in the periodic table below by filling in the atomic number and mass
number in the top and bottom of each empty box, respectively. Use the mass number of the
most abundant isotope for each element, equal to the nearest whole number when the
atomic mass is rounded up or down. Use a completed periodic table for reference. Ask for help
if you need it.
Be
Ne
Si
K
V
Sr
Kr
Ag
Rn
Rn
Atomic Structure and the Periodic Table pg. 10
Identifying Elements by Bohr Model Diagrams
Diagram
Atomic #
A
B
C
D
E
F
G
H
I
J
K
Element
Symbol
1. Figure out the ATOMIC NUMBER of the atom shown in each diagram (A, B, C, etc.) and
write it in the table above under the diagram's letter.
2. Now look at the PERIODIC TABLE and identify each element. Write the symbol (e.g. He,
Ne, etc).
3. Briefly explain the basis for the arrangement of elements in the Periodic Table (in both
rows and columns).
Atomic Structure and the Periodic Table pg. 11
Completing in Bohr Models of Atoms
Complete the diagrams of the following atoms by filling in the number of protons and electrons,
any other blanks below the diagram). Refer to a Periodic Table for reference. By studying the
atomic masses in a Periodic table, determine the approximate number of neutrons in the most
commonly occurring atomic form of these elements. You may need to draw in extra electron
shells for some atoms.
Sodium (Na)
Atomic number 10
Element
14 neutrons (1 more neutron
than protons)
Element
Phosphorous (P)
Sulfur (S)
Chlorine(Cl)
Manganese (Mn)
An atom with a
+15 nuclear charge
Element
An atom with a mass
number of 14 amu
Element
Atomic Structure and the Periodic Table pg. 12
Questions about Elements and Atoms
1. Sodium has only one more proton than neon. How are these two elements different? Why are
they so different?
2. Which elements are called the transitional elements? Name the groups numbers and a few
representative elements.
3. What is an ion and how does it form?
4. In most chemical reactions involving lithium, the lithium atom loses an electron and forms a
charged atom (positive) called an ion. On the other hand, fluorine atoms tend to form
negative ions by gaining an electron. Why do these elements behave so differently in the
formation of ions?
5. What is similar in the electron structure of each of the noble (inert) gases in column VIIIA?
These gases are completely unreactive.
6. What makes the halogens so chemically reactive?
Atomic Structure and the Periodic Table pg. 13
7. What is the definition of an isotope? Write the nuclear symbol for an isotope of carbon-14, a
radioactive isotope of carbon which an atomic mass of 14 amu. Remember that the atomic mass
is equal to the sum of protons and neutrons, where each proton and neutron "weigh" 1 amu each.
(nuclear symbol for C-14)
8. The previous question asked about the radioactive isotope C-14. How is an atom of C-14
different from an atom of C-12, the most common isotope of carbon in nature?
9. Using the Periodic Table as a reference, explain how you know that the most common isotope
in nature of oxygen (O) has 8 neutrons. Begin by explaining what you know from the atomic
mass in the Table, and how the atomic number is used in answering this question.
8. What are the four most common elements found in living things (list them by element
symbol). List four other elements also found in living things.
Atomic Structure and the Periodic Table pg. 14
Crossword of Terms Related to Atomic Structure and the Periodic Table
Crossword Puzzle Clues
ACROSS
4 The tendency for atoms to acquire or
lose electrons such that they have full outer
shells is known as the _______ ______.
6 A group of elements that are short one
electron in making their outer shells full;
highly reactive, often forming poisonous
gases.
10 The electron shell which holds a
maximum of two electrons.
12 An atom that with a lack of or a gain
of one or more electrons, making it
negatively or positively charged.
13 The electron shell which is one short
of an octet in the fluorine atom.
17 The value equal to the number of
protons in the nucleus of an atom.
21 A scientist who determined at atoms
are "mostly air" by firing alpha particles at a
thin sheet of gold foil.
22 A subatomic particle with a positive
charge.
24 The electron shell which has only a
single electron in the sodium atom.
25 A negatively charged subatomic
particle.
26 An energy level or orbital distance
from the nucleus occupied by electrons in an
atom.
Atomic Structure and the Periodic Table pg. 15
DOWN
1 The location of the protons and
neutrons an atom.
2 An interaction between the electrons
in the outermost shells of two or more atoms
(two words).
3 A fundamental type of matter made up
of atoms of a distinct kind.
5 The ______________elements include
manganese (Mn) and copper (Cu) that are
hard, brittle metals and usually good
electrical conductors.
7 The process of giving off energy, as
an atom does when an excited electron falls
from a high to a low energy state.
8 The chart arranged by Dimitri
Mendeleyev made according to the behaviors
(and masses) of the elements.
9 The value equal to the sum of the
protons and neutrons in an atom.
11 Infrared, ultraviolet, and visible light
are all regions of the electromagnetic
___________.
14 The ________ ________ metals (two
words) are a group of elements that have two
electrons in their outermost shell.
15 The expression of an element that
contains the element's abbreviation, atomic
number, and mass number.
16 A subatomic particle with the same
mass as a proton.
18 The group of elements which are
unreactive because they contain full outer
shells of electrons (two words).
19 The weighted average mass of all the
naturally occurring forms of an element,
usually close to the mass number but
recorded as a real (not whole) number in the
Periodic Table.
20 The ______ metals (one word) are a
highly reactive group of elements that form
positive ions by giving away a single electron
during chemical reactions.
23 The last name of the scientist who
developed a model of the atom in which
electrons orbit at distinct distances (shells)
about nucleus.
Atomic Structure and the Periodic Table pg. 16