Elements and the
Periodic Table
Organizing the Elements
Looking for Patterns in the
Elements
• Matter is made up of about 100 different
elements that have a wide variety of
properties.
• Some elements are very reactive - they
form compounds readily with other
elements.
• Other elements are less reactive.
• Others do not form compounds at all.
Mendeleev
• In the 1800’s, scientists began to suspect that
the elements could be organized in a useful
way.
• By 1869, one Russian scientist, Dmitri
Mendeleev recognized a hidden pattern in
the elements.
• Mendeleev studied the evidence, considered
each clue, and looked for patterns.
Mendeleev’s Observations
• One of Mendeleev’s first observations was that some
elements have similar chemical and physical
properties.
• For example, Flourine and chlorine, are both gases
that irritate your lungs if you breathe them.
• Silver and copper are both shiny metals that gradually
tarnish if exposed to air.
• Mendeleev believed that these similarities were
important clues to the pattern he was looking for.
• To help him find the pattern, Mendeleev wrote facts
about the elements on individual paper cards.
• He kept track of all the properties he knew about an
element including melting point, density, and color.
Atomic Mass
• Mendeleev also recorded two other important
properties: Atomic mass and bonding power.
• Atomic mass of an element is the average
mass of one atom of the element. Atomic
masses were determined by comparing
elements to hydrogen, the lightest element.
• Bonding power refers to the number of
chemical bonds an element can form.
Chemical bond was determined by studying
how each element formed compounds with
oxygen.
The First Periodic Table
• Mendeleev tried arranging his cards on
elements in various ways.
• He noticed that patters appeared when
the elements were arranged in order of
increasing atomic mass.
• He also discovered that the bonding
power of the elements from lithium to
flourine change in an orderly way.
Arranging the Table
• Mendeleev discovered that arranging the
elements by increasing atomic mass does not
produce a perfect table.
• He moved cards to positions where they fit
best.
• This left three blank spaces.
• Mendeleev proposed that the blank spaces
would be filled by elements that had not yet
been discovered.
• He even predicted their properties.
Periodic
• In 1869, Mendeleev published the first
periodic table of elements.
• The word periodic means a regular, repeated
pattern.
• In the modern periodic table, the properties of
the elements repeat in each row or period of
the table.
• Within 16 years, chemists discovered all three
of the missing elements and named them
scandium, gallium, and germanium.
Inside an Atom
• Deep within every atom is a core called a
nucleus.
• The model of the atom used today shows the
nucleus as containing smaller particles called
protons and neutrons.
• Outside the nucleus are other particles, called
electrons.
Measuring an Atom
• Because atoms are so small, we cannot
measure them with everyday units of mass.
Scientists created the atomic mass unit to
measure the particles in atoms.
• The mass of a proton or a neutron is about one
atomic mass unit.
• A proton has almost two thousand times the
mass of an electron, which means that most of
an atom’s mass is in its nucleus.
• Thus an atom that contains 3 protons, 4
neutrons, and 3 electrons has the mass of
about 7 atomic mass units.
Electrical Charges
• Protons and electrons also carry electrical
charges.
• Neutrons, as the name implies, are neutral
and carry no electrical charge.
• Protons carry a positive electrical charge.
• Electrons carry a negative electrical charge.
• Electrons also move constantly and rapidly in
the space around the nucleus. This fact is
very important.
Atomic Number
• Every atom of a particular element contains the same
number of protons. For example, every carbon atom
contains 6 protons. Thus an element’s atomic number
(the number of protons in its nucleus) is a unique
property that identifies that element.
• However, the atoms of an element may vary in the
number of neutrons they contain. For example a
carbon atom, may have 5, 6, 7, or 8 neutrons.
• This means that the mass of an atom may vary.
• For this reason, chemists now organize the periodic
table according to atomic number instead of atomic
mass.
The Periodic Table of Today
• Although the periodic table is now
arranged according to atomic number,
the modern version of the table is much
like Mendeleev’s table in many ways.
• It now contains more than 100
elements.
• Let’s look at the table on page 80
together.
Reading the Periodic Table
• Each square of the periodic table usually
includes the element’s atomic number,
chemical symbol, name, and atomic mass.
• Chemical Symbol for an element usually
contains either one or two letters. The last
entry in the square is the atomic mass or the
average mass of an element’s atoms.
• Despite the different masses of each atom, all
iron atoms, for example, react the same way
chemically.
Organization of the Periodic
Table
• An element’s properties can be
predicted from its location in the
periodic table.
• As you look at elements across a row or
down a column, the elements’
properties change in a predictable way.
• This predictability is the reason why the
periodic table is so useful to chemists.
Groups
• The main body of the periodic table is arranged into
18 vertical columns and seven horizontal rows.
• The elements in a column are called a group.
Groups are also known as families.
• Notice that each group is numbered, from Group 1 on
the left of the table to Group 18 on the right.
• The group is given a family name based on the first
element in the column.
• The elements in each group, or family, have similar
characteristics. For example, in Group 1 all of the
elements are metals that react violently with water.
Periods
• Each horizontal row across the table is called a period.
• A period contains a series of different types of
elements from different families, just as a week on a
calendar has a series of different days.
• Unlike the elements in a family, elements in each
period are not alike in properties but there is a pattern
as you move across each period.
• Let’s look at the fourth row on the periodic table. In
this row the elements range from being very reactive
metals to relatively un-reactive metals to an inactive
gas.
Valence Electrons
• It is the electrons that explain bonding power
because electrons can be shared between or
transferred to other atoms.
• This is not true for all electrons.
• When an atom has two or more electrons,
they may be different distances from the
nucleus.
• Only the electrons farthest out can be shared
or transferred.
• Electrons involved in transfer or sharing are
called valence electrons.
Bonding Power
• Elements may have different numbers
of valence electrons.
• The number of valence electrons
determines whether the electron gives
up, shares, or accepts electrons.
• The number of valence electrons an
element has increases from left to right
across a period.
Valence Electrons in Groups
• Atoms that have the same number and arrangement of
valence electrons have similar properties, which is why
the elements fall into a periodic pattern.
• The elements in each group of the periodic table have the
same number and arrangement of valence electrons.
• For example, each of the elements in Group 1, have
atoms with 1 valence electron. In Group 18, each
element except helium has atoms with 8 valence
electrons.
• In the next sections we will be studying the groups of
elements and their properties.