Elements make up the
periodic table.
Section 1.2D
Elements can be organized by
similarities.
• Elements can be
organized by the masses
of their atoms.
• Not all the atoms of an
element have the same
atomic mass – elements
have isotopes
• Atomic mass of an
element is the average
mass of all the
element’s isotopes
Mendeleev’s Periodic Table
• 1800’s several scientists tried to organize the
elements based on their properties
• 1860’s Russian chemist Dmitri Mendeleev – thought
of how he could organize elements based on their
physical and chemical properties
• Arranged cards with the element, its atomic mass,
and properties
Mendeleev’s Periodic Table
• Chart – rows and
columns
• Rows = elements showing
similar chemical
properties; arranged
rows so the atomic
masses increased as you
move down
• Periodic Table – shows a
periodic, or repeating,
pattern of properties of
the elements
• Mendeleev’s Notebook
Predicting New Elements
• Mendeleev left some empty spaces where no known
elements fit the pattern.
• He predicted that new elements that would complete
the chart would eventually be discovered.
• Many chemists found it hard to accept this, but six
years later gallium was discovered and fit in the blank
after aluminum
• In the next 20 years, 2 other elements Mendeleev
predicted were discovered.
The periodic table organizes the atoms of
the elements by properties and atomic
number.
• Modern periodic table differs from Mendeleev’s
– Elements with similar properties are found in columns, not
rows.
– The elements are not arranged by atomic mass but by atomic
number.
Reading the Periodic Table
• Each square of the periodic table gives particular
information of the atoms of an element.
• The number at the top of the square - atomic
number= the number of protons in the nucleus of an
atom of that element
• Chemical symbol – an abbreviation for the element’s
name, which is written below the symbol
• The number below the name – atomic mass = avg.
atomic mass of all the isotopes of the element
Reading the Periodic Table
• Color of the symbol = the physical state of the
element at room temp. (not always these colors)
– White = gas
– Blue = liquid
– Black = solid
The background colors of the squares indicate
whether the element is a metal, nonmetal, or
metalloid
Groups and Periods
• Group = elements in a vertical column of the periodic
table that show similarities in their chemical and
physical properties
• Sometimes a group is called a family of elements,
because these elements seem to be related.
Groups and Periods
• Group 17 – the halogen group
– Tend to combine easily with many other elements and
compounds, especially with the elements in Groups 1 and 2
– Have some similarities, but their physical properties are not
the same (ex. Flourine and chlorine are gases, bromine is a
liquid, and iodine and astatine are solids at room temp.) =
related but not identical
Groups and Periods
• Period – each horizontal row on the periodic table
• Properties of elements change in a predictable way
from one end of a period to the other
• Ex. Period 3 – goes from metals to metalloids to
nonmetals
Trends in the Periodic Table
• An element’s position in the table can give
information about the element.
• Atoms form ions by gaining or losing electrons
– Atoms of elements on the left side of the table: form
positive ions easily (lose electrons)
– Group 1 atoms lose and electron to form ions with one
positive charge (1+) and atoms in Group 2 can lose two
electrons to form ions with a charge of 2+
– Atoms of elements in Group 18 normally do not form ions
– Atoms of elements in Group 17 often gain one electron to
form a negative ion (1-) and atoms in Group 16 can gain two
electrons to form a 2_ ion
– Atoms of Groups 3 to 12 form positive ions, but the charge
can vary
Trends of the Periodic Table
• Atomic size increases as you move down a group
(column) and decreases as you move left to right
across a period
Trends in the Periodic Table
• Density generally increases from the top of a group
to the bottom, but within a period, the elements at
the far left and far right sides of the table are the
least dense, and the elements in the middle are the
most dense (osmium (Os) has the highest known
density and is located at the center of the table)