Download Determining Periodic Trends

Determining Periodic Trends
Purpose
To understand the
periodic nature of the
periodic table – by
knowing an element’s
location on the periodic
table one can determine
the element’s number
of valence electrons and
have an awareness of
its ionization energy,
density, electron affinity,
electronegativity, atomic
radius, and ionic
radius, relative to
other elements
Materials
elements from Periodic
Trends, periodic table,
scissors, tape, white
paper, graph paper
Procedure
Part 1
1. You will be given cards for each of five elements in a certain group.
Each card contains information about the physical properties of the
element. Use the descriptions in the Periodic Trends section below
to place your element cards in what you think is the correct
arrangement. You only have representative or main-group elements.
The transition metals and noble gases are not used in this activity.
2. Use a periodic table to predict the atomic number and period of each
element and write them in the spaces provided on the card.
Part 2
1. As a group, organize all the elements into a periodic table. Tape
the elements onto white paper when you are sure of their placement.
2. Compare your periodic table with the teacher’s key and make any
necessary adjustments.
3. Complete the Analysis and Conclusions section.
Terminology of Periodic Trends
Valence electron: An electron in an outer shell of an atom that can
participate in forming chemical bonds with other atoms. Nonvalence
electrons are tightly bound to the nucleus and are called core electrons.
Octet rule: Atoms tend to gain, lose, or share electrons to reach eight
electrons in their outer electron shells.
Ionization energy: The amount of energy required to remove one
electron from a neutral atom that is in the gaseous state. Elements on
the right side really want to keep their electrons because they are so
close to achieving an octet. Elements on the left side don’t mind losing
an electron. Going down a group, the shielding effect of additional
electrons makes it easier to remove outermost electrons.
Atomic radius: One-half the distance of a single bond between two
atoms of an element. As we go down a group, the principal energy
level increases, causing an increase in the average distance between
the electrons and the nucleus. Atoms get bigger as electrons are added
to the principal energy levels. All the atoms in a given period have their
outermost electrons in the same principal energy level. As we go across a
period, we are adding electrons to the same principal energy level. The
atoms do not get bigger across a period because we are also adding
protons to the nucleus as we move across a period. The additional
protons in the positive nucleus increase the pull on the negative
electrons resulting in smaller atoms across a period.
C.1.33, C.1.34 / Curriculum Framework / Why Are the Elements Arranged That Way?
Indiana Chemistry Standards Resource, November 2003
Ionic radius: One-half the center-to-center distance between two ions in a crystalline ionic compound.
Ions on the left side of the table are smaller than the atoms they come from because they’ve lost
an electron. Ions on the right side are bigger than the atoms they come from because they’ve gained
an electron.
Electronegativity: A measure of attraction of an atom for a pair of shared electrons, a measure of
whether an atom “grabs” more than its fair share of the shared electrons. As you go down a group,
atoms have more total electrons so they don’t really care that much about their outermost ones. As
you go across a period, atoms have more valence electrons and are so close to achieving an octet that
they’ll grab another atom’s electrons.
Electron affinity: The amount of energy absorbed when an electron is added to a neutral gaseous atom
to form a gaseous ion with a 1-charge. It has a negative value if energy is released. Most elements release
energy when they gain an electron. Atoms that really want an extra electron have the most negative
electron affinity. Atoms that don’t really want another electron do not have very negative values.
Analysis and Conclusions
1. Use the words increases or decreases to describe the trends for the following properties of the
representative elements, going from left to right across a period:
• ionization energy
• atomic radius
• ionic radius
• electronegativity
• electron affinity
2. Use the words increases or decreases to describe the trends for the following properties of the
representative elements, going from top to bottom down a group:
• ionization energy
• atomic radius
• ionic radius
• electronegativity
• electron affinity
3. When elements in Group I ionize, are they more likely to gain or lose electrons?
Explain your answer.
4. Explain why Group I elements have ionic radii smaller than their atomic radii.
C.1.33, C.1.34 / Curriculum Framework / Why Are the Elements Arranged That Way?
Indiana Chemistry Standards Resource, November 2003
5. When elements in Group VII ionize, are they more likely to gain or lose electrons?
Explain your answer.
6. Explain why Group VII elements have ionic radii larger than their atomic radii.
7. Compare how the melting point for metals and nonmetals generally changes as you
move down a group.
8. What is the relationship between number of valence electrons and group number?
9. Create a graph of ionization energy versus atomic number and identify the different periods.
Compare the ionization energies of metals to nonmetals.
10. Create a graph of atomic radius versus atomic number and identify the different periods.
What is the trend going down a group? What is the trend as you go across a period?
C.1.33, C.1.34 / Curriculum Framework / Why Are the Elements Arranged That Way?
Indiana Chemistry Standards Resource, November 2003
Determining Periodic Trends
Teacher Directions
There are five element cards in a family/group and there are seven families/groups total.
When placing the class into groups, make sure there are enough groups to create an integral
number of periodic tables.
In Part 2, allow students to use the key to confirm their placement of the elements before
they attempt to draw conclusions about periodic trends.
Answer Key
1.
•
•
•
•
•
ionization energy
atomic radius
ionic radius
electronegativity
electron affinity
increase
decrease
increase
increase
increase
2.
•
•
•
•
•
ionization energy
atomic radius
ionic radius
electronegativity
electron affinity
decrease
increase
increase
decrease
decrease
3.
4.
5.
6.
7.
8.
9.
10.
Group I elements lose their one valence electron to achieve a stable octet.
The loss of the one valence electron means the electron cloud is smaller. In addition, the
positive nucleus has a greater pull on the negative electron cloud, causing the electron cloud
to be drawn closer to the nucleus.
Group VII elements have seven valence electrons and they will gain an electron to achieve
a stable octet.
Group VII elements gain electrons while the total positive charge of the nucleus stays
the same. The electrons are not attracted to the nucleus as strongly as they were before
so the electron cloud spreads out a little. The electron cloud of the ion is also bigger because
there is greater repulsion between the increased number of electrons.
For metals, the melting point decreases down a group. For nonmetals, the melting point
increases down a family.
The group number is equal to the number of valence electrons for the main-group elements.
Compared to nonmetals, metals have lower ionization energies because they give up
electrons easily.
As you go down a group, the atoms get larger. As you go across a period, the atoms
get smaller.
C.1.33, C.1.34 / Curriculum Framework / Why Are the Elements Arranged That Way?
Indiana Chemistry Standards Resource, November 2003