Simply: Distance from the nucleus to the valence electrons.
More officially: “half the distance between the nuclei in a
molecule consisting of identical atoms”
The size of the atoms DECREASES
from left to right across a period.
WHY? The nucleus is gaining more protons to become
larger and more strongly positive. The stronger nuclear
charge attracts the electrons and draws the cloud towards
the nucleus.
Period #2
The size of the atoms
DECREASES from bottom to top
of a group.
WHY? There are fewer energy
levels at the top of the periodic
table. The pull of the nucleus is
shielded as more energy levels are
added.
1A
Use your periodic table and knowledge of atomic radius
to answer the following multiple choice questions.
1. Which of the following atoms will have the
smallest radius?
a. Tin
b. Strontium
c. Selenium
d. Arsenic
2. Which of the following atoms will have a larger
radius than calcium?
a. Aluminum
b. Copper
c. Potassium
d. None
3. In magnesium oxide, MgO, the smaller atom is:
a. Magnesium
b. Oxide
c. Neither
Check your answers. Do you have questions?
1. Which of the following atoms will have the
smallest radius?
a. Tin
b. Strontium
c. Selenium
d. Arsenic
2. Which of the following atoms will have a larger
radius than calcium?
a. Aluminum
b. Copper
c. Potassium
d. None
3. In magnesium oxide, MgO, the smaller atom is:
a. Magnesium
b. Oxide
c. Neither
Cations form by losing electrons to become stable. As
electrons are lost, the atom becomes smaller. Therefore,
cations are smaller than their corresponding neutral atoms.
• Draw the orbital diagram for neutral potassium.
• Draw the orbital diagram for the potassium cation.
• Compare your drawings. Does the trend make sense?
Anions form by gaining electrons to become stable. As
electrons are added, the atom becomes larger. Therefore,
anions are bigger than their corresponding neutral atoms.
• Draw the orbital diagram for sulfur.
• Draw the orbital diagram for the sulfide ion.
• Does the trend make sense?
Energy required to remove an electron from an atom.
Based on what you know about the octet rule, which
atoms might WANT to have electrons removed? Which
ones might NOT WANT to have electrons removed?
The IE INCREASES from left to right
across a period.
WHY? The nucleus is gaining more protons to become
larger and more strongly positive. The nucleus holds tightly
to the electrons. Therefore, HIGH ENERGY is needed to
remove an electron.
The ionization energy of elements
INCREASES from bottom to top of a group.
WHY? There are fewer energy levels at the
top of the periodic table. The pull of the
positive nucleus is NOT shielded by few
energy levels. At the bottom of the table, the
pull of the nucleus IS SHIELDED by the many
energy levels. Therefore, LOW ENERGY is
needed to remove an electron from a large
atom.
Use your periodic table and knowledge of ionization
energy to answer the following multiple choice questions.
1. Which of the following atoms will have the
smallest IE?
a. Tin
b. Strontium
c. Selenium
d. Arsenic
2. Which of the following atoms will have a larger IE
than calcium?
a. Aluminum
b. Copper
c. Potassium
d. None
3. In magnesium oxide, the atom with the smallest
ionization energy is:
a. Magnesium
b. Oxide
c. Neither
Check your answers. Do you have questions?
1. Which of the following atoms will have the
smallest IE?
a. Tin
b. Strontium
c. Selenium
d. Arsenic
2. Which of the following atoms will have a larger IE
than calcium?
a. Aluminum
b. Copper
c. Potassium
d. None
3. In magnesium oxide, the atom with the smallest
ionization energy is:
a. Magnesium
b. Oxide
c. Neither
You have learned about ionization energy
already, but there is more to the story...
1st Ionization Energy - energy required to remove
the first electron from an atom
2nd Ionization Energy - energy required to remove
the second electron from an atom
3rd Ionization Energy - energy required to remove
the third electron from an atom
The pattern on the periodic table requires more
thought for predicting 2nd and 3rd IEs
Example: Aluminum
Draw the orbital diagram for Al.
• 1st IE = 580 kJ/mol (removes the highest energy electron – the lone 3p)
• 2nd IE = 1815 kJ/mol (removes the next highest energy electron – a 3s)
• 3rd IE = 2740 kJ/mol (removes the 3rd highest energy electron – the final
3s)
• 4th IE = 11,600 kJ/mol (removes a “core electron” or a non-valence shell
electron – WILL NOT OCCUR)
2nd Ionization Energy - energy required to remove
the second electron from an atom
Compare the 2nd ionization energy of Mg with the
2nd ionization energy Na.
• Thought process:
•Magnesium needs to lose two electrons to become
stable, and sodium needs to lose one electron to become
stable.
•Who will require more energy to take the second
electron?
• Answer: Sodium is STABLE after losing one electron, and
it will require LOTS of energy to remove a second.
The ability of an atom to draw electrons from another
atom closer to itself when in a chemical bond.
The EN INCREASES from left to
right across a period.
WHY? The nucleus is gaining more protons to become
larger and more strongly positive. The nucleus holds tightly
to the electrons. Therefore, the strong nucleus is able to
draw the bonding electrons closer.
The electronegativity of elements
INCREASES from bottom to top of a group.
WHY? There are fewer energy levels at the
top of the periodic table. The pull of the
positive nucleus is NOT shielded by few
energy levels. At the bottom of the table, the
pull of the nucleus IS SHIELDED by the many
energy levels. Therefore, the shielded
nucleus is unable to draw the bonding
electrons closer.
Notice: Noble gases have NO electronegativity
since they do not form bonds.
Answer the following questions using your periodic table
and understanding of EN.
1. Which of the following has the greatest
electronegativity?
a. Nitrogen
b. Bismuth
c. Silicon
d. Neon
2. In water, H2O, which atom is holding the bonding
electrons closest to itself?
a. Hydrogen
b. Oxygen
c. Neither
Check your answers. Do you have questions before we
move on to ionic radius?
1. Which of the following has the greatest
electronegativity?
a. Nitrogen
b. Bismuth
c. Silicon
d. Neon
2. In water, H2O, which atom is holding the bonding
electrons closest to itself?
a. Hydrogen
b. Oxygen
c. Neither
Applications of Periodic Trends
• Let’s look at 5-3 Review & Reinforcement.
– Which ATOM has the larger atomic radius?
– Which ION has the smaller atomic radius?
– Which atom or ion has the larger ionization
energy?
– Question 33 - Why is there such a large jump in
ionization energy between the second and third
ionization energies for magnesium?
More Applications of Trends
• Let’s look at 5-3 Practice Problems.
– 1-5 Answers
– Question 6: Which is largest: a K atom, a K+1
ion, or an Rb atom?
– Question 7: Which of the following is the
largest: a Cl atom, a Cl- ion, or a Br atom?
– Question 8: Which of the following is the
smallest: a Li atom, a Li1+ ion, or a Na atom?
– Question 9: Which of the following is the
largest: a Te2- ion, an I- ion, or an Xe atom?
Sample Discussion Question
• Which is the smaller atom: oxygen or
nitrogen? Why?
• Which atom has higher ionization
energy: fluorine or chlorine? Why?