Periodic Trends Section 6.3

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Periodic TrendsSection 6.3

• Objectives
• AOD C.3.1 Define atomic radii, ionization energy,
  electronegativity, and energy levels.
• AOD C.3.2 Recognize periodic trends of elements,
  including the number of valence electrons, atomic
  size, and reactivity.

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Atomic Radius

• Def The distance between an atoms nucleus and
  its valence electrons.
• Size of the atom varies from substance to
  substance

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Trends within Periods

• Atomic radius INCREASES as you move to the left
  across a period.
• Due to decreased nuclear charge (total charge in
  the nucleus --- which would be based on WHAT?)
• Decreased nuclear charge has less pull on the
  outermost (highest energy, valence) electrons so
  they can move further away from each other (and
  the nucleus).

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Trends within Groups

• Atomic radius INCREASES as you move down a group.
• Nuclear charge increases, but as electrons are
  added to higher energy levels, they are further
  from the pull of the nuclear protons, AND they
  are shielded by the electrons between them and
  the nucleus.
• Summary Increases left and down!

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Examples

• Which element has the smallest atomic radius?
  Largest atomic radius?
• Iodine (I)
• Bromine (Br)
• Fluorine (F)
• Chlorine (Cl)
• Fluorine
• Iodine

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Ionic Radius

• Ion an atom that gains or loses electrons
• When atoms lose electrons they form positive ions
  and become smaller.
• The electron lost will always be a valence
  electron.
• Loss of valence electrons may leave an empty
  outer orbital which results in a smaller radius.
  (Example Na1)
• Even if the orbital is not emptied, repulsion
  between fewer electrons decreases allowing them
  to be pulled closer to the nucleus.

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Ionic Radius, continued..

• When atoms GAIN electrons they form negative ions
  and they always become larger.
• The addition of an electron to an atom increases
  the repulsion between the valence electrons,
  forcing them to move farther apart (Example
  O-2).
• The result is a larger radius.

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Ionic Radius within Periods

• Size of the positive ions gradually increases
  from right to left across a period.
• The size of the much larger negative ions also
  gradually increases from right to left, until you
  reach the smaller positive ions.
• Ionic radius GENERALLY INCREASES to the left
  across periods.

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Ionic Radius within Groups

• As you move down a group an ions outer electrons
  are in higher principal energy levels resulting
  in a gradual increase in ionic size.
• Ionic radius INCREASES as you move down a group.
• Summary Increases left and down.

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Ionization Energy

• Def the energy required to remove an electron
  from an atom in the gaseous state
• How strongly an atoms nucleus holds on to its
  valence electrons
• High IE indicates atom has a strong hold on its
  electrons
• Low IE indicates an atom loses its outer electron
  more.easily

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Ionization Energy

• Energy required to remove the 1st electron is the
  first ionization energy.
• Energy required to remove the 2nd electron is
  the second ionization energy.
• 1st IE is ALWAYS highest.

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Ionization Energy within Periods

• INCREASES as you move from left to right across a
  period
• The increased nuclear charge of each successive
  element produces an increased hold on the valence
  electrons, as they are all in the same principal
  energy level.

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Ionization Energy within Groups

• INCREASES as you move up a group because the
  valence electrons are closer to the nucleus.
• Summary Increases up and right.

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Octet Rule

• Atoms tend to gain, lose, or share electrons in
  order to acquire a full set of eight valence
  electrons (resembling which elements?).
• Elements on the right side of the periodic table
  (nonmetals) tend to gain electrons in order to
  acquire the 8 valence electrons, forming negative
  ions)
• Elements on the left side of the periodic table
  (metals) tend to lose electrons and form positive
  ions.

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Electronegativity of an Element

• Indicates the relative ability of an atom to
  attract electrons in a chemical bond.
• Noble Gases are not assigned values
• Fluorine is the most electronegative element.
• Fr Cs are the least.
• In a chemical bond the atom with the greater
  electronegativity more strongly attracts the
  shared electrons.

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Electronegativity Trends within Periods Groups

• INCREASES as you move up a group and across a
  period (up and right).
• The lowest electronegativities are found at the
  lower left side
• Highest are found at the upper right side

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Example Problems

• Which element has the highest electronegativity?
  Lowest?
• N- Nitrogen
• P- Phosphorus
• As-Arsenic
• Sb-Antimony
• Bi- bismuth
• Nhighest
• Bi Lowest

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Homework Problems

• Pg. 175
• 56, 57, 59, 62, 63, 65-67