Lesson: 4'2 Periodicity Atomic and ionic radii

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Lesson 4.2 Periodicity - Atomic and ionic radii

• Aim To explain periodic changes in atomic and
  ionic radius.
• Learning Outcomes
• 1. explain why atomic and ionic radii vary
  throughout the periodic table.
• 2. arrange atoms and ions with respect to their
  size.

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Henry Moseley

• H. Moseley
• Elements should be arranged in the order of
  increasing atomic number (proton number) in the
  Periodic Table. (1913)

VIDEO 1
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1) Periodic trends in the size of atom (atomic
radii)

• The size /radius of atom is difficult to be
  defined exactly because the electron cloud has no
  clear boundary.
• The distance between the 2 nuclei in a molecule
  atomic radii

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Example
Radius, r a/2 Size ? volume V 4/3 ?r3 ? V
? r

• Down the group, atomic radii increases
• Across the period, atomic radii decreases

VIDEO 2
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Two factors that affect the atomic radii

• 1) Effective nuclear charge (Zeff) felt by the
  valence electrons.
• 2) Value of the principal quantum number of the
  valence electrons

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i) Effective nuclear charge (Zeff)

• The nuclear charge that actually felt by an
  electron because of the nucleus attraction.
• Value of Zeff
• Zeff z - s
• z no. of proton
• s no. of electrons filled at the
  inner orbital

VIDEO 3
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• Across the period,
• number of proton increase
• the effective nuclear charge increases
• the outer electrons feel an increase in positive
  charge which causes them to be drawn inwards
• the sizes of atom decrease.

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Example
Zeff
Electronic configuration
Atoms
12-102
1s2 2s2 2p6 3s2
12Mg
13-103
1s2 2s2 2p6 3s2 3p1
13Al
14-104
1s2 2s2 2p6 3s2 3p2
14Si
15-105
1s2 2s2 2p6 3s2 3p3
15P
VIDEO 4
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(ii) Value of the principal quantum number of the
valence electrons

• Value of n increase, the larger the orbital.
• The electrons in inner shells shield the
  electrons in outer shells from the nuclear charge
  - screening effect.
• Down a group - sizes of atoms increase .

VIDEO 5
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VIDEO 6
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• This graph shows that
• down the group, sizes of atoms increase.
• atomic radius decreases when across a period and
  up a group in the periodic table.
• The greater the nucleus attraction, the smaller
  the radius.

VIDEO 7
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2) Periodic trends in the ionic radii
VIDEO 8
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cations

• When electrons are removed from the valence
  shell, the electron-electron repulsions decrease,
• Allows the remaining electrons to be pulled
  closer together around the nucleus.
• Cations are smaller than the atoms from which
  they are formed.

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VIDEO 9
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anions

• When electrons are added to an atom, the mutual
  repulsions between them increase.
• This causes the electrons to push apart and
  occupy a larger volume.
• Therefore, negative ions (anions) are larger than
  the atoms from which they are formed.

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Exercise
VIDEO 10
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Isoelectronic series

• Isoelectronic series are groups of atoms and ions
  which have the same electronic configuration.
• Within isoelectronic series,
• the more positive the charge, the smaller the
  species
• the more negative the charge, the larger the
  species.

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P3- gtS2- gt Cl- gt Na gt Mg2 gt Al3 gt Si4
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• Across the period, sizes of cations and
• anions decrease due to the increase of
• Zeff.
• Ionic radii of Na gt Mg 2 gt Al 3 gt Si 4.
• They are isoelectronic (10 e) 1s2 2s2 2p6.
• Zeff increase, ionic radii decrease.

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• Ionic radii of Cl- lt S 2- lt P 3-.
• They are isoelectronic (18 e)
• 1s2 2s2 2p6 3s2 3p6.
• Zeff decrease, ionic radii increase.

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Conclusion

• Ionic and atomic radii
• Across the period
• Down the group