Periodic Table Trends

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Chapter 7

• Periodic Table Trends

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Todays Objectives

• 1. Evaluate periodic trends for atomic and ionic
  sizes, ionization energy and electron affinity
• 2. Calculate Zeff for any atom
• 3. Use Zeff as a tool to predict periodic trends
  in size of atoms and ions as well as ionization
  energy and electron affinity.

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Iso-electronic Atoms

• Atoms (or molecules) that have the same
  electronic configuration
• For example
• F-, Ne and Na are isoelectronic
• Why? What is the electronic structure of each
  one?
• 1s22s22p6 they all have 10 e-

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Summary of What You Learned in Chemistry I

• Slide 5 Atomic Radius Trends
• Slide 6 Ionic Radius Trends
• Slide 7 Ionization energy Trends Ionization
  energy is defined as the energy required to
  remove an electron from a gaseous atom.
• First ionization trends
• Subsequent ionization trends
• Slide 9 Electron Affinity

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Atomic Radius Trends
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Electron is vacated less electron electron
repulsions gt smaller positive ion.
Opposite Electron is added more
electron-electron repulsions gt larger negative
ion.
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First Ionization Energies of the Elements
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Subsequent Ionization Energies
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Electron Affinities

• Electron affinity is the opposite of ionization
  energy.
• Electron affinity is the energy change when a
  gaseous atom gains an electron to form a gaseous
  ion
• Cl(g) e- ? Cl-(g)
• Electron affinity can either be exothermic (as
  the above example) or endothermic
• Ar(g) e- ? Ar-(g)

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Electron Affinities

• Look at electron configurations to determine
  whether electron affinity is positive or
  negative.
• The extra electron in Ar needs to be placed in
  the 4s orbital which is significantly higher in
  energy than the 3p orbital.

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Electron Affinities
Electron is being added to a higher energy
p-orbital.
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Chemical Equations for electron gain or loss

• Na ? Na e-
• Na ? Na e-
• F e- ? F-
• Ca ? Ca e-
• O 2e- ? O2-

1st Ionization
2nd Ionization
Electron affinity
2nd Ionization
Electron affinity
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So how can we more scientifically account for
property variation?

• Use method of effective nuclear charge, Zeff
• Effective nuclear charge is the charge
  experienced by an electron on a many-electron
  atom.
• The effective nuclear charge is not the same as
  the charge on the nucleus because of the effect
  of the
• inner electrons.

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Effective Nuclear Charge

• Electrons are attracted to the nucleus, but
  repelled by the electrons that screen it from the
  nuclear charge.
• The nuclear charge experienced by an electron
  depends on its distance from the nucleus and the
  number of core electrons.
• As the average number of screening electrons (S)
  increases, the effective nuclear charge (Zeff)
  decreases.
• As the distance from the nucleus increases, S
  increases and Zeff decreases.

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Example of Zeff for a 3s electron
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Electron Configuration of Transition Metal Ions

• In general, electrons removed first from the
  highest (least negative) shell first
• However, with transition metals, we generally
  remove the s electrons before the d electrons.
  Doesnt this contradict the general rule?
• Yes, it does and why?

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Electron Configuration of Transition Metal Ions

• First note that removing electrons is NOT the
  reverse process of building the periodic table.
• When building the periodic table, we are adding
  both protons and electrons
• By contrast when we are removing electrons, we
  are only dealing with electrons, with the protons
  remaining constant
• In transition metals, electrons are added to the
  inner d-orbitals, so Zeff remains essentially
  constant, hence size is approx. constant

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How to relate electron force to Zeff

• No need to memorize trends as long as you know
  the Zeff and Coulombs Law
• F a Zeff e/r2 and E a Zeff e/r
• Zeff increases across a period and constant
  within a group (using sophisticated methods)
  incr. down a group
• Use force analysis for atomic/ionic sizes
• Use energy for ionization and electron affinity

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How to relate electron force to Zeff

• Radius (dependent primarily on n) is constant
  across a period and increases down a group
• F a Zeff e/r2
• Zeff a linear effect, radius is squared
• Therefore, the radius will have not much of an
  effect across a period but a greater effect as we
  move down a column.

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Todays Objectives

• Use of Zeff and Coulombs Law to predict property
  variation
• Properties and reactions of metals, nonmetals,
  and metalloids
• Properties of groups 1A (and hydrogen), 2A, 6A,
  7A, 8A

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Property Prediction

• Use force analysis (F a Zeff e/r2 )
• Atomic size decrease across a period
• r constant but Zeff increasing
• Atomic size increase down group
• r increasing radius
• r increasing outweighs Zeff and electron cloud
  not a strongly attracted to nucleus.

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Property Prediction (Contd)

• Use Energy Analysis (E a -Zeff e/r)
• Ionization energy increase across a period
• r constant, but Zeff increasing
• Ionization energy decrease down a group
• r increasing outweighs increase in Zeff

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Property Prediction (Contd)

• Use energy analysis (E a -Zeff e/r)
• Electron Affinity increases across a period
• r constant and Zeff incr, so more energy is
  released
• Electron Affinity relatively constant down a
  group
• r increases at the same rate as Zeff

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Example Zeff Calculation

• Compare the atomic size and first ionization
  energy for boron and nitrogen using Zeff
• Zeff for B 5-2 3
• Zeff for N 7-2 5
• Therefore N should have a smaller atomic size( B
  0.82 vs N 0.75 Å) and a higher I1 (B 800.6 vs N
  1403 kJ/mole)

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Ionization Energy Calculation

• E -2.18 x 10-18 J(Z2/n2)
• was true for Bohr atom.
• Can be derived from quantum mechanical model as
  well
• For a mole of electrons being removed
• E (6.02 x 1023/mol)(2.18 x 10-18 J) (Z2/n2)
• E 1.31 x 106 J/mol(Z2/n2)
• E 1310 kJ/mol(Z2/n2)
• In General for any atom E 1310 kJ/mol(Zeff2/n2)

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Metals, Nonmetals, and Metalloids

• Metals
• Metallic character refers to the properties of
  metals (shiny or lustrous, malleable and
  ductile, oxides form basic ionic solids, and tend
  to form cations in aqueous solution).
• Metallic character increases down a group.
• Metallic character decreases across a period.
• Metals have low ionization energies.
• Most neutral metals are oxidized rather than
  reduced.

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Metals, Nonmetals, and Metalloids

• Metals
• When metals are oxidized they tend to form
  characteristic cations.
• All group 1A metals form M ions.
• All group 2A metals form M2 ions.
• Most transition metals have variable charges.

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Metals, Nonmetals, and Metalloids
Metals
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Metals, Nonmetals, and Metalloids

• Metals
• Most metal oxides are basic (make OH- ions)
• Metal oxide water ? metal hydroxide
• Na2O(s) H2O(l) ? 2NaOH(aq)
• Nonmetals
• Nonmetals are more diverse in their behavior than
  metals.
• When nonmetals react with metals, nonmetals tend
  to gain electrons
• metal nonmetal ? salt
• 2Al(s) 3Br2(l) ? 2AlBr3(s)

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Metals, Nonmetals, and Metalloids

• Nonmetals
• Most nonmetal oxides are acidic
• nonmetal oxide water ? acid
• P4O10(s) H2O(l) ? 4H3PO4(aq)
• Metalloids
• Metalloids have properties that are intermediate
  between metals and nonmetals.
• Example Si has a metallic luster but it is
  brittle.
• Metalloids have found fame in the semiconductor
  industry.

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Reaction page summary

• Active Metals water ? metal hydroxide (base)
  H2
• Metal oxide water ? metal hydroxide (base)
• Non-metal oxide water ? Acid
• Non-metal (Cl2) water ? HCl HOCl
• Metal non-metal ? salt
• Metals O2 ? oxide
• Metals sulfur ? sulfide
• Metals chlorine ? chloride
• Metals H2 ? hydride
• Acid base ? salt water

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Reaction Examples

• 2K 2H2O ? 2KOH H2
• CaO H2O ? Ca(OH)2
• SO3 H2O ?H2SO4
• Cl2 H2O ? HCl HOCl
• Metal non-metal ? salt
• 2Mg O2 ? 2MgO
• Ca S ? CaS
• Zn Cl2 ? ZnCl2
• Ca H2 ? CaH2
• HCl NaOH ? NaCl H2O

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Combining reaction

• Combining 2 and 3
• Acid Metal oxide ? salt water
• Example ZnO 2HCl ? ZnCl2 H2O
• Combining 2 and 3
• Non-metal oxide base ? salt water
• Example SO3 2NaOH ? Na2SO4 H2O

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Reason for reactions

• Metals give up electrons easily and displace H
  from H2O
• Metal oxides The oxide (O2-) forms a more
  stable OH-.
• Non-metal oxides are covalently bonded and will
  split water putting H on the O of the non-metal
  oxide
• Example SO3 H2O ? H2SO4

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Group Trends for the Active Metals

• Group 1A The Alkali Metals
• Alkali metals are all soft.
• Chemistry dominated by the loss of their single s
  electron
• M ? M e-
• Reactivity increases as we move down the group.
• Alkali metals react with water to form MOH and
  hydrogen gas
• 2M(s) 2H2O(l) ? 2MOH(aq) H2(g)

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Group Trends for the Active Metals

• Group 1A The Alkali Metals
• Alkali metal produce different oxides when
  reacting with O2
• 4Li(s) O2(g) ? 2Li2O(s) (oxide)
• 2Na(s) O2(g) ? Na2O2(s) (peroxide)
• K(s) O2(g) ? KO2(s) (superoxide)
• Alkali metals emit characteristic colors when
  placed in a high temperature flame.
• The s electron is excited by the flame and emits
  energy when it returns to the ground state.

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Group Trends for the Active Metals
Group 1A The Alkali Metals
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Group Trends for the Active Metals
Group 1A The Alkali Metals
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Group Trends for the Active Metals
Group 2A The Alkaline Earth Metals
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Group Trends for the Active Metals

• Group 2A The Alkaline Earth Metals
• Alkaline earth metals are harder and more dense
  than the alkali metals.
• The chemistry is dominated by the loss of two s
  electrons
• M ? M2 2e-.
• Mg(s) Cl2(g) ? MgCl2(s)
• 2Mg(s) O2(g) ? 2MgO(s)
• Be does not react with water. Mg will only react
  with steam. Ca onwards
• Ca(s) 2H2O(l) ? Ca(OH)2(aq) H2(g)

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Group Trends for Selected Nonmetals

• Hydrogen
• Hydrogen is a unique element.
• Most often occurs as a colorless diatomic gas,
  H2.
• It can either gain another electron to form the
  hydride ion, H-, or lose its electron to become
  H
• 2Na(s) H2(g) ? 2NaH(s)
• 2H2(g) O2(g) ? 2H2O(g)
• H is a proton.
• The aqueous chemistry of hydrogen is dominated by
  H(aq).

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Group Trends for Selected Nonmetals
Group 6A The Oxygen Group
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Group Trends for Selected Nonmetals

• Group 6A The Oxygen Group
• As we move down the group the metallic character
  increases (O2 is a gas, Te is a metalloid, Po is
  a metal).
• There are two important forms of oxygen O2 and
  ozone, O3. Ozone can be prepared from oxygen
• 3O2(g) ? 2O3(g) ?H 284.6 kJ.
• Ozone is pungent and toxic.

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Group Trends for Selected Nonmetals

• Group 6A The Oxygen Group
• Oxygen (or dioxygen, O2) is a potent substance
  (oxidizing agent) since the O2- ion has a noble
  gas configuration.
• There are two oxidation states for oxygen 2-
  (e.g. H2O) and 1- (e.g. H2O2).
• Sulfur is another important member of this group.
• Most common form of sulfur is yellow S8.
• Sulfur tends to form S2- in compounds (sulfides).

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Group Trends for Selected Nonmetals
Group 7A The Halogens
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Group Trends for Selected Nonmetals

• Group 7A The Halogens
• The chemistry of the halogens is dominated by
  gaining an electron to form an anion
• X2 2e- ? 2X-.
• Fluorine is one of the most reactive substances
  known
• 2F2(g) 2H2O(l) ? 4HF(aq) O2(g) ?H -758.7
  kJ.
• All halogens consists of diatomic molecules, X2.

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Group Trends for Selected Nonmetals

• Group 7A The Halogens
• Chlorine is the most industrially useful halogen.
  It is produced by the electrolysis of brine
  (NaCl)
• 2NaCl(aq) 2H2O(l) ? 2NaOH(aq) H2(g) Cl2(g).
• The reaction between chorine and water produces
  hypochlorous acid (HOCl) which disinfects pool
  water
• Cl2(g) H2O(l) ? HCl(aq) HOCl(aq).
• Hydrogen compounds of the halogens are all strong
  acids with the exception of HF.

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Group Trends for Selected Nonmetals
Group 8A The Noble Gases
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Group Trends for Selected Nonmetals

• Group 8A The Noble Gases
• These are all nonmetals and monatomic.
• They are notoriously un-reactive because they
  have completely filled s and p sub-shells.
• In 1962 the first compound of the noble gases was
  prepared XeF2, XeF4, and XeF6.
• To date the only other noble gas compounds known
  are KrF2 and HArF.