apchem- chapter 7 lecture- periodic trends
DESCRIPTION
Chapter seven lecture for AP Chemistry on periodic trendsTRANSCRIPT
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PeriodicProperties
of the Elements
Development of Periodic Table
• Elements in the same group generally have similar chemical properties.
• Properties are not identical, however.
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PeriodicProperties
of the Elements
Development of Periodic Table
Dmitri Mendeleev and Lothar Meyer independently came to the same conclusion about how elements should be grouped.
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PeriodicProperties
of the Elements
Development of Periodic Table
Mendeleev, for instance, predicted the discovery of germanium (which he called eka-silicon) as an element with an atomic weight between that of zinc and arsenic, but with chemical properties similar to those of silicon.
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PeriodicProperties
of the Elements
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
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PeriodicProperties
of the Elements
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PeriodicProperties
of the Elements
The atomic number depends on the number of protons in the nucleus, while the atomic weight depends (mainly) on the number of both protons and neutrons in the nucleus.
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PeriodicProperties
of the Elements
Periodic Trends
• In this chapter, we will rationalize observed trends inSizes of atoms and ions.Ionization energy.Electron affinity.
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PeriodicProperties
of the Elements
Effective Nuclear Charge
• In a many-electron atom, electrons are both attracted to the nucleus and repelled by other electrons.
• The nuclear charge that an electron experiences depends on both factors.
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PeriodicProperties
of the Elements
• The attraction of an electron to the nucleus depends onThe charge of the nucleusThe distance the electron is from the nucleus.
In large atoms, the electrons in outer shells do not experience the full charge of the nucleus because inner shells of electrons “shield” them from the nucleus.
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PeriodicProperties
of the Elements
Effective Nuclear Charge
The effective nuclear charge, Zeff, is found this way:
Zeff = Z − S
where Z is the atomic number and S is a screening constant, usually close to the number of inner electrons.
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PeriodicProperties
of the Elements
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
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PeriodicProperties
of the Elements
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PeriodicProperties
of the Elements
The 2p electron of a Ne atom.
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PeriodicProperties
of the Elements
1. 2s orbital 2. 2p orbital 3. 3s orbital 4. 3p orbital 5. 3d orbital
In an germanium atom, which electron will experience the greatest effective nuclear charge? An electron in a…
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PeriodicProperties
of the Elements
1. 2s orbital 2. 2p orbital 3. 3s orbital 4. 3p orbital 5. 3d orbital
In an germanium atom, which electron will experience the greatest effective nuclear charge? An electron in a…
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PeriodicProperties
of the Elements
The effective nuclear charge experienced by 3p electrons in phosphorus is
1. +2
2. +3
3. +5
4. +7
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PeriodicProperties
of the Elements
1. +2
2. +3
3. +5
4. +7
Correct Answer:
The effective nuclear charge is given by the equation:
Zeff = Z Swhere Z represents number of protons in the nucleus and S represents the average number of electrons between the nucleus and the electron in question. Here:
Zeff = 15 10 = +5
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PeriodicProperties
of the Elements
Sizes of Atoms
It is difficult to define atomic radii for single atoms because the outside of the electron cloud is ambiguous.
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PeriodicProperties
of the Elements
Sizes of Atoms
The non-bonding atomic radius is defined as one-half of the distance between two atoms when they collide.
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PeriodicProperties
of the Elements
Sizes of Atoms
The bonding atomic radius is defined as one-half of the distance between covalently bonded nuclei.
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PeriodicProperties
of the Elements
Sizes of Atoms
Adding two bonding radii together gives a useful estimate of bond length.
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PeriodicProperties
of the Elements
Sizes of AtomsBonding atomic radius tends to… …decrease from left
to right across a rowdue to increasing Zeff.
…increase from top to bottom of columndue to increasing
value of n
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PeriodicProperties
of the Elements
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
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PeriodicProperties
of the Elements
No
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PeriodicProperties
of the Elements
Order the following according to increasing atomic radius.
1. Ge < Si < Se < Cl 2. Se < Si < Ge < Cl
3. Si < Cl < Ge < Se
4. Cl < Si < Se < Ge
5. Si < Ge < Se < Cl
Ge Si Se Cl
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PeriodicProperties
of the Elements
Order the following according to increasing atomic radius.
1. Ge < Si < Se < Cl 2. Se < Si < Ge < Cl
3. Si < Cl < Ge < Se
4. Cl < Si < Se < Ge
5. Si < Ge < Se < Cl
Ge Si Se Cl
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PeriodicProperties
of the Elements
Based on atomic radii, which of the following bonds would be expected to be the shortest?
1. HH
2. HF
3. FF
4. ClCl
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PeriodicProperties
of the Elements
Correct Answer:
The shortest bonding atomic radius belongs to the H atom, so an H2 molecule will have the shortest HX (where X is any atom) bond distance.
1. HH
2. HF
3. FF
4. ClCl
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PeriodicProperties
of the Elements
Sizes of Ions• Ionic size depends
upon:Nuclear charge.Number of
electrons.Orbitals in which
electrons reside.
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PeriodicProperties
of the Elements
Sizes of Ions• Cations are
smaller than their parent atoms.The outermost
electron is removed and repulsions are reduced.
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PeriodicProperties
of the Elements
Sizes of Ions• Anions are larger
than their parent atoms.Electrons are
added and repulsions are increased.
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PeriodicProperties
of the Elements
Sizes of Ions
• Ions increase in size as you go down a column.Due to increasing
value of n.
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PeriodicProperties
of the Elements
Sizes of Ions
• In an isoelectronic series, ions have the same number of electrons.
• Ionic size decreases with an increasing nuclear charge.
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PeriodicProperties
of the Elements
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
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PeriodicProperties
of the Elements
Order the following according to increasing atomic/ionic radius.
1. C < Li+ < O2- < N3-
2. N3- < O2- < C < Li+
3. Li+ < C < N3- < O2-
4. Li+ < C < N3- < O2- 5. Li+ < C < O2- < N3-
N3- Li+ C O2-
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PeriodicProperties
of the Elements
Order the following according to increasing atomic/ionic radius.
1. C < Li+ < O2- < N3-
2. N3- < O2- < C < Li+
3. Li+ < C < N3- < O2-
4. Li+ < C < N3- < O2- 5. Li+ < C < O2- < N3-
N3- Li+ C O2-
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PeriodicProperties
of the Elements
Which is larger: Na+ or Na?
1. Na+
2. Na
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PeriodicProperties
of the Elements
1. Na+
2. Na
Correct Answer:
Both have the same number of protons, but Na with one more electron will be larger.
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PeriodicProperties
of the Elements
Which is larger: Cl or Cl?
1. Cl
2. Cl
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PeriodicProperties
of the Elements
1. Cl
2. Cl
Correct Answer:
Both have the same number of protons, but Cl− with one more electron will be larger.
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PeriodicProperties
of the Elements
Lecture 7b- Sections 7.4 - 7.5
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PeriodicProperties
of the Elements
Ionization Energy
• Amount of energy required to remove an electron from the ground state of a gaseous atom or ion.First ionization energy is that energy
required to remove first electron.Second ionization energy is that energy
required to remove second electron, etc.
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PeriodicProperties
of the Elements
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PeriodicProperties
of the Elements
• the photoelectric effect
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PeriodicProperties
of the Elements
Ionization Energy
• It requires more energy to remove each successive electron.
• When all valence electrons have been removed, the ionization energy shows a HUGE increase.
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PeriodicProperties
of the Elements
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
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PeriodicProperties
of the Elements
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PeriodicProperties
of the Elements
I2 for the carbon atom is greater.
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PeriodicProperties
of the Elements
Trends in First Ionization Energies
• As one goes down a column, less energy is required to remove the first electron.For atoms in the same
group, Zeff is essentially the same, but the valence electrons are farther from the nucleus.
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PeriodicProperties
of the Elements
Trends in First Ionization Energies
• Generally, as one goes across a row, it gets harder to remove an electron.As you go from left to
right, Zeff increases.
And the electron’s distance from the nucleus decreases.
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PeriodicProperties
of the Elements
Trends in First Ionization Energies
However, there are two apparent discontinuities in this trend.
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PeriodicProperties
of the Elements
Trends in First Ionization Energies
• The first occurs between Groups IIA and IIIA.
• Electron removed from p-orbital rather than s-orbitalElectron farther from
nucleusSmall amount of
repulsion by s electrons.
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PeriodicProperties
of the Elements
Trends in First Ionization Energies
• The second occurs between Groups VA and VIA.Electron removed
comes from doubly occupied orbital.
Repulsion from other electron in orbital helps in its removal.
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PeriodicProperties
of the Elements
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PeriodicProperties
of the Elements
Which will have the highest ionization energy?
• C
• N
• O
• Al
• Si
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PeriodicProperties
of the Elements
Which will have the highest ionization energy?
• C
• N
• O
• Al
• Si
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PeriodicProperties
of the Elements
Which will be the largest?
• I1 of Na
• I2 of Na
• I1 of Mg
• I2 of Mg
• I3 of Mg
I = ionization energy
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PeriodicProperties
of the Elements
Which will be the largest?
• I1 of Na
• I2 of Na
• I1 of Mg
• I2 of Mg
• I3 of Mg
I = ionization energy
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PeriodicProperties
of the Elements
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PeriodicProperties
of the Elements
They have the same electron configuration: [Ar]3d3.
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PeriodicProperties
of the Elements
Electron Affinity
Energy change accompanying addition of electron to gaseous atom:
Cl + e− Cl−
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PeriodicProperties
of the Elements
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PeriodicProperties
of the Elements
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PeriodicProperties
of the Elements
They are equal in magnitude and opposite in sign.
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PeriodicProperties
of the Elements
Trends in Electron Affinity
In general, electron affinity becomes more exothermic as you go from left to right across a row.
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PeriodicProperties
of the Elements
Trends in Electron Affinity
There are again, however, two discontinuities in this trend.
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PeriodicProperties
of the Elements
Trends in Electron Affinity
• The first occurs between Groups IA and IIA.Added electron must
go in p-orbital, not s-orbital.
Electron is farther from nucleus and feels repulsion from s-electrons.
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PeriodicProperties
of the Elements
Trends in Electron Affinity
• The second occurs between Groups 4A and 5A.Added electron must
go in an occupied p-orbital.
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PeriodicProperties
of the Elements
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
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PeriodicProperties
of the Elements
Based on periodic trends, which of the following elements is expected to have the largest (i.e., most negative) electron affinity?
1. K
2. Na
3. Si
4. S
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PeriodicProperties
of the Elements
Correct Answer:
S has most negative electron affinity in this list.
1. K
2. Na
3. Si
4. S
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PeriodicProperties
of the Elements
Lecture 7c- Sections 7.6 - 7.8
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PeriodicProperties
of the Elements
Properties of Metal, Nonmetals,and Metalloids
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PeriodicProperties
of the Elements
Metals versus Nonmetals
Table 7.3 on page 277
Differences between metals and nonmetals tend to revolve around these properties.
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PeriodicProperties
of the Elements
Metals versus Nonmetals
• Metals tend to form cations. (Low I.E.)• Nonmetals tend to form anions. ( exo E.A.)
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PeriodicProperties
of the Elements
Metals
Tend to be lustrous, malleable, ductile, and good conductors of heat and electricity.
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PeriodicProperties
of the Elements
Metals
• Compounds formed between metals and nonmetals tend to be ionic.
• Metal oxides tend to be basic.
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PeriodicProperties
of the Elements
Metal oxide + water metal hydroxideMgO(s) + H2O(l) Mg(OH)2(aq) but only if it dissolves a bit in water
Metal oxide (a.k.a. base anhydride) reactions
Metal oxide + acid water + saltMgO(s) + HCl(aq) MgCl2(aq) + H2O(l)
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PeriodicProperties
of the Elements
Nonmetals
• Dull, brittle substances that are poor conductors of heat and electricity.
• Tend to gain electrons in reactions with metals to acquire noble gas configuration.
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PeriodicProperties
of the Elements
Nonmetals
• Substances containing only nonmetals are molecular compounds.
• Most nonmetal oxides are acidic.
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PeriodicProperties
of the Elements
nonmetal oxide + water acidSO3(g) + H2O(l) H2SO4(aq)
nonmetal oxide (acid anhydride) reactions
nonmetal oxide + base water + salt
CO2(g) + 2NaOH(aq) Na2CO3(aq) + H2O(l)
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PeriodicProperties
of the Elements
Metalloids
• Have some characteristics of metals, some of nonmetals.
• For instance, silicon looks shiny, but is brittle and fairly poor conductor.
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PeriodicProperties
of the Elements
Group Trends
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PeriodicProperties
of the Elements
Alkali Metals
• Soft, metallic solids.• Name comes from
Arabic word for ashes.
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PeriodicProperties
of the Elements
Alkali Metals
• Found only as compounds in nature.• Have low densities and melting points.• Also have low ionization energies.
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PeriodicProperties
of the Elements
Alkali Metals
Their reactions with water are famously exothermic.
2M(s) + H2O(l) 2MOH(aq) + H2(g)
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PeriodicProperties
of the Elements
Alkali Metals• Alkali metals (except Li) react with oxygen to
form peroxides.• K, Rb, and Cs also form superoxides:
K + O2 KO2
• Produce bright colors when placed in flame.
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PeriodicProperties
of the Elements
Alkaline Earth Metals
• Have higher densities and melting points than alkali metals.
• Have low ionization energies, but not as low as alkali metals.
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PeriodicProperties
of the Elements
Alkaline Earth Metals• Be does not react
with water, Mg reacts only with steam, but others react readily with water.
• Reactivity tends to increase down the group.
Mg(s) + 2H2O(g) Mg(OH)2(aq) + H2(g)
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PeriodicProperties
of the Elements
Hydrogen
• Hydrogen is a nonmetal.• It has a high ionization energy
because its single electron experiences no nuclear shielding
• When it does lose it’s electron it isn’t really an atom anymore. It is a proton.
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PeriodicProperties
of the Elements
Group 6A
• Oxygen, sulfur, and selenium are nonmetals.• Tellurium is a metalloid.• The radioactive polonium is a metal.
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PeriodicProperties
of the Elements
Oxygen• Two allotropes:
O2
O3, ozone
• Three anions:O2−, oxideO2
2−, peroxideO2
1−, superoxide
• Tends to take electrons from other elements (oxidation)
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PeriodicProperties
of the Elements
Sulfur
• Weaker oxidizing agent than oxygen.
• Most stable allotrope is S8, a ringed molecule.
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PeriodicProperties
of the Elements
Group VIIA: Halogens
• Prototypical nonmetals• Name comes from the Greek halos and gennao: “salt formers”
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PeriodicProperties
of the Elements
Group VIIA: Halogens
• Large, negative electron affinitiesTherefore, tend to oxidize
other elements easily
• React directly with metals to form metal halides
• Chlorine added to water supplies to serve as disinfectant
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PeriodicProperties
of the Elements
Group VIIIA: Noble Gases
• Astronomical ionization energies• Positive electron affinities
Therefore, relatively unreactive
• Monatomic gases
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PeriodicProperties
of the Elements
Group VIIIA: Noble Gases
• Xe forms three compounds:XeF2
XeF4 (at right)
XeF6
• Kr forms only one stable compound:KrF2
• The unstable HArF was synthesized in 2000.