periodic properties of the elements guess what day it is…hump day!
TRANSCRIPT
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PeriodicProperties
of the Elements
Guess what day it is…Hump day!
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PeriodicProperties
of the Elements
Periodic Trends
• 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
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
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 AtomsBonding atomic radius tends to… …decrease from left to
right across a rowdue to increasing ENC.
…increase from top to bottom of a column
due to increasing value of size of valence shell
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PeriodicProperties
of the Elements
How do we Justify Atomic Radii differences?
Horizontally
Decreases leftright• Effective Nuclear
Charge increases• Resulting in greater
attraction for e- and thus smaller atomic radius
Vertically
Increases top bottom• Size of valence shell
increases with each higher energy level and e- is located farther from nucleus
• e- shielding voids effective nuclear charge argument
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PeriodicProperties
of the Elements
Atomic Radius Practice1. Justify the difference in atomic radius for
Mg and S.
2. Justify the difference in atomic radius for Li and Rb.
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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
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.
Na(g) Na+(g) + e-
Second ionization energy is that energy required to remove second electron, etc.
Na+(g) Na2+
(g) + e-
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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 takes a quantum leap.
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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 due to the size of the valence shell.
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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.
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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
Electron Affinity
Energy change accompanying addition of electron to gaseous atom:
Cl + e− Cl−
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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 IVA and VA.Group VA has no
empty orbitals.Extra electron must
go into occupied orbital, creating repulsion.
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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
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.• Nonmetals tend to form anions.
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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
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
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.
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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 as go down group.
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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.