periodic properties of the elements
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Periodic Properties of the Elements. Development of Periodic Table. Elements in the same group generally have similar chemical properties. Properties are not identical, however. Development of Periodic Table. - PowerPoint PPT PresentationTRANSCRIPT
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Periodic Propertiesof the Elements
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Development of Periodic Table
• Elements in the same group generally have similar chemical properties.
• Properties are not identical, however.
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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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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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Periodic Trends
• In this chapter, we will rationalize observed trends inSizes of atoms and ions.Ionization energy.Electron affinity.
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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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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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Sizes of Atoms
The bonding atomic radius is defined as one-half of the distance between covalently bonded nuclei.
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Sizes of AtomsBonding atomic radius tends to… …decrease from left to
right across a rowdue to increasing Zeff.
…increase from top to bottom of a column
due to increasing value of n
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Sizes of Ions• Ionic size depends
upon:Nuclear charge.Number of
electrons.Orbitals in which
electrons reside.
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Sizes of Ions• Cations are
smaller than their parent atoms.The outermost
electron is removed and repulsions are reduced.
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Sizes of Ions• Anions are larger
than their parent atoms.Electrons are
added and repulsions are increased.
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Sizes of Ions
• Ions increase in size as you go down a column.Due to increasing
value of n.
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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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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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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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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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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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Trends in First Ionization Energies
However, there are two apparent discontinuities in this trend.
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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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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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Electron Affinity
Energy change accompanying addition of electron to gaseous atom:
Cl + e− Cl−
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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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Trends in Electron Affinity
There are again, however, two discontinuities in this trend.
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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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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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Properties of Metal, Nonmetals,and Metalloids
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Metals versus Nonmetals
Differences between metals and nonmetals tend to revolve around these properties.
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Metals versus Nonmetals
• Metals tend to form cations.• Nonmetals tend to form anions.
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Metals
Tend to be lustrous, malleable, ductile, and good conductors of heat and electricity.
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Metals
• Compounds formed between metals and nonmetals tend to be ionic.
• Metal oxides tend to be basic.
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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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Nonmetals
• Substances containing only nonmetals are molecular compounds.
• Most nonmetal oxides are acidic.
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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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Group Trends
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Alkali Metals
• Soft, metallic solids.• Name comes from
Arabic word for ashes.
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Alkali Metals
• Found only as compounds in nature.• Have low densities and melting points.• Also have low ionization energies.
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Alkali Metals
Their reactions with water are famously exothermic.
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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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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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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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Group 6A
• Oxygen, sulfur, and selenium are nonmetals.• Tellurium is a metalloid.• The radioactive polonium is a metal.
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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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Sulfur
• Weaker oxidizing agent than oxygen.
• Most stable allotrope is S8, a ringed molecule.
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Group VIIA: Halogens
• Prototypical nonmetals• Name comes from the Greek halos and gennao: “salt formers”
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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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Group VIIIA: Noble Gases
• Astronomical ionization energies• Positive electron affinities
Therefore, relatively unreactive
• Monatomic gases
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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.