Transcript Document

Section 8.4
Ions: Electron Configurations and Sizes
Periodic Table Allows Us to not only
predict electron configurations, but
many trends including  Atomic size
 ion radius
 ionization energy
 electronegativity
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Section 8.4
Ions: Electron Configurations and Sizes
• Atomic size varies consistently through the
periodic table.
– Down a group, the atoms become larger
– Across a period, atoms become smaller
• What influences atomic size?
– The number of electrons in an atom
• How? Two opposing forces:
– principal quantum number, n
– the effective nuclear charge, Zeff
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Section 8.4
Atomic Radii
Ions: Electron Configurations and Sizes
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Section 8.4
Ions: Electron Configurations and Sizes
Principal Quantum Number, n
• As n increases, electrons are farther
from the nucleus, therefore atoms are
larger
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Section 8.4
Ions: Electron Configurations and Sizes
Effective Nuclear Charge
• The charge experienced by an electron
• Not the same as nuclear charge (Z; atomic number;
number of p+) because of the effect of the inner
electrons
– “Shielding”
– Outer electrons are attracted to the nucleus, but repelled
by the inner electrons that shield them from the nucleus
– Zeff is less than Z
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Section 8.4
Ions: Electron Configurations and Sizes
Effective Nuclear Charge
• Zeff for a given electron depends on:
– Distance of the electron from the nucleus
– Number of core electrons
• Zeff increases across a period
– Z increases across a period
– Shielding stays the same because only adding electrons to the
outer shell (no change to inner shell)
– Therefore, Zeff increases
• As Zeff increases, outer electrons are held more tightly,
therefore atoms are smaller.
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Section 8.4
Ions: Electron Configurations and Sizes
Atomic Radii
• Electrons are moving in atomic
orbitals
• Overlap of orbitals = sphere
• Radius of sphere = atomic radius
– Measurable quantity
– One-half distance between identical
adjacent nuclei
– Metal = metallic radius
– Nonmetal = covalent radius
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Section 8.4
Ions: Electron Configurations and Sizes
Ionic Radii
• Ionic radius = estimated size
of ion in a crystalline ionic compound
• Cations are always smaller than their neutral parent atoms
– Electron is removed
– Electron repulsions decrease
– Nucleus-electron interaction increases
– Electrons pulled closer to the nucleus and atom is smaller
• Anions are always larger than their neutral parent atoms
– Electron is added
– Election repulsions increase
– Electrons occupy more space and atom is larger
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Section 8.4
Ions: Electron Configurations and Sizes
Trends in Ionic Size
• Increases down a group
• Decreases across a period
• Decreases with increasing + charge (isoelectronic ions -a series
of ions/atoms containing the same number of electrons)
example:
N3- > O2- > F- > Na+ > Mg2+ > Al3+
• Decreases with increasing + charge (multiple ions of the same
element)
example:
Au3+ < Au+ < Au
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Section 8.4
Ions: Electron Configurations and Sizes
Ionic Radii
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10
Section 8.4
Atomic
Radii Configurations and Sizes
Ions: Electron
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Section 8.4
Ions: Electron Configurations and Sizes
Problem
• Just by looking at the periodic table, rank the
following atoms from largest to smallest: Cl,
Se, Br.
• Se > Br (Se is to the left in period 4)
• Br > Cl (Br is below Cl in group 17)
• From largest to smallest: Se > Br > Cl
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Section 8.4
Ions: Electron Configurations and Sizes
•
Ionization
Energy
(IE)
Minimum amount of energy required to remove an
electron from an isolated gaseous atom
– measure of an element’s ability to form positive ions
• First ionization energy (IE1): removes outermost eatom + IE1 → ion+ + e-
• Second ionization energy (IE2): removes next eIon+ + IE2 → ion2+ + e-
• IE2 > IE1
• Atoms with low IE1 tend to form cations
• Atoms with high IE1 tend to form anions
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Section 8.4
Ions: Electron Configurations and Sizes
•
Trends
in
Ionization
Energy
Generally increases as you go across a period
– Smaller atomic size, more difficult to remove e-
– Lowest = alkali metals
– Highest = noble gases
– Important exceptions at Be & Mg, N & P
• Generally decreases as you go down a group
– Larger atomic size, easier to remove e-
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Section 8.4
Ionization
Energy
Ions: Electron Configurations and Sizes
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Section 8.4
Ions: Electron Configurations and Sizes
•
Electron
Affinity
Energy change that occurs when an electron is added to an
isolated gaseous atom
– measure of an element’s ability to gain an electron (ability to form
an anion)
• Equation: atom + e- → ion-
(DE = Electron affinity)
• Electron affinity is usually negative
– DE < 0
– exothermic process
• Atoms with small negative EA tend to form cations
• Atoms with large negative EA tend to form anions
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Section 8.4
Ions: Electron Configurations and Sizes
Trends in Electron Affinity
• Generally become more negative across a period
– Smaller atomic size, more attraction of e- to nucleus
– Noble gases have EA > 0
– Important exceptions at Be & Mg, N & P
• No trends within groups
– Greater attraction of e- to nucleus with smaller atoms, but
greater e- repulsion as well
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Section 8.4
Ions: Electron Configurations and Sizes
Electron Affinity
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