Section 8.4 Ions: Electron Configurations and Sizes Return to TOC Periodic Table Allows Us to not only predict electron configurations, but many trends.

Section 8.4

Ions: Electron Configurations and Sizes

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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

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• 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

Section 8.4

Ions: Electron Configurations and Sizes

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Atomic Radii

Section 8.4

Ions: Electron Configurations and Sizes

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Principal Quantum Number, n• As n increases, electrons are farther

from the nucleus, therefore atoms are larger

Section 8.4

Ions: Electron Configurations and Sizes

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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

Section 8.4

Ions: Electron Configurations and Sizes

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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.

Section 8.4

Ions: Electron Configurations and Sizes

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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

Section 8.4

Ions: Electron Configurations and Sizes

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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

Section 8.4

Ions: Electron Configurations and Sizes

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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

Section 8.4

Ions: Electron Configurations and Sizes

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Ionic Radii

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Section 8.4

Ions: Electron Configurations and Sizes

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Atomic Radii

Section 8.4

Ions: Electron Configurations and Sizes

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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

Section 8.4

Ions: Electron Configurations and Sizes

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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 e-

atom + IE1 → ion+ + e-

• Second ionization energy (IE2): removes next e-

Ion+ + IE2 → ion2+ + e-

• IE2 > IE1

• Atoms with low IE1 tend to form cations

• Atoms with high IE1 tend to form anions

Section 8.4

Ions: Electron Configurations and Sizes

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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-

Section 8.4

Ions: Electron Configurations and Sizes

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Ionization Energy

Section 8.4

Ions: Electron Configurations and Sizes

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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- (E = Electron affinity)

• Electron affinity is usually negative– E < 0

– exothermic process

• Atoms with small negative EA tend to form cations

• Atoms with large negative EA tend to form anions

Section 8.4

Ions: Electron Configurations and Sizes

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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

Section 8.4

Ions: Electron Configurations and Sizes

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Electron Affinity