7.3 Sizes of Atoms and Ions

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7.3 Sizes of Atoms and Ions

• By Meredith Veit, Molly Spanfelner, Julie Gaul
  Julia Shaner

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What will you learn in this section?

• Sizes of Atoms
• Apparent Radii (Nonbonding)
• Bonding Radii
• Periodic Trends in Radii Size
• Trends in Ionic Size

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Sizes of Atoms

• Definite size cannot be measured
• Exact location of electrons cannot be determined
• Edges of atoms and ions appear fuzzy

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Apparent Radii (Nonbonding)

• Apparent Radii distance from the nucleus to the
  outermost electrons
• Gives a rough estimate of atomic size
• Also called the Nonbonding Radius

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Bonding Radii

• Chemically bonded atoms are closer together than
  atoms during a collision
• Bonding Radii distance separating the nuclei
  during a chemical bond
• Shorter than the nonbonding radius
• Also called covalent radius
• Bond Length Radius of Element 1 Radius of
  Element 2

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Periodic Trends in Atomic Radii
Radius Increases Top to Bottom
Radius Increases Right to Left
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Periodic Trends Cont.

• Top to Bottom
• Increase in size
• Increase in Principal Quantum Number (n) of outer
  electrons
• Electrons are farther from nucleus
• Right to Left
• Increase in size
• Decrease in effective nuclear charge (the net
  positive charge experienced by an electron in a
  many electron atom)
• Zeff Protons Nonvalence Electrons
• More protons create a greater pull on the
  electrons, bringing them closer to the nucleus
  and making the atom smaller (and vice versa)

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Trends in Ionic Size

• Depends on
• Nuclear charge
• Number of electrons it possesses
• Orbitals in which outer shell electrons reside
• Isoelectronic Series- Series of atoms, ions, and
  molecules having the same number of electrons
• Cations are smaller than their parent atoms
• Anions are larger than their parent atoms

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Example of Isoelectronic Series

• Example
• O-2
• F-
• Na
• Mg2
• Al3
• All have 10 electrons
• Radius decreases as nuclear charge increases
• Electrons are strongly attracted to the nucleus