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Chapter 8

Sections 4 & 5

Ions

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Atomic Atomic SizeSize

Atomic Atomic SizeSize

• Size goes UPSize goes UP on going down a group. on going down a group. • Because electrons are added further Because electrons are added further

from the nucleus, there is less from the nucleus, there is less attraction. This is due to additional attraction. This is due to additional energy levels and the energy levels and the shielding effectshielding effect. . Each additional energy level “shields” Each additional energy level “shields” the electrons from being pulled in the electrons from being pulled in toward the nucleus.toward the nucleus.• Size goes DOWNSize goes DOWN on going across a on going across a

period.period.

• Size goes UPSize goes UP on going down a group. on going down a group. • Because electrons are added further Because electrons are added further

from the nucleus, there is less from the nucleus, there is less attraction. This is due to additional attraction. This is due to additional energy levels and the energy levels and the shielding effectshielding effect. . Each additional energy level “shields” Each additional energy level “shields” the electrons from being pulled in the electrons from being pulled in toward the nucleus.toward the nucleus.• Size goes DOWNSize goes DOWN on going across a on going across a

period.period.

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Atomic SizeAtomic SizeAtomic SizeAtomic Size

Size Size decreasesdecreases across a period across a period owing to increase in the positive owing to increase in the positive charge from the protons. Each added charge from the protons. Each added electron feels a greater and greater + electron feels a greater and greater + charge because the protons are pulling charge because the protons are pulling in the same direction, where the in the same direction, where the electrons are scattered.electrons are scattered.

LargeLarge SmallSmall

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Which is Bigger?

•Na or K ?

•Na or Mg ?

•Al or I ?

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Which is Bigger?

•Na or K ? K

•Na or Mg ? Na

•Al or I ? I

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Ion SizesIon SizesIon SizesIon Sizes

Li,152 pm3e and 3p

Li+, 60 pm2e and 3 p

+Does the size goDoes the size goup or down up or down when losing an when losing an electron to form electron to form a cation?a cation?

Does the size goDoes the size goup or down up or down when losing an when losing an electron to form electron to form a cation?a cation?

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Ion SizesIon SizesIon SizesIon Sizes

• CATIONSCATIONS are are SMALLERSMALLER than the than the atoms from which they come.atoms from which they come.

• The electron/proton attraction has The electron/proton attraction has gone UP and so size gone UP and so size DECREASESDECREASES..

Li,152 pm3e and 3p

Li +, 78 pm2e and 3 p

+Forming Forming a cation.a cation.Forming Forming a cation.a cation.

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Ion SizesIon SizesIon SizesIon Sizes

F,64 pm9e and 9p

F- , 136 pm10 e and 9 p

-Does the size go up or Does the size go up or down when gaining an down when gaining an electron to form an electron to form an anion?anion?

Does the size go up or Does the size go up or down when gaining an down when gaining an electron to form an electron to form an anion?anion?

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Ion SizesIon SizesIon SizesIon Sizes

• ANIONSANIONS are are LARGERLARGER than the atoms from than the atoms from which they come.which they come.

• The electron/proton attraction has gone DOWN The electron/proton attraction has gone DOWN and so size and so size INCREASESINCREASES..

• Trends in ion sizes are the same as atom sizes. Trends in ion sizes are the same as atom sizes.

Forming Forming an anion.an anion.Forming Forming an anion.an anion.F, 71 pm

9e and 9pF-, 133 pm10 e and 9 p

-

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Trends in Ion SizesTrends in Ion SizesTrends in Ion SizesTrends in Ion Sizes

Figure 8.13Figure 8.13

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Which is Bigger?

•Cl or Cl- ?

•K+ or K ?

•Ca or Ca+2 ?

• I- or Br- ?

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Which is Bigger?

•Cl or Cl- ? Cl-

•K+ or K ? K

•Ca or Ca+2 ? Ca

• I- or Br- ? I-

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All you have to do is add or subtract All you have to do is add or subtract electrons from the atom’s configuration!electrons from the atom’s configuration!

Electron ConfigurationsElectron ConfigurationsElectron ConfigurationsElectron Configurations

Mg

Mg2+

O

O2-

14Trends in Ionization Trends in Ionization EnergyEnergy

Trends in Ionization Trends in Ionization EnergyEnergy

• IE increases across a IE increases across a period because the period because the positive charge increases.positive charge increases.

• Metals lose electrons Metals lose electrons more easily than more easily than nonmetals.nonmetals.

• Nonmetals lose electrons Nonmetals lose electrons with difficulty (they like to with difficulty (they like to GAIN electrons).GAIN electrons).

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Trends in Ionization Trends in Ionization EnergyEnergy

Trends in Ionization Trends in Ionization EnergyEnergy

• IE decreases down a IE decreases down a group group

• Because size increases Because size increases (Shielding Effect)(Shielding Effect)

• Reducing ability generally Reducing ability generally increases down the increases down the periodic table. periodic table.

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Which has a higher 1st ionization energy?

•Mg or Ca ?

•Al or S ?

•Cs or Ba ?

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Which has a higher 1st ionization energy?

•Mg or Ca ? Mg

•Al or S ? S

•Cs or Ba ? Ba

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Electronegativity, Electronegativity,

is a measure of the ability of an atom is a measure of the ability of an atom in a molecule to attract electrons to in a molecule to attract electrons to itself.itself.

Concept proposed byConcept proposed byLinus PaulingLinus Pauling1901-19941901-1994

Concept proposed byConcept proposed byLinus PaulingLinus Pauling1901-19941901-1994

19Periodic Trends: Electronegativity

• In a group: Atoms with fewer energy levels can attract electrons better (less shielding). So, electronegativity decreases DOWN a group of elements.

• In a period: More protons, while the energy levels are the same, means atoms can better attract electrons. So, electronegativity increases RIGHT in a period of elements.

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ElectronegativityElectronegativity

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Energy Effects

• The change in energy when separated gaseous ions are packed together to form an ionic solid.

• M+(g) + X-(g) MX(s)

• Lattice energy is negative (exothermic) from the point of view of the system.

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Lattice Energy

• To determine which compound will have the highest lattice energy, take into consideration the following:– The size of the ions in the compound

» The smaller the size, the greater the lattice energy

– The charge of the ions in the compound

» The greater the charge, the greater the lattice energy

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Calculating ∆Hf• We can take advantage of the fact the energy

is a state function and break the reaction into steps, the sum of which is the overall reaction.

• Let’s do #41 Na(s) + ½ Cl2 (g) NaCl(s)

Given the following:

Lattice energy -786 kJ/mol

Ionization energy for Na 495 kJ/mol

Electron affinity for Cl -349 kJ/mol

Bond energy of Cl2 239 kJ/mol

Enthalpy sublimation for Na 109 kJ/mol

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Process

Step 1: Sublimation of Na

Na(s) Na(g) 109 kJ/mol

Step 2: Ionization of Na

Na (g) Na+ (g) + e- 495 kJ/mol

Step 3: Dissociation of Cl2

½ Cl2 (g) Cl(g) 119.5 kJ/mol

Step 4: Formation of Cl-

Cl (g) + e- Cl-(g) -349 kJ/mol

Step 5: Formation of NaCl

Na+(g) + Cl-(g) NaCl(s) -786 kJ/molNa(s) + ½ Cl2 (g) NaCl(s) -411.5 kJ/mol