Chapter 4: The Periodic Table Section 3: Trends in the Periodic Table

Chapter 4: The Periodic Chapter 4: The Periodic TableTable

Section 3: Trends in the Periodic Section 3: Trends in the Periodic TableTable

Key TermsKey Terms Ionization energyIonization energy – removes an – removes an

electron from an atom or ionelectron from an atom or ion Electron shieldingElectron shielding – when inner – when inner

electrons cancel some of the + charge electrons cancel some of the + charge of the nucleus & lessen its attraction of the nucleus & lessen its attraction of outermost electronsof outermost electrons

Trend – predictable change in a Trend – predictable change in a particular directionparticular direction

Key Terms 2Key Terms 2

Atomic radiusAtomic radius – depends on volume – depends on volume occupied by atom’s electron cloudoccupied by atom’s electron cloud

Bond radiusBond radius – half the distance – half the distance between nuclei of atoms that are between nuclei of atoms that are bonded togetherbonded together

ElectronegativityElectronegativity – measure of how – measure of how much an atom in a chemical com-much an atom in a chemical com-pound can attract electronspound can attract electrons

Key Terms 3Key Terms 3

Electron affinityElectron affinity – change in – change in energy when a neutral atom gains energy when a neutral atom gains an electron an electron

There are no existing bonds as There are no existing bonds as with electronegativity.with electronegativity.

Things To Know/AnswerThings To Know/Answer

What are the periodic trends in What are the periodic trends in ionization energyionization energy, & how are they , & how are they affected by atomic structures?affected by atomic structures?

Answer these questions for Answer these questions for atomic atomic radiusradius, , electronegativityelectronegativity, , ionic sizeionic size, , electron affinityelectron affinity, , melting pointsmelting points (mp) (mp) & & boiling pointsboiling points (bp). (bp).

Periodic TrendsPeriodic Trends

All in a group can be explained All in a group can be explained by electron configurations.by electron configurations.

Reactivity in alkali metals rise Reactivity in alkali metals rise from top to bottom in group 1.from top to bottom in group 1.

Periodic Trends 2Periodic Trends 2

When one adds enough energy to When one adds enough energy to overcome the attraction between overcome the attraction between protons & electrons in an atom, it protons & electrons in an atom, it becomes charged (ion) and an becomes charged (ion) and an electron escapes. See page 133 electron escapes. See page 133 Figure 16.Figure 16.

A + ionization energy A + ionization energy →→ A A++ + e + e--

Ionization EnergyIonization Energy

Ionization energyIonization energy decreases down- decreases down-ward in a group because the number ward in a group because the number of energy levels increases downward.of energy levels increases downward.

Increasing energy levels have Increasing energy levels have increasing distance from the nucleus increasing distance from the nucleus where the positive charge that attracts where the positive charge that attracts ee-- in the atom is. in the atom is.

Ionization Energy 2Ionization Energy 2

The farther an eThe farther an e-- is from the nu- is from the nu-cleus, the less attraction protons cleus, the less attraction protons there have on the ethere have on the e--..

Also, the higher an Also, the higher an ee--’s’s energy energy level is the more full levels of level is the more full levels of ee--’s’s there are between it & the nucleus.there are between it & the nucleus.


These eThese e--’s “in the middle” reduce the ’s “in the middle” reduce the positive attraction that extends from positive attraction that extends from the nucleus through the atom the nucleus through the atom ((electron shieldingelectron shielding).).

Outermost electrons are less tightly Outermost electrons are less tightly held for this reason.held for this reason.


Ionization energyIonization energy increases as increases as you move across a period. See you move across a period. See Figure 17 on page 134.Figure 17 on page 134.

This is because protons increase 1 This is because protons increase 1 at a time rightward on a row, but at a time rightward on a row, but the energy level stays the same. the energy level stays the same.

Ionization Energy 5 / Atomic RadiusIonization Energy 5 / Atomic Radius

Increasing eIncreasing e--’s get crowded & repel ’s get crowded & repel each other; this counteracts the each other; this counteracts the positive attrac-tion of the nucleus.positive attrac-tion of the nucleus.

Here, electrons can only get so close Here, electrons can only get so close before increased positive attraction before increased positive attraction can-not overcome ecan-not overcome e-- to e to e- - repulsion.repulsion.

For this reason, For this reason, atomic radiusatomic radius stops stops decreasing rightward in a row.decreasing rightward in a row.

Atomic Radius 2Atomic Radius 2

Increasing atomic # across a row pro-Increasing atomic # across a row pro-duces a much bigger rise in positive duces a much bigger rise in positive attraction than rise in distance of eattraction than rise in distance of e--’s ’s from the nucleus.from the nucleus.

This pattern also causes decreased This pattern also causes decreased atomic radius left to right on a row.atomic radius left to right on a row.


Rising + attraction also pulls Rising + attraction also pulls electrons closer to the nucleus. electrons closer to the nucleus.

Added inner energy levels are Added inner energy levels are present downward in groups & present downward in groups & add distance from the nucleus.add distance from the nucleus.


Electron shielding also blocks Electron shielding also blocks rises in + attraction & yields rises in + attraction & yields similar attraction down the group.similar attraction down the group.

These effects cause increasing These effects cause increasing atomic radius downward in a atomic radius downward in a group.group.

ElectronegativityElectronegativity

Electronegativity is relative attraction Electronegativity is relative attraction of electrons by nuclei in bonded of electrons by nuclei in bonded atoms where they “play tug of war” atoms where they “play tug of war” with their shared electrons.with their shared electrons.

Linus Pauling, one of America’s Linus Pauling, one of America’s most famous chemists, made a scale most famous chemists, made a scale of electronegativity values.of electronegativity values.

Electronegativity 2Electronegativity 2

In the scale, he assigned F 4.0 since it In the scale, he assigned F 4.0 since it attracts electrons in bonds most then attracts electrons in bonds most then Pauling calculated values for other Pauling calculated values for other elements relative to this one.elements relative to this one.

ElectronegativityElectronegativity decreases down a decreases down a group mostly because higher energy group mostly because higher energy levels are farther from the nucleus.levels are farther from the nucleus.


Nuclei cannot attract valence Nuclei cannot attract valence electrons in these distant energy electrons in these distant energy levels well.levels well.

For this reason, an element like Cs For this reason, an element like Cs has a nucleus w/ more protons but has a nucleus w/ more protons but weak attraction of a valence electron weak attraction of a valence electron on its 6th energy level.on its 6th energy level.


However, an element like Li attracts However, an element like Li attracts a valence electron on its 3a valence electron on its 3rdrd energy energy level more strongly.level more strongly.

This makes Li more electronegative This makes Li more electronegative than Cs.than Cs.

Electronegativity increases sharply Electronegativity increases sharply rightward across a period.rightward across a period.


This trend arises because no change This trend arises because no change in electron shielding happens across a in electron shielding happens across a row since no electrons get added to row since no electrons get added to inner energy levels.inner energy levels.

As atomic # rises quickly, nuclear As atomic # rises quickly, nuclear charge does also and can attract bond charge does also and can attract bond electrons much more strongly.electrons much more strongly.


Adding inner electrons downward Adding inner electrons downward in a group increases electron in a group increases electron shielding.shielding.

This keeps effective nuclear This keeps effective nuclear charge mostly the same.charge mostly the same.

Electronegativity 7Electronegativity 7 Slight drops in electronegativity Slight drops in electronegativity

result.result. This is b/c distance from the nucleus This is b/c distance from the nucleus

is the key factor not nuclear charge. is the key factor not nuclear charge. Slight rises in distance from the Slight rises in distance from the

nucleus downward in a group have nucleus downward in a group have much less effect than boosts in nu-much less effect than boosts in nu-clear charge rightward across a row.clear charge rightward across a row.

Other Periodic TrendsOther Periodic Trends

Effective nuclear charge & electron Effective nuclear charge & electron shielding explain most periodic trends shielding explain most periodic trends including ionic size & electron including ionic size & electron affinity.affinity.

Ionic size follows trends of atomic Ionic size follows trends of atomic radii for the same reasons.radii for the same reasons.

Other Periodic Trends 2Other Periodic Trends 2

Metals tend to lose one or more Metals tend to lose one or more electrons & become cations (+ions); electrons & become cations (+ions); whereas, nonmetals tend to gain ewhereas, nonmetals tend to gain e--’s ’s & form anions (-ions).& form anions (-ions).

Electron affinity follows electronega-Electron affinity follows electronega-tivity trends (decrease down a group tivity trends (decrease down a group but increase right across a series) for but increase right across a series) for the same reasons.the same reasons.


Mp & bp do not generally rise or fall Mp & bp do not generally rise or fall but reach 2 different peaks as but reach 2 different peaks as dd & & pp orbitals fill.orbitals fill.

For example, Cs has low mp & bp For example, Cs has low mp & bp b/c it only has 1 valence electron for b/c it only has 1 valence electron for bonding; it is far left in 6th period.bonding; it is far left in 6th period.


As the electron # increases across a As the electron # increases across a row, more bonds can form & require row, more bonds can form & require more energy to break.more energy to break.

This effect peaks near the middle of This effect peaks near the middle of dd-block elements at W and Re b/c the -block elements at W and Re b/c the dd orbitals are half filled. orbitals are half filled.


Further eFurther e--’s pair in ’s pair in dd orbitals beyond orbitals beyond W & decrease the # of unpaired eW & decrease the # of unpaired e--’s ’s that help strengthen bonds between that help strengthen bonds between atoms by forming multiple bonds.atoms by forming multiple bonds.

More rightward, Hg & Rn have much More rightward, Hg & Rn have much lower mp & bp b/c lower mp & bp b/c dd orbitals are full. orbitals are full.

Other Periodic Trends 6Other Periodic Trends 6 Past Hg, mp & bp rise again as elec-Past Hg, mp & bp rise again as elec-

trons start filling trons start filling pp orbitals until these orbitals until these are half filled. are half filled.

Beyond half filled status, mp & bp Beyond half filled status, mp & bp drop again b/c the drop again b/c the pp orbitals get full orbitals get full and unable to help strengthen bonds.and unable to help strengthen bonds.

By Rn, By Rn, pp orbitals are full also so mp orbitals are full also so mp & bp are unusually low.& bp are unusually low.

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Chapter 4: The Periodic Table Section 3: Trends in the Periodic Table