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Periodic Trends

OBJECTIVES:

Interpret group trends in atomic

radii, ionic radii, ionization

energies, m.p., b.p.,

electronegativity and chemical

properties

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Trends in Atomic Size

First problem: Where do youstart measuring from?

The electron cloud doesnt havea definite edge.

They get around this by

measuring more than 1 atom at atime.

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Atomic Size

Atomic Radius = half the distance betweentwo nuclei of a diatomic molecule.

}

Radius

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Trends in Atomic Size

Influenced by three factors:1. Energy Level

Higher energy level is further

away.

2. Charge on nucleus

More charge pulls electrons incloser.

3. Shielding effect e e repulsion

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Group trends

As we go downa group...

each atom hasanother energylevel,

so the atomsget bigger.

HLi

Na

K

Rb

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Periodic TrendsAs you go across a period, the

radius gets smaller.

Electrons are in same energy level.

More nuclear charge.Outermost electrons are closer.

Na Mg Al Si P S Cl Ar

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Overall

Atomic Number

AtomicRadius(nm)

H

Li

Ne

Ar

10

Na

K

Kr

Rb

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Trends in Ionization Energy

The amount of energy requiredto completely remove a mole ofelectrons from a mole of

gaseous atoms.Removing an electron makes a

+1 ion.

The energy required to remove (1mole of) the first electron iscalled the first ionization energy.

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Ionization Energy

The second ionization energy isthe energy required to remove (1mole of) the second electron(s).

Always greater than first IE.

The third IE is the energy

required to remove a thirdelectron.

Greater than 1st or 2nd IE.

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Symbol First Second Third

HHe

Li

BeB

C

N

OF

Ne

13122731

520

900

800

1086

1402

13141681

2080

5247

7297

1757

2430

2352

2857

33913375

3963

11810

14840

3569

4619

4577

53016045

6276

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Symbol First Second Third

HHe

Li

BeB

C

N

OF

Ne

13122731

520

900

800

1086

1402

13141681

2080

5247

7297

1757

2430

2352

2857

33913375

3963

11810

14840

3569

4619

4577

53016045

6276

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What determines IE

The greater the nuclear charge,the greater IE.

Greater distance from nucleusdecreases IE

Filled and half-filled orbitals have

lower energy, so achieving themis easier, lower IE.

Shielding effect

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ShieldingThe electron in the

outermost energylevel experiences

more inter-electronrepulsion (shielding).

Second electron has

same shielding, if itis in the same period

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Periodic trends

All the atoms in the same periodhave the same energy level.

Same shielding.But, increasing nuclear charge

So IE generally increases from

left to right.Exceptions at full and 1/2 full

orbitals.

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FirstIonizationenergy

Atomic number

He

He has a greater IEthan H.

same shielding

greater nuclearcharge

H

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Atomic number

H

He

Li has lower IEthan H

Outer electron

further away outweighs greater

nuclear charge

Li

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Atomic number

H

He

Be has higher IEthan Li

same shielding


Li

Be

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Atomic number

H

HeB has lower IE

than Be

same shielding


p orbital is slightly

more diffuse and itselectron easier to

remove

Li

Be

B

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Atomic number

H

He

Li

Be

B

C

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Atomic number

H

He

Li

Be

B

C

N

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Atomic number

H

He

Li

Be

B

C

N

O

Breaks thepattern, becausethe outer electronis paired in a p

orbital andexperiences inter-electron

repulsion.

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Atomic number

H

He

Li

Be

B

C

N

O

F

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Atomic number

H

He

Li

Be

B

C

N

O

F

Ne

Ne has a lower

IE than He

Both are full,

Ne has moreshielding

Greater

distance

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FirstIoniza

tionenergy

Atomic number

H

He

Li

Be

B

C

N

O

F

Ne

Na has a lower

IE than Li

Both are s1

Na has moreshielding

Greater

distanceNa

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FirstIoniza

tionenergy

Atomic number

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Driving Force

Full Energy Levels require lots ofenergy to remove their electrons.

Noble Gases have full orbitals.

Atoms behave in ways toachieve noble gas configuration.

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Trends in Electron Affinity

The energy change associated withadding an electron (mole ofelectrons) to a (mole of) gaseousatom(s).

Easiest to add to group 7A.

Gets them to full energy level.

Increase from left to right: atomsbecome smaller, with greater nuclearcharge.

Decrease as we go down a group.

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Ionic size

Anions form by gainingelectrons.

Anions are bigger that the atomthey come from.

Nonmetals form anions.

Anions of A groups elementshave noble gas configuration.

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Configuration of Ions

Ions have noble gas configurations(not transition metals).

Na is: 1s22s22p63s1

Forms a 1+ ion: 1s22s22p6

Same configuration as neon.

Metals form ions with theconfiguration of the noble gasbefore them - they lose electrons.

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Group trends

Adding energy level

Ions get bigger asyou go down.

Li1+

Na1+

K1+

Rb1+

Cs1+

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Periodic Trends

Across the period, nuclearcharge increases so they getsmaller.

Energy level changes betweenanions and cations.

Li1+

Be2+

B3+

C4+

N3-

O2- F1-

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Size of Isoelectronic ions

Iso- means the same

Iso electronic ions have thesame # of electrons

Al3+ Mg2+ Na1+ Ne F1- O2- and N3-

all have 10 electrons

all have the configuration:1s22s22p6

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Size of Isoelectronic ions

Positive ions that have moreprotons would be smaller.

Al3+

Mg2+

Na1+Ne F

1- O2- N

3-

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Group Trend

The further down a group, thefarther the electron is away, andthe more electrons an atom has.

More willing to share.

Low electronegativity.

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Ionization energy, Electronegativity,

and Electron Affinity INCREASE

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Atomic size increases,

shielding constant

Ionic size increases

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