Lecture 0802Trends on thePeriodic Table

PERIODIPERIODIC C

TRENDSTRENDS

PERIODIPERIODIC C

TRENDSTRENDS

Li

Na

K

Effective Nuclear Charge

Z*The 2s electron PENETRATES

the region occupied by the 1s electron.

2s electron experiences a higher positive charge than expected.

EffectiveEffective Nuclear Charge, Nuclear Charge, Z*Z*

Atom Z* e- in Valence Orbitalsest measured

Li 1 +1.28 Be 2 ------- B 3 +2.58 C 4 +3.22 N 5 +3.85 O 6 +4.49 F 7 +5.13

Increase in Increase in Z* across a Z* across a periodperiod

General Periodic Trends Atomic and ionic sizeAtomic and ionic size

Ionization energyIonization energy Electron affinityElectron affinity

Higher effective nuclear chargeElectrons held more tightly

Larger shells.Electrons held lesstightly.

Atomic Radius

Is taken as the covalent radius for non-metallic elements and as the metallic radius for metals

Atomic Radius Covalent radius is one-half

the distance between the nuclei of two identical atoms that are singly bonded to one another.

Chlorine Bond Length

Atomic Radius Covalent radii for elements

whose atoms do not bond to one another can be estimated by combining radii of those that do with the distances between unlike atoms in various molecules.

Atomic Radius

Metallic radius is one-half the closest internuclear distance in a metallic crystal.

Prediction!

0

50

100

150

200

1 3 5 7 9 11 13 15 17 19

Atomic Number

Ato

mic

Rad

ius

(pm

)

Atomic Size

Atomic Size

Size goes UP on going down a group.

Because electrons are added further from the nucleus, there is less attraction.

Size goes DOWN on going across a period.

Size goes UP on going down a group.

Because electrons are added further from the nucleus, there is less attraction.

Size goes DOWN on going across a period.

Atomic SizeAtomic SizeSize decreases across a period

owing to increase in Z*. Each added electron feels a greater and greater + charge.

LargeLarge SmallSmall

Trends in Atomic SizeTrends in Atomic Size

0

50

100

150

200

250

0 5 10 15 20 25 30 35 40

Li

Na

K

Kr

He

NeAr

2nd period

3rd period 1st transitionseries

Radius (pm)

Atomic Number

0

50

100

150

200

250

0 5 10 15 20 25 30 35 40

Li

Na

K

Kr

He

NeAr

2nd period

3rd period 1st transitionseries

Radius (pm)

Atomic Number

Sizes of Transition Elements

Sizes of Transition Elements

3d subshell is inside the 4s subshell.3d subshell is inside the 4s subshell. 4s electrons feel a more or less 4s electrons feel a more or less

constant Z*.constant Z*. Sizes stay about the same andSizes stay about the same and

chemistries are similar!chemistries are similar!

General Periodic Trends Atomic and ionic size

Ionization energy Electron affinity

Higher effective nuclear chargeElectrons held more tightly

Larger shells.Electrons held lesstightly.

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 when up or down when losing an electron to losing an electron to form a cation?form a cation?

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

Ion SizesIon Sizes

CATIONS are SMALLER than the atoms from which they come.

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

Li,152 pm3e and 3p

Li +, 78 pm2e and 3 p

+Forming a

cation.Forming a

cation.

Ion SizesIon Sizes

F,64 pm9e and 9p

F- , 136 pm10 e and 9 p

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

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

Ion SizesIon Sizes

ANIONS are LARGER than the atoms from which they come.

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

Forming an anion.

Forming an anion.F, 71 pm

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

-

Trends in Ion Sizes

Trends in Ion Sizes

Trends in ion sizes are the same as atom sizes.

Redox Reactions

Redox Reactions

Why do metals lose electrons in their reactions?

Why does Mg form Mg2+ ions and not Mg3+?

Why do nonmetals take on electrons?

Why do metals lose electrons in their reactions?

Why does Mg form Mg2+ ions and not Mg3+?

Why do nonmetals take on electrons?

Ionization Energy (General)

Is the energy required to remove the outermost electron from an atom or a positive ion in the ground state.

First Ionization Energy

Energy required to remove the first electron from a neutral atom in the gaseous state.

Ionization EnergyIonization Energy

Mg (g) + 738 kJ Mg+ (g) + e-

Prediction!

0

500

1000

1500

2000

2500

1 3 5 7 9 11 13 15 17 19

Atomic Number

1st

Ion

izat

ion

En

erg

y (

kJ/m

ol)

Trends in Ionization EnergyTrends in Ionization EnergyTrends in Ionization EnergyTrends in Ionization Energy

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 350

500

1000

1500

2000

2500

1st Ionization energy (kJ/mol)

Atomic NumberH Li Na K

HeNe

ArKr

Atomic RadiiAtomic Radii

Trends in Ionization Energy

Trends in Ionization Energy

IE increases across a period because Z* increases.

Metals lose electrons more easily than nonmetals.

Metals are good reducing agents.

Nonmetals lose electrons with difficulty.

Trends in Ionization Energy

Trends in Ionization Energy

IE decreases down a group

Because size increases. Reducing ability

generally increases down the periodic table.

Remember Li, Na, K

Second Ionization Energy

Energy needed to remove the outermost electron from a +1 ion.

Energy needed to remove the second electron from a neutral atom.

Ionization EnergyIonization EnergyMg (g) + 738 kJ Mg+ (g) + e-

MgMg+ + (g) + 1451 kJ (g) + 1451 kJ Mg Mg2+2+ (g) + e (g) + e-

Mg+ has 12 protons and only 11 electrons. Therefore, IE for Mg+ > Mg.

0

1000

2000

3000

4000

5000

6000

7000

8000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Atomic Number

2nd

Io

niz

atio

n E

ner

gy

(kJ/

mo

l)

Ionization EnergyIonization EnergyMg (g) + 735 kJ Mg+ (g) + e-

Mg+ (g) + 1451 kJ Mg2+ (g) + e-

MgMg2+2+ (g) + 7733 kJ (g) + 7733 kJ Mg Mg3+3+ (g) + e (g) + e--

Energy cost is very high to dip into a shell of lower n.

This is why ox. no. = Group no.

General Periodic Trends Atomic and ionic size

Ionization energy Electron affinity

Higher effective nuclear chargeElectrons held more tightly

Larger shells.Electrons held lesstightly.

Electron Affinity A few elements GAIN

electrons to form anions. E.A. is the energy released or

absorbed when an electron is added to the valence level of a gas-phase atom.

A(g) + e- A-(g) E.A. = ∆E

Prediction!

Trends in Electron AffinityTrends in Electron Affinity

Electron Affinity of Oxygen

∆E is EXOthermic because O has an

affinity for an e-.

[He] O atom

EA = - 141 kJ

+ electron

O [He] - ion

Electron Affinity of Nitrogen

∆E is zero for N- due to electron-electron repulsions.

EA = 0 kJ

[He] N atom

[He] N- ion

+ electron

See Figure 8.12 and Appendix F

Affinity for electron increases across a period (EA becomes more negative).

Affinity decreases down a group (EA becomes less negative).

Atom EAAtom EAFF -328 kJ-328 kJClCl -349 kJ-349 kJBrBr -325 kJ-325 kJII -295 kJ-295 kJ

Atom EAAtom EAFF -328 kJ-328 kJClCl -349 kJ-349 kJBrBr -325 kJ-325 kJII -295 kJ-295 kJ

Trends in Electron Affinity

General Periodic Trends Atomic and ionic size

Ionization energy Electron affinity

Higher effective nuclear chargeElectrons held more tightly

Larger shells.Electrons held lesstightly.