Periodic Relationships Among the Elements

Chapter 8

General Periodic TrendsGeneral Periodic Trends• Atomic and ionic sizeAtomic and ionic size• Ionization energyIonization energy• ElectronegativityElectronegativity• Electron AffinityElectron Affinity

Higher effective nuclear chargeElectrons held more tightly

Larger orbitals.Electrons held lesstightly.Shielding Effect!

Effective nuclear charge (Zeff) is the “positive charge” felt by an electron.

Na

Mg

Al

Si

11

12

13

14

10

10

10

10

1

2

3

4

186

160

143

132

ZeffCoreZ Radius

Zeff Z – number of inner or core electrons

8.3

Effective Nuclear Charge (Zeff)

8.3

increasing Zeff

incr

eas

ing

Ze

ff

8.3

8.3

Atomic Radii

8.3

Atomic SizeAtomic SizeAtomic SizeAtomic Size

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

nucleus, there is less attraction. This is due to nucleus, there is less attraction. This is due to additional energy levels and the additional energy levels and the shielding effectshielding effect. . Each additional energy level “shields” the Each additional energy level “shields” the electrons from being pulled in toward the nucleus.electrons from being pulled in toward the nucleus.

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

nucleus, there is less attraction. This is due to nucleus, there is less attraction. This is due to additional energy levels and the additional energy levels and the shielding effectshielding effect. . Each additional energy level “shields” the Each additional energy level “shields” the electrons from being pulled in toward the nucleus.electrons from being pulled in toward the nucleus.

Atomic SizeAtomic SizeAtomic SizeAtomic SizeSize Size decreasesdecreases across a period owing to across a period owing to

increase in the effective nuclear charge.increase in the effective nuclear charge.

LargeLarge SmallSmall

Cation is always smaller than atom from which it is formed.Anion is always larger than atom from which it is formed.

8.3

8.3

8.3

Ionization energy is the minimum energy (kJ/mol) required to remove an electron from a gaseous atom in its ground state.

I1 + X (g) X+

(g) + e-

I2 + X+ (g) X2+(g) + e-

I3 + X2+(g) X3+

(g) + e-

I1 first ionization energy

I2 second ionization energy

I3 third ionization energy

8.4

I1 < I2 < I3

8.4

Filled n=1 shell

Filled n=2 shell

Filled n=3 shellFilled n=4 shell

Filled n=5 shell

8.4

General Trend in First Ionization Energies

8.4

Increasing First Ionization Energy

Incr

easi

ng F

irst

Ioni

zatio

n E

nerg

y

Electron affinity is the negative of the energy change that occurs when an electron is accepted by an atom in the gaseous state to form an anion.

X (g) + e- X-(g)

8.5

F (g) + e- F-(g)

O (g) + e- O-(g)

H = -328 kJ/mol EA = +328 kJ/mol

H = -141 kJ/mol EA = +141 kJ/mol

8.5

8.5

Group 1A Elements (ns1, n 2)

M M+1 + 1e-

2M(s) + 2H2O(l) 2MOH(aq) + H2(g)

4M(s) + O2(g) 2M2O(s)

Incr

easi

ng r

eact

ivity

8.6

Group 7A Elements (ns2np5, n 2)

X + 1e- X-1

X2(g) + H2(g) 2HX(g)

Incr

easi

ng r

eact

ivity

8.6

Group 8A Elements (ns2np6, n 2)

8.6

Completely filled ns and np subshells. Highest ionization energy of all elements.No tendency to accept extra electrons.

Properties of Oxides Across a Period

basic acidic

8.6

More basic

Less basic More acidicLess acidic

Electronegativity, Electronegativity, is a measure of the ability of an is a measure of the ability of an

atom in a molecule to attract atom in a molecule to attract electrons to itself.electrons to itself.

Concept proposed byConcept proposed byLinus PaulingLinus Pauling1901-19941901-1994

Concept proposed byConcept proposed byLinus PaulingLinus Pauling1901-19941901-1994

Periodic Trends: Electronegativity

• In a group: Atoms with fewer energy levels can attract electrons better (less shielding). So, electronegativity increases UP 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.

ElectronegativityElectronegativity