Periodic Relationships Among the Elements
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Transcript of Periodic Relationships Among the Elements
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
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
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
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
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 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.
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.