Transcript Effective Nuclear Charge

Periodicity 1
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Classification of the Elements
Elements in the same group tend to have similar chemical and physical properties.
There is a change in chemical and physical properties across a period. The
repeating pattern of these properties shown by different periods is known as
periodicity.
General Periodic Trends
•
•
•
•
Atomic and ionic size
Ionization energy
Electronegativity
Electron Affinity
Higher effective nuclear charge
Electrons held more tightly
Larger orbitals.
Electrons held less
tightly.
Shielding Effect!
Effective nuclear charge (Zeff) is the “positive charge” felt
by an electron.
Z
Core
Zeff
Radius
Na
11
10
1
186
Mg
12
10
2
160
Al
13
10
3
143
Si
14
10
4
132
8.3
Effective Nuclear Charge (Zeff)
increasing Zeff
increasing Zeff
8.3
8.3
8.3
Atomic Radii
8.3
Atomic Size
• Size goes UP on going down a group.
• Because electrons are added further from the
nucleus, there is less attraction. This is due to
additional energy levels and the shielding
effect. Each additional energy level “shields”
the electrons from being pulled in toward the
nucleus.
Atomic Size
Size decreases across a period owing to
increase in the effective nuclear charge.
Large
Small
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
Melting Points
•
Melting points depends on the structure and the forces that hold the atoms
together.
•
Metallic bonding is strong, so the melting temperature is high. The strength of
the bond depends on the number of electrons each atom contributes to the
delocalised electrons. The greater the amount of electrons contributed, the
stronger the bond therefore the higher the melting point.
• Group 4 elements like carbon and silicon form giant covalent structures.
What this means is there are lots of bond to break before melting. So as a
result the melting temperature is very high in comparison simple covalent
molecules.
Ionization Energy
The first ionization energy is the energy required to remove one electron from
an atom in its gaseous state, measured in kJ mol-1
http://ibchem.com/IB/ibnotes/full/ato_htm/12.2.htm
http://ibchem.com/IB/ibnotes/brief/ato-hl.htm#lew
X(g) + I1 => X +(g) + eX1+(g) + I2 => X2+(g) + e-
X 2+ + I3 => X3+(g) + eI1 < I2 < I3
8.4
Factors that affect IE
1. Atomic radius
As the distance of the outer electrons from the nucleus increases,
the attraction of the positive nucleus for the negatively
charged electrons falls.
2. Nuclear Charge
When the nuclear charge becomes more positive due to more
protons, its attraction on all electrons increases causing IE to
increase.
3. Shielding Effect
The valence electrons are repelled by all the other electrons in the
atom. They are shielded from the attraction of the nucleus by
shielding effect.
Filled n=1 shell
Filled n=2 shell
Filled n=3 shell
Filled n=4 shell
Filled n=5 shell
8.4
Evidence for Sublevels
General Trend in First Ionization Energies
Increasing First Ionization Energy
Increasing First Ionization Energy
8.4
Discrepancies
B
1s2 2s22p1
Be
1s2 2s2
The first decrease in the period is the result of a change in the sublevel
from which the electron is lost and a change in electron shielding.
Although 2s and 2p are on the same energy level, the energy difference is
large.
A single 2p is more shielded by inner electrons than the 2s
Similar for Mg and Al
• The second decrease occurs between nitrogen and
oxygen.
N
1s22s22p3
O
1s2 2s22p4
N+
1s22s2 2p2
O+
1s2 2s2 2p3
The two electrons in the 2p sublevel of oxygen, experience
The IE drops.
severe repulsion.
Examples IE
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)
F (g) + e-
F-(g)
DH = -328 kJ/mol
EA = +328 kJ/mol
O (g) + e-
O-(g)
DH = -141 kJ/mol
EA = +141 kJ/mol
8.5
8.5
8.5
Group 1A Elements (ns1, n  2)
M+1 + 1e-
2M(s) + 2H2O(l)
4M(s) + O2(g)
2MOH(aq) + H2(g)
2M2O(s)
Increasing reactivity
M
8.6
Group 2A Elements (ns2, n  2)
M+2 + 2e-
Be(s) + 2H2O(l)
Mg(s) + 2H2O(g)
M(s) + 2H2O(l)
No Reaction
Mg(OH)2(aq) + H2(g)
M(OH)2(aq) + H2(g) M = Ca, Sr, or Ba
Increasing reactivity
M
8.6
Group 7A Elements (ns2np5, n  2)
X2(g) + H2(g)
X-1
2HX(g)
Increasing reactivity
X + 1e-
8.6
Group 8A Elements (ns2np6, n  2)
Completely filled ns and np subshells.
Highest ionization energy of all elements.
No tendency to accept extra electrons.
8.6
Properties of Oxides Across a Period
basic
acidic
8.6
Electronegativity, EN
EN is a measure of
the ability of an
atom in a molecule
to attract electrons
to itself.
Concept proposed by
Linus Pauling
1901-1994
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.
Electronegativity