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Title: CHEMICAL PERIODICITY
Description: In the context of chemistry and the periodic table, periodicity refers to trends or recurring variations in element properties with increasing atomic number. Periodicity is caused by regular and predictable variations in element atomic structure.
Description: In the context of chemistry and the periodic table, periodicity refers to trends or recurring variations in element properties with increasing atomic number. Periodicity is caused by regular and predictable variations in element atomic structure.
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CHAPTER 5 β CHEMICAL PERIODICITY
CHAPTER GOALS
1
...
Atomic Radii
3
...
Electron Affinity
5
...
Electronegativity
Chemical Reactions and Periodicity
7
...
Oxygen & the Oxides
Oxygen and Ozone
Reactions of Oxygen and the Oxides
Combustion Reactions
Combustion of Fossil Fuels and Air Pollution
THE PERIODIC TABLE
-
-
The Periodic Table is one of the first things a
student of Chemistry encounters
...
Scientists
consider
it
an
indispensable reference
...
At an 1866 meeting of the
Chemical Society at Burlington House, England,
J
...
R
...
LAW OF OCTAVES
It stated that when the known elements were listed by
increasing atomic weights, those that were eight places
apart would be similar, much like notes on a piano
keyboard
...
In the 1860s, little information was
available to illustrate relationships among the
elements
...
Mendeleevβs discovery was the result of many
years of hard work
...
He put the statistics of each element on a
small card and pinned the cards to his laboratory
wall, where he arranged and rearranged them
many times until he was sure that they were in the
right order
...
He predicted the properties of these substances
(gallium, scandium, and germanium)
...
Spectroscopic and other discoveries have filled in
the blanks left by Mendeleev and added a new
column consisting of the noble gases
...
The perplexing ftransition elements were sorted out and given a
special place, along with many of the radioactive
elements created by atomic bombardment
...
Elements that had similar behavior were put in
groups together which helped to correlate
existing data
...
With the gaps in
the table, Mendeleev said that there must be
elements that go in these spots, and he predicted
some of these properties
...
MORE ABOUT THE PERIODIC TABLE
-
Establish a classification scheme of the elements
based on their electron configurations
Elements in the same group behave similarly
is because they have the same valence
electrons
...
Noble Gases
-
All of them have completely filled electron shells
...
He
1s2
Ne
(He) 2s2 2p6
Ar
(Ne) 3s2 3p6
Kr
(Ar) 4s2 3d10 4p6
Xe
(Kr) 5s2 4d10 5p6
Rn
(Xe) 6s2 4f14 5d10 6p6
Representative Elements
-
These are elements in A groups on periodic chart
...
These elements have fairly regular variations in
their properties
...
Atomic radii increase within a column going from
the top to the bottom of the periodic table
...
How does nature make the elements smaller
even though the electron number is increasing?
d-Transition Elements
-
Elements on periodic chart in B groups
Sometimes called transition metals
Each metal has d electrons
...
They exhibit smaller variations from row-to-row
than the representative elements
...
Electrons are being added to f orbitals
...
Consequently, very slight variations of properties
from one element to another
...
-
The reason the atomic radii decrease across a
period if due to shielding or screening effect
...
The inner electrons block the nuclear chargeβs
effect on the outer electrons
...
Consequently, the outer electrons feel a stronger
effective nuclear charge
...
-
Se, S, O, Te
Plan: All these elements are from group Via nonmetals
...
Solution: The order of increasing atomic radii is O < S <
Se < Te
Example: Arrange these elements based on their atomic
radii
...
From
the figure of atomic radii, you should determine the
increasing radii
...
-
Ga, F, S, As
-
Ions with the same electron configuration, will
have the radii decrease as the atomic number
increases
...
Anions (negative ions) are always larger than
their neutral atoms
...
From
the figure of atomic radii, you should determine the
increasing radii
...
And so traditionally we
have to view the atoms as bonded together and this is
called the BONDING ATOMIC RADIUS
...
On the other hand, Neon has 10+ charge in its
nucleus since it has 10 protons and electrons
...
And so since
this 3+ charge is trying to attract the 3 electrons
in the orbitals, the force is too weak to
compress themselves closer to the nucleus so
they stay roaming around the nucleus
...
And so as we move from left to right of the
periodic table, atoms get smaller
...
IONIC RADIUS
-
-
Electrons repel each other so adding an electron
makes an atom bigger, taking one away makes it
smaller
...
Increasing nuclear charge attracts the electron
and decreases the radius
...
Increasing electron numbers in highly charged
ions cause the electrons to repel and increase the
ionic radius
...
It will always be an
electron in the outermost shell
...
So the farther away an electron is from the
nucleus, the easier it is to pull it away
...
-
-
-
Francium, a very large atom with only one
valence electron (7s1) will be easy to ionize
because the electron is so far away from the
nucleus
...
Losing the electron means the
outermost shell is gone and the one below is
completely full so elements in group 1 will easily
loose 1 electron
...
For this reason, it requires much more energy to
ionize helium
...
Elements can have successive ionization
energies for removing more than one electron
...
-
ο·
full outermost shell
...
But if oxygen looses
an electron, it will gain the same special stability,
nitrogen has
...
All deviations from the ionization energy trend
can be explained by discrepancies in orbital
symmetry like this one
...
Symbolically:
πππ¨π¦(π ) + energy
Ex: Mg (g) + 738kJ/mol
ο·
β
π’π¨π§+
(π) + π
β
Mg +
(π) + e
Second ionization energy ( πππ )
The amount of energy required to remove the
second electron from a gaseous 1+ ion
...
There are just a couple exceptions to the ionization
energy trend, but we can rationalize them
...
But something like oxygen which deeps
downward from Nitrogenβs ionization energy does
so because of orbital symmetry
...
This gives nitrogen
a special stability just like elements that have a
π’π¨π§+ + energy
π’π¨π§π+ + πβ
Ex: Mg + + 1451 kJ/mol
Mg π+ + eβ
PERIODIC TRENDS FOR IONIZATION ENERGY:
1
...
2
...
Important exceptions at Be & Mg, N & P, etc
...
3
...
IE1 , for Li>IE1 for Na, etc
...
-
Sr, Be, Ca, Mg = Sr < Ca < Mg
B, O, Be, N = Be < B < N < O
First, second and third, etc
...
Disregarding the noble gases as their shells are full,
electron affinity increases up and right
...
Notice that the energy increases
enormously when an electron is removed from a
completed electron shell
...
Group
and
element
πππ
(kJ/mol)
πππ
(kJ/mol)
πππ
(kJ/mol)
πππ
(kJ/mol)
IA
Na
IIA
Mg
IIIA
Al
IVA
Si
496
738
578
786
4562
1451
1817
1577
6912
7733
2745
3232
9540
10,550
11,580
4356
Looking at the elements at Groups Ia and IIa, these
elements donβt want to gain electrons, they would rather
loose them
...
The reason why Na forms Na+ and not Na2+ is that the
energy difference between IE1 and IE2 is so large
...
The same trend is persistent throughout the series
...
Al forms Al3+
ELECTRON AFFINITY
-
-
-
-
Electron Affinity is the amount of energy
absorbed when an electron is added to an
isolated gaseous atom to form an ion with a 1charge
...
Sign conventions for electron affinity
...
If electron affinity < 0 energy is released
...
Symbolically:
General periodic trend for electron affinity is
-
The values become more negative from left to
right across a period on the periodic chart
...
Measuring electron affinity values is a difficult experiment
...
Al, Mg, Si, Na
Si < Al < Na < Mg
ELECTRONEGATIVITY
-
Electronegativity is a measure of an atomβs ability
to attract shared electrons to itself
...
Electronegativity increases up and right
because a smaller atom like Fluorine with more
protons for its energy level or a higher ππππ =
effective nuclear charge will hold electrons best
Title: CHEMICAL PERIODICITY
Description: In the context of chemistry and the periodic table, periodicity refers to trends or recurring variations in element properties with increasing atomic number. Periodicity is caused by regular and predictable variations in element atomic structure.
Description: In the context of chemistry and the periodic table, periodicity refers to trends or recurring variations in element properties with increasing atomic number. Periodicity is caused by regular and predictable variations in element atomic structure.