Transcript TRENDS in the PERIODIC TABLE

TRENDS in the PERIODIC TABLE
A trend is a pattern
or a repetition of
particular
properties.
Trends of the
periodic table
include
Atomic Mass,
Atomic Radius,
Net Nuclear Charge,
Ion Size,
Metallic or Non-Metallic
Property,
Electro-negativity,
1st ionization energy
The periodic table
is arranged
in a certain way
to keep elements with
similar properties close
together
GROUPS vs. PERIODS
Groups go up and down.
Periods go left and right.
Groups share many similarities.
Periods show periodically
(regularly) changing properties.
THERE ARE
SEVERAL BASIC TRENDS
(or patterns)
THAT WE NEED TO
RECOGNIZE AND
UNDERSTAND.
These Trends are…
1. Atomic Mass
(in amu)
2. Atomic Radius
(size in picometers)
3. Net Nuclear Charge
(how many protons are in the nucleus of an atom?)
4. Ion Size
(cations and anions)
5. Metal Properties and Non Metallic Properties
6. Electronegativity
7. 1st Ionization Energy levels
Demetri Mendeelev
• Developed the first
real periodic table
which ours is based
upon.
• He was able to
predict the missing
elements and their
properties once the
table was put
together.
Mendeelev was a genius!
Many of his predictions came true, such as the existence of
“eka-aluminum” which we know as gallium.
A missing element in his table made him believe that an
element with certain properties belonged there.
By looking specifically for this missing
“eka-aluminum”,
chemists were able to discover gallium.
His predictions were quite close to the actual values of the
various properties he quoted, such as mass, density, ionic
formula, and others.
Our FIRST Trend
Atomic Mass
Atomic mass goes higher from element
to element, with few exceptions.
The Group Trend is that atomic
mass increases.
The Period Trend is that atomic
mass increases too.
Because of specific properties...
our Periodic Table has a non-regular shape.
The elements are arranged by properties rather than a
way just make the table be uniform in shape.
The Alkali Metals
are in Group 1
but Hydrogen is not in this group.
Alkaline Earth Metals
are in Group 2
TRANSITIONAL METALS
are in Groups 3 - 12
INNER
TRANSITIONAL METALS
are at the bottom and fit in as shown
NON-METALS
are at the right hand side of the table
Noble Gases
are at the far right, group 18
OUR SECOND TREND
Atomic Radius or Atom Size
If you follow along Table S
for atomic radius, you find:
The Group Trend for atomic
radius is that it increases
down a group.
The Period Trend for atomic
radius is that it decreases
left to right.
Period Trend for Atomic Radius
Li
2-1
Be
2-2
••
B
2-3
C
2-4
N
2-5
O
2-6
F
2-7
Ne
2-8
•
•
•
•
•
•
Atoms get smaller as you go across a period.
They gain electrons in the same energy levels, not getting bigger.
The additional positive charge from the additional protons pulls the
electron orbital slightly tighter for each atom going across the period.
SHOWS Atomic Sizes for Groups and Periods
Someone, somehow, determined that “the most metallic metal” was
Francium, bottom left hand corner of the Periodic Table.
They also came to the conclusion that Helium was to become
known as the “most non-metallic” element.
You have to remember that, and you have to be able to compare up
to 4 elements at a time and see which is closer to either Fr or He,
and decide which is the most metallic, or most non-metallic.
The NEXT TREND is
1st IONIZATION ENERGY
First Ionization Energy,
It is the amount of energy needed to remove
a valence electron from an entire mole of atoms
and make them all into +1 ions.
For example…
To turn a mole of Li atoms
into a mole of Li+1 ions,
it would take (look at Table S now)
520 kJ/mole
Time for an Example…
• Na has a first ionization energy of 496 kJ/mol
• That means to take an outer electron from an
entire mole of sodium atoms, it would take 496
kJ for the mole.
FIRST IONIZATION
ENERGY FOR SOME
SELECTED ATOMS
Na
Mg
Al
736 kJ/mol
578 kJ/mol
Si
P
496 kJ/mol
787
kJ/mol
1012
Going across the 3rd
period, the trend for
1st Ionization Energy
is to INCREASE.
what about Mg
to Al then???
The Mg - Al EXCEPTION
is one of those unusual
places where the periodic
table cannot manage to be
perfect for
all properties.
Still, “the trend” is that first
ionization energy increases
when going across any
period.
It’s a trend to wear a
tuxedo to the high
school prom. But it’s an
exception to wear a
color like these guys!
Ionization Energy
• just so you know, there are first, second
and third ionization energy levels.
• each is measured with the unit kJ/mol
• the “first” is the energy required to
remove the first electron
• the “second” is to remove an additional
electron from the mole of ions (+1
cations into +2 cations, this is not in
Regents Chemistry.)
Another Trend – Ionic Sizes
Atomic size
• atoms get
bigger going
down a
group
• atoms get
smaller
going across
a period
• Cations are always smaller than
atoms because the cations lose
a whole orbital when they form.
• Anions are bigger than the atoms
they started as, because by
adding electrons into the outer
orbital, they must stretch a bit
larger to accommodate those
extra negative charges that push
against each other.
K+1
+1
Cs
-1
Br
is larger than
+1
K
is bigger than Cl
when going down a group
the ions get bigger
Cs +1
Cs+1
-1
This is true for
cations & anions
GOING ACROSS A PERIOD
CATION
S get
ANIONS get
smaller
smaller
too.
Electro-negativity
the measure of the attraction an
atom has to gain an electron
in a chemical reaction.
It’s measured on the
Linus Pauling electro-negativity scale.
Fluorine and E-N
• Fluorine tops out the
scale at 4.0
• Pauling set this standard,
because he could.
• It’s a totally arbitrary
scale, based upon
Fluorine and at 4.0 just
because. All other atoms
are compared to that
one.
• All the other electronegativity values are
relative to Fluorine’s
Electro-negativity is the amount of pull that
an atom has for another electron in a bonding
situation. Fluorine has the greatest desire of
all atoms for that electron gain. Fluorine is
given the rating of 4.0 on the E-N scale, the
highest Electronegativity of all elements.
Electro-negativity is on Table S. You don’t have to memorize
the trend, you can look it up anytime you want to.
• Going down
a group the
trend is towards
LOWER E-N
values.
• Going across a
period the trend
is towards higher
E-N values.
It is all about HOW CLOSE IS THE ATOM TO FLUORINE
which determines the relative electro-negativity.