Transcript Periodic trends

Periodic Trends
An Introduction to Using the Periodic Table
Introduction
Mendeleev put the periodic table together by grouping
elements together by properties.
All alkali metals (Li, Na, K, and so forth) are soft, silvery
metals with low density and high reactivity with water.
All halogens are non-metals with high reactivity to metals
and organic compounds.
We now know that there are other properties that are periodic
as well.
Introduction
As we examine the trends
in the properties of
elements, we will be talking
about the changes when ...
we move down a group
we move across a period
Periodic Trends
We will be examining the
trends in ...
atomic radius
ionization energy
electron affinity
ionic radius
electronegativity
Atomic Radius
Monoatomic binary molecules:
atomic radius, r, is ½ the
distance, d, between the
nuclei of the two atoms.
e.g., hydrogen, H2:
d
d = 0.74 Å
r = (0.74 Å)/2 = 0.37 Å
Atomic Radius
Multiatomic molecules:
r is d minus known radius
e.g., methane, CH4:
distance between C and H
dCH
dC-H = 1.14 Å
C
H
rC = dC-H - rH
atomic radius of H
• rC = 1.14 Å- 0.37 Å = 0.77 Å
Atomic Radius
Trend:
decreases as we move from
left to right
increases as we move from
top to bottom
Atomic Radius
Trend:
decreases as we move from
left to right
increases as we move from
top to bottom
Why is this so?
As we move across the period, we add more protons
(increasing the + charge of the nucleus) while we add
more electrons to the same shell.
The added + charge draws the electrons closer
Atomic Radius
Trend:
decreases as we move from
left to right
increases as we move from
top to bottom
Why is this so?
As we move down a group or family, we are adding
electrons to new shells at a much greater distance from
the nucleus.
This causes the atom to increase in size.
Ionization Energy
Ionization energy (IE) is the
energy needed to remove an
electron from an atom.
As we move across a period,
the distance of the electron
from the nucleus decreases.
The amount of energy needed
to remove an electron
increases. (Why?)
Energy
Energy
++
Ionization Energy
Ionization energy (IE) is the
energy needed to remove an
electron from an atom.
As we move down a group, the
distance of the electron from
the nucleus increases.
The amount of energy needed
to remove an electron
Energy
decreases. (Why?)
+
Ionization Energy
Trend:
increases as we move from
left to right
decreases as we move from
top to bottom
Electron Affinity
Electron affinity is the change of
energy in an atom when an
extra electron is added.
As we move across a period,
the distance of the electron
from the nucleus decreases.
The amount of energy needed
to add an electron decreases.
(Why?)
We say that affinity of the
element for the electron has
increased.
−−
Energy
Energy
Electron Affinity
Ionization energy (IE) is the
energy needed to remove an
electron from an atom.
As we move down a group, the
distance of the electron from
the nucleus increases.
The amount of energy needed
to add an electron increases.
(Why?)
Energy
We say that affinity of the
element for the electron has
decreased.
−
Electron Affinity
Trend:
increases as we move from
left to right
decreases as we move from
top to bottom
Remember that the affinity of the element increases if the
amount of energy to add an electron decreases.
Ionic Radius
Ionic radius is just what it says:
it is the radius of the most
common ion.
This is a little complicated
since elements to the left
(metals) are usually cations
(+ charged) and the elements
to the right (non-metals) are
usually anions (− charged).
−
+
In general, the ionic radius of the cations decrease as we move
from left to right for the same reason that the atomic radius
decreases as we move from left to right. (Why is that?)
Ionic Radius
Ionic radius is just what it says:
it is the radius of the most
common ion.
This is a little complicated
since elements to the left
(metals) are usually cations (+
charged) and the elements to
the right (non-metals) are
usually anions (− charged).
−
+
Similarly, the ionic radius of the anions decrease as we move
from left to right. (Why is that?)
Ionic Radius
Ionic radius is just what it says:
it is the radius of the most
common ion.
This is a little complicated
since elements to the left
(metals) are usually cations (+
charged) and the elements to
the right (non-metals) are
usually anions (− charged).
−
+
However, when we change from cations to anions, there is a
jump in ionic radius. (Why?)
Ionic Radius
Ionic radius is just what it says:
it is the radius of the most
common ion.
This is a little complicated
since elements to the left
(metals) are usually cations (+
charged) and the elements to
the right (non-metals) are
usually anions (− charged).
−
+
In general, the ionic radius of all ions increase as we move from
top to bottom for the same reason that the atomic radius
increases as we move from top to bottom. (Why is that?)
Ionic Radius
Trend:
generally, decreases as we
move from left to right
increases as we move from
top to bottom
Electronegativity
Electronegativity is the
tendency of an element to draw
electron density toward itself
when bonded to a different
element.
This makes electronegativity
different from other properties.
it is a property of an element bonded with other elements.
The other properties we have looked at are only related to the
element itself.
Electronegativity
In general, electronegativity
increases as the size of the
element decreases.
Electronegativity does not apply
to the noble gas elements.
(Why?)
This means that F is the most electronegative element (at 4.0)
and Fr is the least electronegative element (at 0.7).
The scale for electronegativity is arbitrary and the values are
only for comparison of relative magnitudes.
Electronegativity
Trend:
increases as we move from
left to right
decreases as we move from
top to bottom
Summary
As we move from left to right:
atomic radius decreases
ionization energy increases
electron affinity increases
ionic radius decreases
electronegativity increases
As we move from top to bottom:
atomic radius increases
ionization energy decreases
electron affinity decreases
ionic radius increases
electronegativity decreases