Transcript The Periodic Table - Midland Park School District

Section 6.3 - Periodic Trends
Objectives
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Compare period and group trends of
several properties.
Relate period and group trends in atomic
radii to electron configuration.
Periodic Trends
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When the properties of an element change
in a predictable way, we call it a “trend”.
In the periodic table, there are trends
observed within a group (from top to
bottom) and across a period (from left to
right).
Periodic Trends
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Atomic radius is
defined as half the
distance between the
nuclei of identical
atoms that are
chemically bonded
together.
Atomic radius is a
measure of atomic
size
Trends in Atomic Radius WITHIN A GROUP
WHY does radius increase WITHIN A GROUP?
1. As the number of the energy level of the valence
electrons increases, the size of the energy level
(and its orbitals) increases because there is more
space to occupy. Therefore, the size of the atom
increases.
2. Valence electrons in higher energy levels are
further from the nucleus & feel the pull of the
positively charged nucleus less and less.
3. Outer energy level electrons are also shielded
from increasing positive nuclear charge by
electrons in the inner energy levels.
Trends in Atomic Radius ACROSS A PERIOD
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Across a period, atomic radius decreases
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Increasing atomic number means an increase
in nuclear charge across the period
Since the energy level remains the same, the
valence electrons do not move further away.
(There is NO shielding.) Therefore, the
increase in positive charge in the nucleus
pulls on the increasing number of electrons
with equal force.
The electrons are pulled to the nucleus and
radius decreases.
Prac. Probs. pg. 189 #16-18
Trend in Ionic Radius
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An ION is an atom or a bonded group of atoms
that has a positive or negative charge
When atoms lose electrons, they form
positively charged ions (the number of protons
will be greater than the number of electrons)
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Positive ions have empty orbitals so the ion will
always be smaller than the atom.
In addition, the remaining electrons experience
less repulsion so they can get closer to each other
and to the nucleus.
Ionic Radius Trends
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When an atom gains electrons, it becomes
negatively charged (more electrons than protons).
The ion will be larger than the atom because
1) The pull on each electron will be smaller
2) Increased electron repulsion causes an increase
in radius
Ionic Radius Trend Across a Period
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Negative ions are
always larger than
positive ions.
As charge on positive
ions increases, ionic
radius decreases.
As charge on negative
ions decreases, ionic
radius decreases.
Ionic Radius Trend Within a Group
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As atomic number increases (top to
bottom), ionic radius increases for both
positive & negative ions.
This is because there is an increase in
energy levels down a group.
Trends in Ionization Energy
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Ionization energy is the energy required to
remove an electron from a gaseous form
of that atom.
Think of it as an indication of how strongly
an atom’s nucleus holds onto its valence
electrons - a high value means the atom
has a strong hold on its electrons - they
are not likely to form positive ions!
Types of Ionization energy
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First ionization energy: energy required to
remove the first valence electron
Second ionization energy: energy required to
remove the second valence electron from a
+1 ion
Third ionization energy?
First Ionization Energy Trends
First Ionization Energy Trends
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Ionization Energy INCREASES across a
period. An increasing nuclear charge
produces an increased hold on valence
electrons.
Ionization Energy DECREASES within a
group (top to bottom). Valence electrons
are further away from the nuclear positive
charge and thus easier to remove.
Periodic Trends
Decreasing Ionization Energy
Ionization Energy Trends
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Open your books to page 192.
In Table 5, you will see that the energy
required for each successive ionization
always increases.
For each element there is an ionization
for which the required energy jumps
dramatically.
Ionization Energy Trends
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Find this ionization for Boron.
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This means a boron atom can “easily” lose
the first, second, and third valence
electrons but it is extremely hard to remove
the 4th. Therefore, very unlikely that it
will lose the fourth electron.
Ionization Energy Trends
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Boron has 3 valence electrons and will
“easily” form a +3 ion. (It will NOT form
a +4 ion!)
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The ionization at which the large jump
in energy occurs is related to the atom’s
number of valence electrons.
The Octet Rule
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Sodium atom (Na)
1s22s22p63s1
Sodium ion (Na+)
1s22s22p6
The sodium ion has the same electron
configuration as neon, a noble gas.
Filled s and p orbitals of the same
energy level are unusually stable.
Octet Rule – atoms tend to gain, lose
or share electrons in order to acquire a
full set of eight valence electrons.
The Octet Rule
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Useful for determining types of ions likely
to form
Left side of table (METALS) - will LOSE
electrons. (Will form positive ions.)
Right side of table (NONMETALS) - will
GAIN electrons to acquire an octet. (Will
form negative ions.)
Trends in Electronegativity
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The electronegativity of an element
indicates the relative ability of its atoms to
attract electrons in a chemical bond.
Noble gases have essentially NO
electronegativity.
EN is expressed in terms of a numerical
value of 4.0 or less; see pg. 194
Trends in Electronegativity