Download Lesson 33 Periodic Trends

Lesson 21 Periodic Trends
Learning Targets:
I can predict changes in atomic radius,
shielding effect, ionization energy, electron
affinity, and electronegativity, based on element
positions on the periodic table.
PA Science and Technology Standards:
3.1.10.C; 3.4.10.A; 3.4.12.A; 3.4.10.B
I. The periodic table contains vertical and horizontal
trends
a.
Trends in the table can be found in the
horizontal periods and the vertical groups
or families.
b.
Knowing these trends allows you to predict
the behavior of elements when you don’t
know much about them.
II. Atomic Radius increases within a family
a.
b.
c.
d.
e.
Because the nucleus is so small, the element’s size is
determined by the electron cloud surrounding it
Often these measurements are made by measuring the
distance between two nuclei in a diatomic molecule,
and then dividing by two.
Atomic radius is defined as one-half the distance from
center to center of two like atoms.
Spherical representations of the atoms are useful,
although not completely accurate.
Trend 1: Atomic radius increases as you progress down
through the elements in each group
i. Because of the addition of an energy level every
time you go to the next period, it makes sense that the
atomic radius increases.
f.
Elements with large numbers of electrons show
the shielding effect
– the reduction of the attractive force between a
nucleus and its outer electrons due to the
blocking effect of the inner electrons
g.
h.
i.
Trend 2: Shielding effect increases as you
progress down through the elements in a group.
As you move across a period, you add electrons to
the same level, but don’t create any new levels.
Trend 3: Shielding effect does not change as you
move across a period.
III. Atomic size decreases from left to right across a period
a.
b.
c.
d.
e.
Trend 4: Atomic radii generally decrease as you
move across a period from left to right
As you add electrons within the same period, no
new principle energy levels are added.
The nucleus has a greater positive charge, and
therefore can pull on the electrons more, and they
end up closer to the nucleus.
As you move further across the period, the
electrons end up being pulled closer together by the
nucleus
Because electrons repel each other, there comes a
point where the repulsion of the electrons among
themselves and the attraction of the nucleus for them
levels out, and the atomic radius does not continue
to shrink
f.
Tables illustrating atomic radii:
Atomic radii in the case of ion
formation(Ionic Radius)
+ ions will decrease in size
- ions will increase in size
IV. Ionization energy follows a periodic trend.
a.
b.
c.
d.
e.
Atoms are normally electrically neutral.
An ion forms when an atom loses or gains
electrons
Ion – an atom or groups of atoms that has gained
or lost one or more electrons to acquire a net
electric charge.
The amount of energy needed to pull one electron
away from an atom is known as the ionization
energy
Ionization energy - the amount of energy needed
to remove an electron from a specific atom or ion
in its ground state in the gas phase.
f.
g.
h.
i.
Trend 5: Ionization energy generally
decreases as you move down through a
group.
As you move down in a group, the
electrons are further away from the
nucleus, and therefore, easier to remove.
Trend 6: Ionization energy generally
increases as you move across a period.
As you move across a period, the electrons
in the outer level are held in place with a
stronger attractive force from the nucleus
Table of Ionization Energies:
V. Electron affinity decreases within a family and increases within a period.
a.
Electron affinity is the energy change that accompanies the
addition of an electron to an atom in the gas phase.
b.
Trend 7: Electron affinity values generally become more negative
(read as “increases”) as you move from left to right across a
period.
c.
This is because as you move left to right, the atom has more
attractive charge from the nucleus for electrons, the shielding
effect doesn’t change, and the electrons are close to the nucleus.
d.
Trend 8: Electron affinity tends to become less negative (read as
“decreases”) as you move from top to bottom within a group.
e.
As the atom gets larger, the attractive force gets larger also, but
the distance more than offsets the larger attraction. It becomes
harder for the atom to attract electrons.
VI. Electronegativity decreases within a family and increases within
a period
a.
Electronegativity is the tendency for an atom to attract
electrons to itself when it is combined with another
atom.
b.
It is an arbitrary scale, with Fluorine (the most
electronegative) being assigned 4, and 0 being the
lowest, or no attraction.
c.
The noble gases are not assigned a value, as they do not
form a significant number of compounds.
d.
It is related to electron affinity and ionization energy
e.
Trend 9: Electronegativity values generally
decrease going down a group
f.
As you go down a group, the atom becomes larger,
and therefore cannot exert as strong a pull on a
new electron.
g.
Trend 10: Electronegativity values generally
increase going across a period
h.
As you go across, the shielding effect doesn’t
change, but the positive attraction of the nucleus
does.
Summary of Periodic Trends