Transcript We know that Rb+ is smaller than Rb because it loses an electron

PERIODIC TRENDS

According to Bohr’s model of the atom, atoms
are very similar to solar systems.

The nucleus of an atom acts like the sun of a
solar system. The positive electrical charge of
the protons provides the “gravity” of the atom.

The electrons of an atom are like the planets of
a solar system. They have “orbits” or regions in
which they can travel (called energy levels or
shells), but they do not travel outside their
regions.
 Atoms are three dimensional. They are not flat.
Therefore, the regions in which an electron can
travel are not flat circles. We now call these
three dimensional regions “electron clouds.”
This was an improvement over Bohr’s Model.

In our solar system, each planet has its own
orbit, each a different distance from the sun.
However, multiple electrons can share the
same energy level in an atom.

In fact, most atoms would like to have eight
electrons sharing the outer orbit, known as the
valence shell. This would be like eight planets
all sharing Neptune’s orbit.

The distance of a planet from the sun depends on
the gravity of the sun, the mass of the planet, and
the speed the planet is traveling (kinetic energy).
All electrons have the same mass, so for electrons,
mass has no effect on distance from the nucleus.
The distance depends only on the “gravity” of the
nucleus and the energy of the electron.

If a solar system has a large sun with strong
gravity, the planets will be closer to the sun
than in a solar system with a small sun with
weak gravity.

Stop and think: which element from each pair
represents a solar system with more gravity?
A)
 B)
 C)
 D)
 E)

Li or S
Ir or Ca
As or P
Ba or Pb
F or Fr

The radius of a solar system is the distance
from the center of the sun to the farthest
planet. For an atom, the atomic radius is the
distance from the center of the nucleus to the
farthest electron.

You have discovered that as each element
progresses from left to right on the periodic table, it
gains protons. Thus, its total positive charge
(gravity) increases. When this happens, the pull on
the electrons is greater, so the electrons move
closer. The atomic radius SHRINKS from left to
right on the table.


So what happens when you go DOWN the
chart?
Each period (or row) as you go down the chart
represents an additional orbit in your solar system.
This is a new shell or energy level of electrons and
creates what is called electron shielding. Since you
have gained a whole orbit, this will automatically
make the radius bigger, in spite of the magnetic
pull of the protons.

Now you know the trends for atomic radius: from
left to right, the radius decreases due to a greater
nuclear magnetic pull on the electrons. From top
to bottom, the radius increases due to gaining
additional energy levels and electron shielding.
The trend looks much like this:

Stop and think! : Based on the strength of the
“gravity” or positive charge of the nucleus of the
atom, which of each pair will have the SMALLEST
atomic radius? Remember: high gravity pulls
planets closer!

A)
B)
C)
D)
E)




Al vs Cl
Sn vs Y
He vs H
K vs Ca
N vs Ne

Stop and think! Which of these will have the
LARGEST atomic radius?
A)
 B)
 C)
 D)

B, Al, or In?
V, Cr, or Mn?
I, Se, or P
Mg, Ar, or Fe


So, what happens when an atom gains or
loses electrons to form an ion?
Remember, the octet rule states that atoms want to
have 8 electrons in their valence (or outer) shells
(orbits). In order to accomplish this, atoms form
ions. An ion is an atom with a charge because its
electrons and protons are no longer equal.

An atom that loses electrons is called a cation.
This atom initially had the same number of protons
as electrons. Once it loses electrons to follow the
octet rule, it will have more protons than electrons.
It will then have a positive charge.

Stop and think: If an element such as
calcium has two valence electrons, then how
does losing electrons satisfy the octet rule?
When it loses two electrons, it
will also lose its entire valence
shell. Thus, instead of four
shells, calcium will be left with
only three. The third shell is now
the valence shell, and it contains
eight electrons.

Since a cation loses a shell when losing
electrons, its radius as an ion gets smaller.
This new radius is called the ionic radius.
Since metals act as cations, metals will have
a smaller ionic radius than atomic radius.
Unlike metals, nonmetals usually
gain electrons in order to fill their
current valence shell with eight
electrons.
Since they will then have more electrons than
protons, they have a negative charge.
These negatively charged ions are called

anions:
A Negative ION.

Because the nonmetal atom is gaining electrons
without gaining additional protons, the negative
electrons will push each other away (like charges
repel), but there is no extra force pulling the
electrons in toward the nucleus. The electrons
take advantage of the extra space outside the
atom to push further away, slightly expanding the
radius.
Stop and think!
 List the following atoms in order of
increasing size: Rb, Rb+, F, F-, Te.

We know that Rb+ is smaller than Rb because it loses
an electron and has one less shell. Another reason is
that since Rb+ has the same number of protons but
less electrons, the electrons are held more tightly in
Rb+. We know that Te is smaller that Rb because of the
trend of decreasing radius to the right within a period.
Since Te has one shell more than Rb+ is it bigger.
We know that F and F- are both smaller than the rest
because they have less shells.
Now between F and F-: F- has one more electron but the
same number of protons, which causes the radius to be
bigger.
FINAL ANSWER: F,
F-, Rb+, Te, Rb
ELECTRONEGATIVITY

Electronegativity is the tendency of atoms to gain electrons.
Since nonmetals want to gain electrons to satisfy the octet rule
while metals want to lose electrons, nonmetals have a higher
electronegativity than metals.

Nonmetals are located in the upper right corner of the
periodic table. Thus, electronegativity increases

left to right
and bottom to top.

IONIZATION ENERGY


Ionization energy is the energy needed to remove an
electron from a neutral atom.
Because metals would rather lose electrons to satisfy
the octet rule, it is easy to remove electrons. They
have a low ionization energy. However, as they gain
protons, the magnetic pull on the electrons also
increases, making it more difficult to remove them.
This is why ionization energy
increases from left to right.

Nonmetals want to gain electrons, not lose
them! Thus, they have very high ionization
energies. These are located at the top right
of the periodic table. Thus, the ionization
energy, like electronegativity, increases from
bottom to top
and left to right.
HOW DOES IT ALL WORK TOGETHER?
STOP AND THINK!

Put the elements C, K, N, Sr, and P in order by:
1)
 2)
 3)
 4)

Increasing atomic radius
Decreasing electronegativity
Decreasing ionic radius
Increasing ionization energy
STOP AND THINK!
1) If an atom has 3 valence electrons, will its
radius increase or decrease when forming an
ion?
2) If an atom has two valence electrons, will it
have a higher or lower ionization energy than
most of the other elements in the same period?
3) If an atom has 2 valence electrons, will it have
a higher or lower electronegativity than atoms
with 6 valence electrons?
Assignment: write a paragraph explaining
how one of the four trends works and what
causes the trend from left to right and top to
bottom.