Trends in the Periodic Table
This packet will help us to understand some trends of characteristics of elements in the periodic table.
Trends in a Group
1. Valence electrons
Pick any of the 18 groups and examine the number of valence electrons as you go
down the group. What did you notice?
__________________________________________________________
This is why all the elements in a group react similarly. Since they all have the same valence
structure, they all do the same thing with their electrons when they bond.
2. Nuclear charge
What two particles are in the nucleus of an atom? _____________________________
Which of these causes the nucleus to have a charge? ___________________________
So the nuclear charge is the same as the ______________________________________________
Pick any group and look at the nuclear charges in that group. As you go down a group, the nuclear
charge ____________, because you have an increase in ______________.
3. Ionization Energy
Ionization energy describes the atom’s ability to hold onto its own electrons. You
would assume that as you move down a group, since you have more protons, you
would have a higher ionization energy. Using Table S, list the ionization energies of
the following elements of group 1
(11)Na ___________ (19)K _______________ (37)Rb _______________ (55)Cs________________
The reason for this has to do with the size of the atom. As you move down a group,
you add one full energy level. This means that the valence electrons are farther from
the nucleus. In large atoms, even though they have lots of protons, the valence
electrons are too far away from the nucleus to be held tightly. For example:
Li
e-
Fr
e-
Fr (7 valence shells)
Li (2 valence shells)
Notice how much farther the Fr nucleus is from its valence electrons? Even though it
is a larger nucleus, Fr does not hold onto its valence electrons strongly. We need only
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add a small amount of energy to remove the valence electrons of large atoms like
Francium.
4. Electronegativity
Electronegativity tells us how strongly the atom pulls on electrons of other atoms.
Again, follow the electronegativity trend as you move down one group on Table S.
What happens to the electronegativity?
(11)Na ___________ (19)K _______________ (37)Rb _______________ (87)Fr________________
The reasons for this are the same as for ionization energy. Since the atoms get larger
as you move down, the nuclei must reach farther and farther out to attract other
electrons from other atoms. For example, if Li and Fr both wanted to attract the
circled electron:
Li
e-
Fr
See how much farther the Fr nucleus needs to reach out to attract the electron?
Even though it has a larger nucleus, it is too far away to pull on another atom’s
electrons.
Trends in a Period
The easiest way to observe trends in a period is to avoid periods with d or f blocks. While the trends still
exist in these periods, it is harder to observe. In this section, try to limit yourself to elements in the first three
periods.
1. Valence electrons
Look at the elements in the second period. As you move from left to right, the
number of valence electrons _____________.
This is why each element in a period reacts differently from the others.
2. Nuclear Charge
Look at the atoms of the third period. Since nuclear charge is the same as the
number of protons, as you move left to right in a period, the nuclear charge
_____________. This will become important in the next two sections.
3. Ionization Energy
In a period, all elements have the same number of energy levels. With this in
mind, look at the following elements
(11)Na _______ (13)Al __________ (14) Si_________________
as you move from left to right, the ionization energy
__________.
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This is due to the increase in the number of protons. Since we have the same
number of energy levels, the more protons we have, the stronger the atom can hold
onto its valence electrons. For example
Li
+3
e-
Fr
+9
e-
Fluorine’s higher nuclear charge helps it hold onto its electrons better.
4. Electronegativity
As you may have guessed, this follows the same trend as ionization energy, for
the same reason, with one exception. Look at the same elements from period 2
(11)Na _______ (12)Mg__________ (13)Al __________ (14) Si_________________
as you move from left to right in a period, electronegativity_______________.
Again, this is due to the increase in nuclear charge. Atoms have the same
number of energy levels, so the increase in protons allows them to pull on
another atom’s electrons stronger. For example.
e-
K
+19
Br
+35
The bromine has many more protons, so it pulls harder on the extra
electrons.
The exception to this trend is the Noble gases (He, Ne, Ar, Kr). What
do you notice about the electronegativity of most noble gases?
_______________________________________________
This is because the noble gases have a full valence shell. They are
stable and do not require any more electrons, so there is no attraction for
electrons.
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Notes
III Trends in the Periodic Table
A. Trends in Groups
1. Valence electrons
As you go down a group, the number of valence electrons
_______________________
Because of this, all elements in a group ________________________________
____________________
2. Nuclear charge
Nuclear charge is the same as the ________________________________________
As you move down a group, the nuclear charge ___________ because there are more
_______________________.
3. Ionization Energy
This is the energy needed to _______________________________________
As you move down a group, ionization energy ___________________
This is because the nucleus of larger atoms _______________________________
__________________________________________
4. Electronegativity
How strongly an atom __________________________________
As you move down a group, electronegativity ___________________________
This is because the nucleus of larger atoms ____________________________
______________________________________________
B. Trends in Periods
1. Valence electrons
As you move from left to right in a period, the number of valence electrons
____________________________
This is why atoms in a period ____________________________________
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2. Nuclear charge
As you move from left to right in a period, nuclear charge __________________________
This is because _________________________________________________
3. Ionization Energy
As you move left to right in a period, ionization energy ___________________
This is due to the increase in ____________________________
Since atoms have the same ______________________, a larger ____________________
allows them to hold onto their own electrons stronger.
4. Electronegativity
As you move left to right in a period, electronegativity ___________________
This is due to the increase in ____________________________
Since atoms have the same ______________________, a larger ____________________
allows them to attract another atoms electrons stronger.
Exception - __________________ breaks the trend. These atoms have no electronegativity
because
_________________________________________________
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