Download Periodic Trends

Periodic Trends
A periodic trend is a trend that
repeats.
Which of these actions is periodic?
A. School starts in August.
B. A hurricane causes massive
power outages and shuts down the
schools for two weeks.
All the periodic trends can be understood in
terms of three basic rules:
1. Electrons are attracted to the protons in
the nucleus.
a. The closer an electron is to the nucleus,
the more strongly it is attracted.
B. The more protons in the nucleus, the
more strongly an electron is attracted.
2. Electrons are repelled by other electrons
in an atom. If there are other electrons
between a valence electron and the nucleus,
the valence electron will be less attracted
to the nucleus. That’s called shielding.
Definition: Shielding – the effect of inner
electrons on the attraction between the
valence electrons and the nucleus
3. Completed shells are very stable. Atoms
prefer to add or subtract valence electrons to
get 8 electrons in the outer energy level.
Exception: Helium is stable with 2 electrons in
its outer (and only) energy level.
Trend Number One: Atomic radius
Trend Number One: Atomic radius
Moving from left to right across a period, atomic
radius decreases.
Why?
As you move across a row, the effective nuclear
charge increases. Effective nuclear charge is the
charge “felt” by the valence electrons after you have
taken into account the number of shielding electrons
that surround the nucleus.
Which is larger, an atom of sulfur or an atom of
argon?
Trend: Atomic radius
Moving down a group, atomic radius increases.
Why?
As you move down a group, there are more
energy levels occupied by electrons.
Which is larger, an atom of oxygen or an atom
of sulfur?
Trend: Atomic radius
Metallic ions are smaller than atoms.
Why?
Metals lose electrons to form ions, and an
entire energy level disappears.
Which is larger, a sodium atom or a sodium
ion?
Trend: Atomic radius
Non-metals form negatively charged ions.
Negatively charged ions are larger than atoms.
Why?
Adding electrons to an energy level increases the
repulsion between the electrons, so they spread
out more.
Which is larger, a chlorine atom or a chlorine ion?
Practice Questions
Use the periodic table to choose the
largest atom or ion:
a. Al, Ga, Ge
b. N, F, P
c. F atom, F ion, Cl atom, Cl ion
Trend Number Two: Ionization Energy
But, wait!
What is ionization energy?
Ionization energy is the amount of energy
required to remove an electron from an atom
in the gaseous state.
Trend Number Two: Ionization Energy
Moving across a period, ionization energy
increases.
Why?
As you move across a period, the effective
nuclear charge increases, so the electrons are
more attracted to the nucleus.
Which has a higher ionization energy, oxygen
or fluorine?
Trend: Ionization Energy
Moving down a group, ionization energy
decreases.
Why? As you move down a group, the valence
electrons are farther from the nucleus and
held more loosely. It takes less energy to
remove a “loose” electron.
Trend: Electronegativity
Electonegativity refers to how strongly the
nucleus of an atom attracts the electrons
from other atoms it is bonding with.
Trend: Electronegativity
Moving from left to right across a period,
electronegativity increases.
Why? 2 reasons:
1. The effective nuclear charge increases as
you move across a period.
2. The closer the nucleus is to the outside of
the atom, the more likely it is to attract an
electron that is in a chemical bond.
Atoms get smaller going left to right, so the
nucleus is closer to the outside of the atom.
Electronegativity
Moving down a group, electronegativity decreases.
Why? As you move down a group, the atoms get larger
and larger, so the nucleus is farther away from the
bonding electrons.
Which element is the most electronegative? Fluorine