Download 2015 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?
Easy way to remember the size
trends:
Helium atoms are the smallest!
Therefore, atoms must get larger as you go
down a group, and larger as
you go to the left.
Practice Questions
Use the periodic table to choose the largest
atom:
a. Al, Ga, Ge
b. N, F, P
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.
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
of other atoms in a bond.
Trend: Electronegativity
Moving from left to right across a period,
electronegativity increases.
Why?
The effective nuclear charge is increasing
as you move from left to right. The atoms
not only attract their own electrons more
strongly, they attract electrons from other
atoms more strongly.
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