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Periodic Trends
Chemistry I
Mrs. Hayes
Group Trends
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Group one, Alkali metals
Group two, Alkaline earth metals
Groups 3-12, Transition metals
Group 17, Halogens
Group 18, Noble gases
At the bottom: Lanthanides and Actinides,
also known as Rare earth metals
The Octet Rule
• All atoms want their outer (valence) shell to have
eight electrons.
• The first shell (closest to nucleus) always holds
two electrons.
• As you move away from the nucleus, the second
shell will hold eight, the third shell will hold eight,
the next two shells will hold 18.
• As long as the outer shell has eight, the atom is
stable.
Orbitals or Shells
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Bohr's model of the atom showed
electrons orbiting the nucleus.
The quantum mechanical model instead
shows energy levels called “orbitals” or
“shells.”
Each energy level corresponds to a period
on the periodic table.
Level 1, the ground state, is the same as
period 1 on the table and so on...
Organizing Orbitals
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Orbitals are organized on the periodic
table according to the shape, energy and
number of electrons.
This organization is called an “electron
configuration.”
There are four sections in the
configuration; s, p, f and d.
Compare your periodic table to the next
slide.
Ions
• Ions are formed when an element either
gives up electrons from their outer
(valence) shell or gains electrons in their
valence shell so their outer shell has eight.
• This transfer of electrons creates an
element that is no longer neutral (same
number of protons and electrons)
• Cations are ions with a positive charge
• Anions are ions with a negative charge
Cations
(The “t” in “cation” looks like a plus sign)
• All metals form positive cations when they
donate electrons from their valence shell.
• Alkali metals, for example, have one extra
electron in their valence shell and would like to
have a full outer shell.
• It is easier to give away one than gain seven!
• This leaves them with one more proton than
electron, so they have a positive charge of +1.
Anions (“A-negative-ion”)
• Anions are formed by non-metals when
they take electrons from metals to fill their
outer shell with a total of eight.
• More electrons than protons means they
are no longer neutral. They are now more
negative.
• A halogen, for example, that has one more
electron is more negative with a -1 charge.
Your Turn!
• What charge will a potassium ion have?
– K+1
• What charge will a fluorine ion have?
– F-1
• What charge will a magnesium ion have?
– Mg+2
• What about noble gases?
– Already stable!
Atomic Radii
• What does the atomic number for a neutral
atom represent?
• What is the atomic radius?
• What is the trend toward size of the atomic
radii as you move across the periodic
table?
Atomic Radii Decrease as You
Move Across a Period
• The atomic number increases as you move
left- to-right across the periodic table.
• The atomic radius decreases as you move
left-to-right, because the greater positive
charge in the nucleus pulls the electrons
closer..
• Draw arrows on your periodic table and
label the trend.
Atomic Radii Increase as you Move
Down a Group
• The energy levels increase as you move
down the periodic table (from period 1 to 7)
• Each energy shell of electrons enlarges
the overall radius.
Ionization Energy
• The IE is the minimum amount of energy
required to remove an electron from the
outer shell of a neutral atom.
• When several e- are removed (example,
group 2), the energy required to remove
the first e- is called the “first ionization
energy”.
• As more e- are removed, the energy is
called the second IE, and so on.
Ionization Energy
What is this ion? 4 protons, 3 electrons…
Ionization Energy Trend
• IE decreases as you move down the periodic
table because the electrons are farther away
from the nucleus and therefore are easier to
remove.
• IE increases as you move across from left to
right because the more electrons there are in
the valence shell, the more energy it takes to
remove one.
• Draw arrows on your periodic table and label
this trend.
Electron Affinity
• The measure of the change in energy
when you add an electron to an atom.
• EA increases as you move from left to
right on the periodic table because you
produce more stable anions as you move
to the right.
• The elements to the right have a larger
positive charge in the nucleus, so attract
the electrons more easily.
Electron Affinity Trend Stops at
Noble Gases
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Noble gas valence shells are full.
Noble gases are very stable.
They do not want additional electrons.
Their EA values are therefore positive.
Electronegativity
• Electronegativity refers to the ability of an
atom to attract the electrons of another
atom to it when those two atoms are
associated through a bond.
• This differs from electron affinity in that EA
relates to the attraction of electrons by an
unbonded atom becoming an anion.
Electronegativity Trends
• Electronegativity generally increases as
you move left to right across the periodic
table.
• Electronegativity generally increase as you
move up a group.
Electronegativity Trends
Fluorine, F
Fluorine, in group 17 and period 2, is the
element with the highest
electronegativity.
Noble Gases, Lanthanides,
Actinides
• Noble gases have a full valence shell, so
do not normally attract e-.
• Lanthanides and actinides posses a more
complex chemistry and therefore do not
follow electronegativity trends.