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Section 6.3
Periodic Trends
Review
 What happens to the size of the atoms in
the periodic table as we move from left
to right across the periodic table? Why?
 What happens to their atomic radius as
we move from top to bottom in a
column? Why?
 What is the element with the largest
atomic radius on the periodic table.
Ion Formation
 The driving force that makes reactions
happen is ion formation.
 Ions form when atoms gain or lose
electrons.
 When electrons are gained or lost the
resultant ion has a positive or negative
charge.
Cations
 When atoms lose electrons and form positively
charged ions, they become smaller, and are
called cations.
 They become smaller because the loss of the
electrons means that the number of protons is
greater than the number of electrons.
 Therefore, the electrons will be pulled more
tightly to the nucleus, and the outer electrons will
feel the pull of the nucleus more strongly than
before.
Anions
 When atoms gain electrons they form
negatively charged ions, called anions.
 The added electron makes the ionic
radius increase, because of repulsion
and the weaker pull of the nucleus on
each electron.
Trend in Atomic Radius
 As you move from left to right across the
periodic table, your positively forming
ions, Groups 1A-4A get smaller, but
Groups 5A- 8A get larger.
 As you move from top to bottom the ions
get larger in each column because of the
addition of energy levels.
Trend
Ionization Energy
 To form a positive ion, an electron must be
removed from a neutral atom.
 Removing the electron requires energy. That
energy must overcome the attraction between
the positive charge in the nucleus and the
negative charge of the electron.
 This energy, known as ionization energy is
defined as the energy required to remove an
electron from an atom in the gaseous state.
First Ionization Energy
 The first ionization energy is the amount of
energy required to remove the first electron
from the outer shell of the atom.
 Remember that as you move across a period,
you increase atomic number and therefore add
more positive charge to the atom.
 That addition makes it more difficult to remove
the electron and therefore more energy is
required.
Ionization Energy
Trend in Ionization Energy
 For the first ionization energy, as you
move across a period, from left to right,
the ionization energy increases.
 As you move down a group, the
ionization energy generally decreases.
Why?
Trend
Additional Ionization Energies
 It is also possible to remove electrons
after removing the first, or furthermost
electron from the electron cloud.
 However, once you remove the first
electron, much more energy is required
to remove the second, and third, and
fourth, etc.
Octet Rule
 Remember, when Newland tried to
design his periodic table he came up
with a law of octaves, which wasn’t
accepted.
 We see using ionization energies that his
predictions were every correct.
 When atoms/ions have eight electrons in
their outer shell, they are much more
stable.
Octet Rule
 The octet rule has come to be one of
the most important principles in
chemistry.
 It says that atoms tend to gain, lose or
share electrons in order to acquire a full
set of eight valence electrons.
 Use this to predict what kinds of ions will
form: elements on the left will form
positive ions and elements on the right
negative ions.
Electronegativity
 The electronegativity of an element
tells about its ability to attract electrons
in a chemical bond.
 The more electronegative an element,
the more electron loving it is.
 Noble gases form few compounds and
so virtually lack electronegativity.
Trend
 As you move from top to bottom down a
group, electronegativity generally
decreases.
 As you move from left to right,
electronegativity generally increases.
 So what is the most electronegative
element?
Trend