Transcript Periodic Trends

Chemistry Notes:
Periodic Trends
Atomic Size
Atomic
size increases down
a group.
Atomic size decreases from
left to right across a period.
Why increasing down a group?
As
you go down a group
more energy levels cause
the atom to be larger.
Shielding also affects the
size; electrons in closer
energy levels cause less pull
to electrons outside of it.
Why decreasing across a period?
Across
a period no new
energy levels are added, but
more protons are.
The added positive charge
gives more pull to the
nucleus, so it pulls electrons
towards the center.
Which is bigger?
Li
or Na?
Se or As?
N or Si?
What is the biggest element?
What is the smallest element?
Valence Electrons
Valence
electrons increase
from left to right across a
period.
Valence electrons are the
same in the same group.
Before we go on...
An
ion is an atom that loses or
gains electrons to have a full
outside energy level.
In doing so, it becomes
charged--ceasing to be
electrically neutral.
Before we go on...
A
positively charged ion is
called a cation, a negatively
charged ion is called an
anion.
Oxidation Number
The oxidation number is the
charge that an element has
after it becomes an ion.
Metals tend to lose electrons
to get a positive charge,
nonmetals gain electrons and
become negative
Ion Size
Ion
size is essentially the
same trend as atomic size,
except anions are always
bigger than cations.
Why are anions bigger?
Anions
have more electrons
than the atom it is made
from, so there will be more
repulsion between them,
causing increase in radius.
Why are anions bigger?
Cations
have less electrons,
so they will have one less
energy level than the
element made from.
Which is bigger?
Cations
or anions?
3N or F ?
3+
3Ga or As ?
C4+ or C4-?
Ionization Energy
Ionization
energy is the
amount of energy it takes to
pull an electron from the
outside shell of an atom.
Ionization Energy
st
1
Ionization energy is the
amount of energy to pull
the first electron away, the
nd
2 ionization energy is the
amount of energy to pull
away a second electron, etc.
st
1
Ionization Energy
Ionization
energy increases
up a group.
Ionization energy increases
left to right across a period.
Why decreasing down a group?
As
you go down a group, the
valence electrons are farther
away from the nucleus, and
so it takes less energy to pull
them away from the atom.
Why increasing across a period?
The
more electrons in the
outside energy level, the less
eager an atom will be to lose
an electron.
Which has a higher I.E.?
K
or Ca?
Mg or Ba?
Br or Sb?
I or Rb?
nd
2
The
Ionization Energy
second ionization
energy follows the same
general trend as the first,
except that all the elements
in the first group have the
highest IE.
nd
2
This
Ionization Energy
is because after the first
row loses one electron it has a
full valence, making as stable
as possible. It takes much
more energy to take away a
second electron.
Electronegativity
The
electronegativity of
an element is the tendency
for the atoms of the
element to attract an
electron when combining
with other elements.
Electronegativity
The
scale is based on
several factors, including
the ionization energy of the
element. It can be used to
predict the type of bonds.
Electronegativity
Electronegativity
increases
up a group.
Electronegativity increases
from left to right across a
period.
Electronegativity
The
exception to the general
trend for electronegativity is
that the noble gases have a
value of zero!
This is because they don’t
attract electrons, since they
already have a full outer shell.
Which has a higher electronegativity?
S
or P?
Al or Ga?
Cl or Ar?
C or Mg?
Cs or He?