Transcript Ch5 Periodic trends

Periodic Trends
Mrs. Wood
Chemistry
Part I – Atomic Size
Atomic Radius

Measures as distance from nucleus to
nucleus and divided by 2.

Unit commonly used is pm
picometer= 10-12m

Example: iodine atomic radius 140pm

How does atomic radius change
across a period?

It is smaller to the right.

Why?

More protons in the nucleus higher
electrical force pulls electrons closer to
nucleus. No addition of energy level.
How does atomic radius change
down a group?

It is larger down the group.

Why?

The addition of energy level.Valence
electrons are at higher energy levels
and are not bound as tightly to the
nucleus because they are screened or
shielded ( pushed away) by other
electrons in inner levels.
Note

There are some exceptions. Example
column 13 and transition metals.
The Periodic Table and Atomic
Radius
Example:
Which is larger: a lithium atom or a
fluorine atom?
A lithium atom
Example:


Which is larger: an arsenic atom or a
sulfur atom?
An arsenic atom
Part II – Ionization Energy
Ionization energy

Ionization energy is the amount of
energy needed to remove an electron
from an atom.



First ionization energy (neutral)
Second ionization energy
etc
Ion

Positive ion ---removal of electron(s)

Negative ion--- addition of electron(s)
How does ionization energy
change down a group?


The first ionization energy decreases as
you move down a group.
Why?


The size of the atom increases.
Electron is further from the nucleus.
How does ionization energy
change across a period?


The first ionization energy increases as
you move from left to right across a
period.
Why?


Nuclear charge increases while shielding is
constant.
Attraction of the electron to the nucleus
increases.
Ionic size


Metallic elements easily lose electrons.
Non-metals more readily gain electrons.
How does losing or gaining an electron
effect the size of the atom (ion) ?
Positive ions

Positive ions are always smaller that the
neutral atom. Loss of outer shell
electrons.
Negative Ions

Negative ions are always larger than
the neutral atom. Gaining electrons.
Ion size trends in periods.


Going from left to right there is a
decrease in size of positive ions.
Starting with group 5, there is sharp
increase followed by a decrease in the
size of the anion as you move from left
to right.
Ion size trends in columns.

Ion size increases as you move down a
column for both positive and negative
ions
Electronegativity: the ability of
an atom in a bond to pull on the
electron. (Linus Pauling)
Electronegativity



When electrons are shared by two atoms
a covalent bond is formed.
When the atoms are the same they pull on
the electrons equally. Example, H-H.
When the atoms are different, the atoms
pull on the electrons unevenly. Example,
HCl
Trends in Electronegativity


Electronegativity generally decreases as you
move down a group.
Electronegativity of the representative
elements (s & p block) increases as you
move across a period.
Electronegativities of
Some Elements
Element
F
Cl
O
N
S
C
H
Na
Cs
Pauling scale
4.0
3.0
3.5
3.0
2.5
2.5
2.1
0.9
0.7
Note


Most electronegative element is F
(EN 4.0)
Least electronegative stable element is Cs
(EN 0.7)
Electron Affinity


The energy change that occurs when an
electron is acquired by a neutral atom
Most release energy (becoming more
stable)
Trends in Electron Affinity



Typically increases across a period with
exceptions in gr 2 and 14
Negative value means releases energy
Many variations in group trends
Nuclear charge increases
Shielding increases
Atomic radius increases
Ionic size increases
Ionization energy decreases
Electronegativity decreases
Summary
Shielding is constant
Atomic Radius decreases
Ionization energy increases
Electronegativity increases
Nuclear charge increases
Electron Affinity increases