Transcript Chapter 6

Understanding Periodic
Trends
CHAPTER 6
Timeline
1829 J.W. Dobereiner



Classification system in which elements are
grouped in triads
3 elements with similar chemical properties
Didn’t work for all of the elements
1869-Russian Chemist, Dmitri Mendeleev:


Designed a Periodic Table 1869
Elements are listed in order of increasing atomic
mass
Timeline continued
1913-British Scientist, Henry Moseley:



Designed the Modern Periodic Table
Elements are listed in order of increasing atomic #
Based on Periodic Law: physical & chemical
properties of elements are periodic functions
of their atomic #s
PERIODICITY
There are general trends in the
properties of atoms and their
ions.
These trends can be explained
using the periodic table and the
electron configurations of the
atoms.
Atomic Radii
atom
Half the distance between the
nuclei in a molecule consisting
of identical atoms
Increases as you move down a group
e- s
are added to higher energy levels
Shielding effect: inner e-s shield the outer
e-s from the attractive force of the nucleus
Decreases as you move across a period
Increasing
# of protons attract
valence electrons more
Stronger nuclear charge
I
D
Atomic Radii
D
I
Atomic Radii
Practice
Which
has a
larger
atomic
radii?
Li or
Ne?
Li
More protons, so
Ne is smaller
Ionization Energy
Energy needed to remove loosely held electron
from outer energy level
I.E.
e
electron
2 1
Li
2
positive ion
Decreases as you move down a group
effect: inner e-s shield the outer e-s from the
attractive force of the nucleus
Shielding
atom so it is easier to remove valence e-s
Nucleus can’t “hold” onto e-s that are far away
Bigger
Increases as you move across
valence e-s, more
energy needed to
remove e-s
More
D
I
Ionization Energy in kJ/mol
D
I
Ionization Energy
Practice
Which
has more
ionization
energy?
He
or
Xe?
(kJ/mol)
He
Xe
2372 kJ/mol 1170 kJ/mol
Ionic Size: Cation(+) vs. Anion(-)
Increases as you move down a group
• more energy levels greater the size
Cations decrease as you move across a period
Anions decrease as you move across a period
Cation
(D)
I
Anion
(D)
Ionic Size: Cation(+) vs. Anion(-)
Cations lose
electrons and get
smaller
Anions gain
electrons and get
bigger
Ionic Size
Practice
Which
has a
larger
ionic
size?
Li
or
Cs?
Cs
Electronegativity
The ability of an atom to attract e-s to itself when it is
chemically combined with another element.
Decreases as you move down a group

Bigger the atom, the harder it is for the nucleus to attract e-s
Increases as you move across a period
 More valence e-s, easier to gain e-s than lose
(MAGIC 8)
 Noble gases are omitted because they don’t form
I
many compounds
D
Electronegativity
I
D
Electronegativity
Practice
Li
or
F?
Which is more
electronegative?
F
More valence e-s
Shielding Increases
Ionization Energy Decreases
Electronegativity Decreases
Atomic Radii Increases
PERIODICITY
Atomic Radii Decreases
Ionization Energy Increases
Electronegativity Increases
Shielding is Constant
Ionic Size (cation)
Decreases
Ionic Size (anion)
Decreases
PERIODICITY