Download The Periodic Law

Chapter14: Periodic Trends
Modern Chemistry; Holt, Rinehart, & Winston
Dmitri Mendeleev (1834-1907)
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
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Russian chemist
Arranged his periodic table
according to atomic mass so that
elements with similar properties
were in the same group
Predicted the properties of
elements that had not yet been
discovered using his periodic table
Henry Moseley (1887-1915)
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
English chemist
Proved Mendeleev’s
arrangement of the periodic
table to be correct – only, the
periodic table was arranged
according to atomic number,
not atomic mass
The Periodic Law

States that when elements are arranged
in order of increasing atomic number,
their physical and chemical properties
show a periodic pattern
Atomic Radii

Atomic radius – the distance between
the center of the nucleus and the outer
most energy level
Atomic Radius
Distance between nuclei
Atomic Radius

Decreases across a row
Why?
Protons are added to the nucleus
moving across a period from left to
right
 This increases the charge of the
nucleus (effective nuclear charge – Zeff)
 As a result, it compacts the atom

Group Trends

Increase down a group
Why?
The addition of shells increases the
electrons’ distance from the nucleus
and the size of the atom
 This makes the atom larger

n=3
n=2
n=1
Ionization Energy

The energy required to remove one
electron from a neutral atom of an
element creating an ion
Period Trends
Increase across a period
 Why?

Zeff increases across the period
 There is stronger attraction between
the positive and negative particles
more difficult to remove them.

Group Trends
Decrease down the group
 Why?
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Electrons are far away from positive
nucleus, making it easier to remove
Electronegativity


A measure of the ability of an atom in a
chemical compound to attract a
bonding pair of electrons
A MEASURE OF GREEDINESS!
Trends

Increase across a period

Decrease down a group

EXCEPTIONS: NOBLE GAS (GROUP 18).
They do not have an electronegativity
value because they do not form
chemical compounds.