Effective Nuclear Charge,
Shielding, and Trends on the
Periodic Table
Vocabulary:
• effective nuclear
charge (Zeff)
• shielding
• atomic radius
• ionization energy
• electronegativity
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Trend
 What is a trend?
 a pattern or direction
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Periodic trends
 The elements have trends, too. For example:
 Look at the zig-zag line on your periodic table. Metals are
on the left, nonmetals on the right, and the metalloids are
adjacent to the line.
Periodic Trends
We’re going to look at trends on the
periodic table for physical properties
of elements. These are:
atomic radius
electronegativity
ionization energy
Periodic Trends
You can make predictions about certain
behavior patterns of an atom and its
electrons based on the position of the atom
in the periodic table.
All periodic trends can be understood in
terms of 2 basic rules that describe the
relationship between protons and electrons
in an atom, which is referred to as
“Effective Nuclear Charge, Zeff”.
1. Attractions
Electrons are attracted to protons in the
nucleus of the atom.
The closer an electron is to the nucleus,
the more strongly it is attracted,
increasing effective nuclear charge.
The more protons in a nucleus, the more
strongly an electron is attracted,
increasing effective nuclear charge.
Effective Charge (Zeff)
A good estimate for the effective
charge (Zeff) across a period attracting
a valence electron can be calculated
using the following equation:
Zeff = Z – core electrons
Zeff of Period 4
K
Ca
Sc
Ti
V
Cr
•The number of protons increases as you go
across a period. More protons = more
attraction of the electrons to the nucleus.
•Increased Zeff as you go across the period.
Calculate Zeff for K, Ca, and Sc.
2. Electron Shielding and
Repulsion
Electrons repel other electrons.
Electrons are repelled by other electrons
in an atom, decreasing effective nuclear
charge.
Effective nuclear charge decreases when
electron levels exist between the valence
level and the nucleus of the atom. This is
commonly referred to as electron
shielding.
Shielding in the alkali metals
Increased
shielding
Decreased
attraction of
valence
electrons
Shielding: the
blocking of the
nuclear charge to
the valence
electrons by the core
electrons. More
energy levels = more
shielding. (Valence
electrons are
circled.)
Trends: Atomic Radius
The atomic radius of an atom is the
approximate distance from the nuclei of an
atom to its valence electrons.
Typically measured as half the distance
between the nuclei of identical bonded
atoms.
From the online lab, what trend in atomic
radius was observed when going left to right
across a period on the periodic table? Why?
From the online lab, what trend in atomic
radius was observed when going top to
bottom in a group on the periodic table?
Why?
Trends: Ionization Energy
Electrons are attracted to the
nucleus of an atom, so it takes
energy to remove an electron.
The energy required to remove
the outermost electron from an
atom is called the first ionization
energy.
The energy required to remove
the next furthest electron is
called the second ionization
energy, and so on.
Trends: Ionization Energy
What trend do you predict in ionization energy
when going left to right across a period on the
periodic table? Why?
What trend do you predict in ionization energy
when going top to bottom in a group on the
periodic table? Why?
Trends: Electronegativity
Electronegativity refers to how
strongly the nucleus of an atom
attracts the electrons of other
atoms in a bond (noble gases are
not measured based on
electronegativity).
The scale is from 0-4 with 0
meaning no attraction for valence
electrons and 4 as the highest
attraction.
When an atom gains or loses an
electron, the atom is called an ion.
Periodic Table Trends
Explaining Trends
LEFT TO RIGHT
 Moving from left to right on the periodic table, protons are
added to the nucleus and electrons are added to the
valence level. Due to the addition of protons to the nucleus
of an atom Zeff increases (although electron repulsion
increases slightly with additional electrons there is no
increase in electron shielding) . This causes atomic radius
to decrease, and all other trends to increase.
TOP TO BOTTOM
 Moving from top to bottom on the periodic table, new shells
(energy levels) of electrons are added to the atom. This
pushes the valence electrons further away from the
nucleus and increases the electron shielding in the atom
(protons are also being added, but the shielding far
outweighs this attraction). This results in a decreases in
Zeff causing atomic radius to increase, and all other trends
to decrease.
Connecting Trends and Atomic
Properties
Explain why the atomic radius of neon is…
1.
a)
b)
Less than a neutral magnesium atom.
Greater than the common ion formed by magnesium.
2.
Which element, potassium or calcium, has a higher first
ionization energy? Which has a higher second ionization
energy? Justify your answers.
3.
Explain why the electronegativity of oxygen is higher than
the electronegativity of chlorine.