Chemical properties of an element are largely determined by their valence electrons. Elements with similarly filled valence
shells have similar properties. For example, all the alkali metals have a electron configuration ns
1
. All the halogens have an
electron configuration ending with ns
2
np
5
. What is a common characteristic shared by all the halogens?
What is a common characteristic shared by the noble gasses? What is their generic configuration?
We now explore trends in chemical properties that are related to electron configuration and valence shells.
1. Consider the chart of atomic radii to the left. As you move
down a group (for example, the alkali metals) does the atomic
radius increase or decrease?
2. Explain the trend you observed in question 1 in terms of
quantum number n (principle energy level).
3. As you move across a period (row) does the atomic radius
increase or decrease?
In order to understand the periodic trend of atomic radius across a row, we need to understand the concept of effective
nuclear charge (Z
eff
). Z
eff
is the charge that valence electrons experience after core electrons shield the nuclear charge.
Valence electrons do not shield each other from the charge. Z
eff
=is calculated using the following equation:
Z
eff
= Atomic number(Z) - core electrons (S)
Na Al P Cl Ar
Electron
configuration
# Valence
electrons
# Core
electrons
# protons 11 13 15 17 18
Z
eff
4. As you go across a row (period) in the periodic table, does the positive charge experienced by the valence electrons
(Z
eff
) increase decrease, or stay the same?
5. Would the attraction of valence electrons of argon to its nucleus be the same, greater than, or less than the
attraction of the valence electrons of sodium to a sodium nucleus?
Effective Nuclear Charge (Z
eff
) and Atomic Radius
Created by Emily Heider
Complete the table and calculate Zeff for the following elements.
Informal Writing Task – Answer the following questions using grammatically correct English sentences.
A) Explain the physical reasons for the trends in atomic radii down a column in the periodic table.
B) Considering the trend of Z
eff
(effective nuclear charge) explain the trend of atomic radius across a period
in terms of effective nuclear charge and Coulombic attraction (positive is attracted to negative).
6. Write electron configurations for the transition metals Sc (atomic # 21) and Ti (atomic #22). Calculate Z
eff
for
these two transition metals.
7. The atomic radii of transition metals do not decrease significantly across a row. As you add electrons to the
d-orbital are you adding core electrons or valence electrons?
8. Does Z
eff
increase, decrease or stay the same for transition metals in a row?
9. Contrast the radius trend for transition metals across a row with that of the main group elements across a
row.
Created by Emily Heider
