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Effective Nuclear Charge (Zeff) And Atomic Radius

Effective Nuclear Charge (Z

) and Atomic Radius

eff

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

. All the halogens have an

electron configuration ending with ns

. 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

). Z

is the charge that valence electrons experience after core electrons shield the nuclear charge.

eff

Valence electrons do not shield each other from the charge. Z

=is calculated using the following equation:

eff

= Atomic number(Z) - core electrons (S)

eff

Complete the table and calculate Zeff for the following elements.

Electron

configuration

# Valence

electrons

# Core

electrons

# protons

eff

4. As you go across a row (period) in the periodic table, does the positive charge experienced by the valence electrons

) increase decrease, or stay the same?

eff

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?

Created by Emily Heider

Effective Nuclear Charge (Zeff) And Atomic Radius