Chemistry Lesson

Tug-of-War in the Atom: Periodic Trends Through Coulomb's Law

Shared by A STEM Educator

10th Grade5E Model50 min

Learning Objective

Students will analyze how effective nuclear charge and electron shielding govern atomic/ionic radii, ionization energy, and electronegativity across periods and groups, construct graphical representations of these trends, and apply Coulomb's law reasoning to predict relative chemical reactivity.

Lesson Overview

Students use Coulomb's law as a lens to explain periodic trends in atomic/ionic radii, ionization energy, and electronegativity, connecting effective nuclear charge and shielding to the reactivity of alkali metals versus halogens.

Materials

Paper and pencils
Whiteboard and markers
Phone or basic calculator

Scaffolded Task Progression

1Task 1

The table below shows two species. Without calculating anything, rank them from largest to smallest radius and justify your ranking in one sentence.

Species	Protons	Electrons
$Na$	11	11
$Na^{+}$	11	10
$Mg^{2 +}$	12	10

Which is largest? Which is smallest? Why?

2Task 2

Using the data your teacher wrote on the board, sketch Graph 1 (atomic radius vs. atomic number for

Z = 3

Z = 18

) on your paper. Then answer:
(a) Describe the trend within Period 2 (Li to Ne).(b) What happens to radius when you cross from Ne (

Z = 10

) to Na (

Z = 11

)? Explain using the idea of electron shells.(c) Estimate

Z_{e f f}

for a Period 2 element using the shortcut

Z_{e f f} \approx Z - (core electrons)

. Calculate

Z_{e f f}

for Li, C, and F. What pattern do you see?

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