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Zeff and the Shrinking Atom: A Periodic Mystery
Noah Suter
Created on October 18, 2025
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Transcript
Zeff and the Shrinking Atom: A Periodic Mystery
START
Noah Suter
Class Handout
Research Question
How does Zeff change across Period 2, and how does it correlate with atomic radius and ionization energy?
Background Information
In chemistry, Zeff, or effective nuclear charge, is the net positive charge an electron experiences from the nucleus, accounting for the shielding effect of inner-shell electrons. It is calculated as Zeff=Z-S, where Z is the total number of protons (atomic number) and where S is the number of inner or core electrons shielding the designated electron. As atomic number increases across a period, the number of protons increases while shielding remains mostly constant. This results in a stronger attraction between the nucleus and outer electrons. This lab investigates Zeff changes across Period 2 and investigates its impact on atomic structure and reactivity via data analysis and CER reasoning.
Hypothesis
If atomic number increases across a period, then Zeff will increase due to stronger nuclear attraction and constant shielding, resulting in a smaller atomic radius and higher ionization energies.
Materials
• Periodic table• Values for atomic number, electron configuration, atomic radius, ionization energy • Spreadsheet or graphing software
Procedures/Variables
1. Select Period 2 elements: Li, Be, B, C, N, O, F, Ne.2. Record atomic number, electron configuration, atomic radius, and ionization energy. 3. Calculate Zeff using the simplified formula: Zeff= Z - S- where Z is atomic number and S is the number of core electrons. 4. Graph Zeff vs. atomic number. 5. Graph atomic radius and ionization energy vs. atomic number. 6. Analyze how Zeff correlates with these properties Independent Variable- Element (atomic number) Dependent Variable- Zeff, atomic radius, ionization energy Control Group- Periodic Table (Period 2), data source, method of experiment
Data Table
Analysis
As atomic number increased across Period 2, the effective nuclear charge (Zeff) also increased because the number of protons rose while the number of core electrons stayed the same. This stronger nuclear pull drew valence electrons closer to the nucleus, resulting in a smaller atomic radius and a higher ionization energy. The data table shows that Zeff increased from Li to Ne, correlating with a steady decrease in atomic size and a sharp rise in the energy required to remove an electron.
Conclusion
This investigation showed that the effective nuclear charge (Zeff) increased across Period 2, which lead to a decrease in atomic radius and an increase in ionization energy. This trend occurred because the number of protons increased while the number of electrons remained constant, strengthening the attraction between the nucleus and the valence electrons. The data supports this relationship, which confirms that Zeff is a key factor in driving multiple periodic trends and helping explain the structure and behavior of atoms that are across a period.