Chemistry / Unit 3: Electrons and the Periodic Table
3.6 Periodic Trends
Notes
Properties change in predictable ways across periods and down groups.
- Atomic radius: increases down a group (more energy levels) and decreases across a period (more protons pull electrons closer).
- Ionization energy (energy to remove an electron): decreases down a group, increases across a period.
- Electronegativity (pull on shared electrons): decreases down a group, increases across a period. Fluorine is highest (3.98).
- Ions: cations are smaller than their atoms; anions are larger.
- Rule of thumb: everything except radius increases toward the upper right (fluorine); radius is largest at the lower left.
Practice
Which atom has the largest atomic radius?
Potassium is lowest in group 1 of these choices, so it has the most energy levels.
Which element has the highest first ionization energy?
Ionization energy increases across period 3; argon, a noble gas, holds its electrons most tightly.
Which is larger, and why?
An anion is larger than its neutral atom; a cation is smaller because it loses its outer level.
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