Periodic Trends and the Logic of the Periodic Table
Reading the Table
Periodic organization
Elements are arranged by increasing atomic number, with similar valence electron structures recurring across periods. This repetition creates predictable trends in atomic radius, ionization energy, electron affinity, and electronegativity.
Across a period vs down a group
Across a period
- Effective nuclear charge generally increases
- Atomic radius decreases
- Ionization energy generally increases
Down a group
- More electron shells are added
- Atomic radius increases
- Valence electrons are farther from the nucleus
Effective nuclear charge
Shielding by inner electrons reduces the pull felt by outer electrons. A useful qualitative idea is effective nuclear charge, , which increases across a period and helps explain many trends.
Core periodic trends
Which element is expected to have the highest electronegativity among the following?
Chlorine is far to the right on the periodic table and has a strong tendency to attract electrons.
Correct answer: Cl
Why do atoms generally get larger as you move down a group?
Each step down a group adds a principal energy level, increasing size and shielding.
Correct answer: Because additional electron shells are added
Prediction strategy
When comparing two elements, first locate them on the table, then ask whether you are moving across a period or down a group, and finally consider shielding and nuclear charge.
Ionization energy is the energy required to do what?
First ionization energy is the energy needed to remove one electron from a gaseous neutral atom.
Correct answer: Remove an electron from a gaseous atom