Periodic Law and Periodic Trends
The periodic table is a predictive model: electron structure explains why properties repeat and how they change.
Learning goals
- State the modern periodic law in terms of atomic number.
- Use effective nuclear charge and shielding to explain trends.
- Compare atomic and ionic radii, ionization energy, and electronegativity.
- Predict metallic character and the acid-base behavior of oxides.
- Distinguish a general trend from an absolute rule.
1. Why properties repeat
The modern periodic law states that the properties of elements and their compounds change periodically with atomic number $Z$. Increasing $Z$ adds a proton and an electron. When a new principal energy level begins, a similar outer-electron pattern reappears, so chemical behavior also repeats. Elements in one group share related valence configurations; a period records the progressive filling of a principal level and its sublevels.
The table therefore connects microscopic electron structure to macroscopic observations such as reactivity, conductivity, bond polarity, and oxide character.
2. Two competing causes
Effective nuclear charge $Z_{eff}$ is the net attraction experienced by a valence electron after repulsions and shielding by other electrons are considered. Across a period, proton number rises while added electrons enter the same principal level, so shielding changes less than nuclear charge. $Z_{eff}$ generally increases and pulls electrons closer.