Chemistry · Ch 3 — Classification of Elements and Periodicity in Properties

Summary

Moseley’s law established that atomic number ( $Z$ ), not atomic mass, is the fundamental property determining element identity.
The modern periodic law states that physical and chemical properties are periodic functions of atomic number.
The long-form periodic table has 18 groups and 7 periods; elements are arranged by increasing $Z$ into $s$ , $p$ , $d$ , and $f$ blocks based on the subshell being filled.
Periodicity arises from the recurrence of similar valence-shell electronic configurations after certain intervals ( $2, 8, 8, 18, 18, 32$ ).
Atomic radius decreases across a period (increasing $Z_\text{eff}$ ) and increases down a group (adding shells).
Ionisation enthalpy ( $\Delta_i H$ ) increases across a period and decreases down a group; exceptions occur for half-filled and fully filled subshells (e.g., $N > O$ , $Mg > Al$ ).
Electron gain enthalpy ( $\Delta_\text{eg} H$ ) becomes more negative across a period (except noble gases) and less negative down a group; $O$ has a less negative value than $S$ due to electron repulsion in the compact $2p$ subshell.
Electronegativity (Pauling scale) increases across a period and decreases down a group; $F$ is the most electronegative element. …