Periodicity - Periodic trends

Effective Nuclear Charge ($Z_{eff}$): The net positive charge experienced by valence electrons. It is calculated as $Z_{eff} = Z - S$, where $Z$ is the atomic number and $S$ is the number of shielding electrons. $Z_{eff}$ increases across a period but remains relatively constant down a group.

Atomic Radius: The distance from the nucleus to the outermost electron shell. It decreases across a period due to increasing $Z_{eff}$ pulling electrons closer, and increases down a group due to the addition of new principal energy levels ($n$).

Ionic Radius: Cations (e.g., $Na^{+}$) are smaller than their parent atoms because they lose an electron shell and have increased $Z_{eff}$. Anions (e.g., $Cl^{-}$) are larger than their parent atoms due to increased inter-electron repulsion in the valence shell.

First Ionization Energy ($IE_1$): The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous $1+$ ions. Trend: Increases across a period, decreases down a group.

Electronegativity: A relative measure of the attraction an atom has for a shared pair of electrons in a covalent bond. Trends follow the $Z_{eff}$ pattern, peaking at Fluorine ($F$).

Electron Affinity ($EA$): The energy change when one mole of electrons is added to one mole of gaseous atoms. Values generally become more exothermic (more negative) across a period.

Melting Points: Depend on the type of bonding. In Period 3, melting points increase from $Na$ to $Al$ (metallic bonding strength), peak at $Si$ (giant covalent structure), and then drop for $P_4$, $S_8$, $Cl_2$ (weak London dispersion forces).

Oxide Periodicity: Period 3 oxides transition from basic ($Na_2O$, $MgO$) to amphoteric ($Al_2O_3$) to acidic ($SiO_2$, $P_4O_{10}$, $SO_3$, $Cl_2O_7$).