Periodic Classification of Elements - Explain modern periodic table trends and metallic vs non-metallic properties

Modern Periodic Law: The physical and chemical properties of elements are a periodic function of their atomic number ($Z$). The atomic number represents the number of protons in the nucleus of an atom.

Valency: The valency of an element is determined by the number of valence electrons present in the outermost shell. Across a period, valency increases from $1$ to $4$ and then decreases to $0$. Down a group, valency remains constant because the number of valence electrons is the same.

Atomic Size (Atomic Radius): It is the distance between the center of the nucleus and the outermost shell of an isolated atom. Across a period, atomic radius decreases because the effective nuclear charge ($Z_{eff}$) increases, pulling the electrons closer. Down a group, atomic radius increases because new shells are added, which outweighs the increase in nuclear charge.

Metallic Character (Electropositivity): It is the tendency of an atom to lose electrons. Metals are found on the left side of the periodic table. Metallic character decreases across a period (due to increasing nuclear charge) and increases down a group (due to increasing atomic size and easier electron loss).

Non-metallic Character (Electronegativity): It is the tendency of an atom to gain electrons. Non-metals are found on the right side. Non-metallic character increases across a period and decreases down a group.

Nature of Oxides: Generally, oxides of metals are basic in nature (e.g., $Na_2O$, $MgO$), while oxides of non-metals are acidic in nature (e.g., $SO_2$, $CO_2$).