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## I. Microscopic Structures of Zinc (Three Hierarchical Structures)
### 1. Atomic Structure
(1) Basic parameters: Zinc is an element in Group IIB zinc subgroup of the 4th Period with an atomic number of 30 and a relative atomic mass of 65.38. Its valence electron configuration is 3d¹⁰4s²; all inner orbitals are fully filled, with only two 4s electrons in the outermost shell.
(2) Ionization characteristics: Zinc loses two outer electrons to form Zn²⁺ ions, which have an ionic radius of merely 0.083 nm and an 18-electron outer shell configuration, endowing Zn²⁺ with prominent polarizing power among divalent cations.
### 2. Crystal Structure
Zinc adopts a hexagonal close-packed (hcp) crystal structure. At 25°C, its lattice constants are a = 0.2665 nm and c = 0.4947 nm, with an axial ratio c/a = 1.856, higher than the theoretical value 1.633 for ideal hexagonal close packing. The larger atomic spacing along the c-axis exerts impacts on the mechanical properties of zinc.
### 3. Ionic Structure
The small radius and 18-electron outer shell structure of Zn²⁺ grant zinc ions stronger polarizing power than most divalent cations, which fundamentally governs the chemical reaction characteristics of zinc.
## II. Thermodynamic Properties of Zinc (Four Indicators: Heat Capacity, Entropy, Vapor Pressure and Surface Tension)
### 1. Heat Capacity (Molar Heat Capacity at Constant Pressure, Cp,m)
(1) Gaseous zinc: Its heat capacity remains constant at 20.80 J/(K·mol), unaffected by temperature fluctuations.
(2) Solid zinc: Its heat capacity rises linearly with temperature, with exclusive fitting formulas for two separate temperature ranges. The measured value at room temperature (298 K) is 25.35 J/(K·mol). The heat capacity difference between solid and liquid zinc near the melting point is minimal.
(3) Liquid zinc: Its heat capacity gradually declines as temperature rises, decreasing by approximately 0.8 J/(K·mol) per 100 K temperature increase. For metallic solids, heat capacity rises with heating and peaks near phase transition critical points, ranging from 29 to 33 J/(K·mol).
### 2. Entropy (Entropy of Fusion/Vaporization and Temperature-dependent Entropy Formula)
(1) Phase transition entropy: The enthalpy of fusion is 6700 J/mol, corresponding to an entropy of fusion of 9.65 J/(K·mol), falling within the universal entropy of fusion range for metals (9.6–10.0 J/(K·mol)). At the boiling point of 1180 K, the entropy of vaporization reaches 97.32 J/(K·mol), complying with Trouton's rule.
(2) Temperature-variable entropy: Empirical formulas for entropy calculation are available for solid, liquid and gaseous zinc respectively, applicable to the temperature ranges corresponding to each state of matter.
### 3. Vapor Pressure
(1) Variation pattern: The vapor pressure of zinc rises exponentially with increasing temperature, growing moderately at low temperatures and surging sharply once the temperature exceeds 1023 K. For solid zinc-vapor and liquid zinc-vapor two-phase systems, a unique equilibrium vapor pressure corresponds to each specific temperature.
(2) Calculation methods: The Kelley equation is applied for liquid zinc vapor pressure calculation, while the Barrow-Dodsworth equation is used for solid zinc. Calculation results from these formulas match measured data with high consistency. Vapor pressure serves as a critical parameter for controlling zinc volatilization loss during hot-dip galvanizing.
### 4. Surface Tension
(1) Fundamental rule: Surface tension is determined by metallic bonds (cohesive force). Rising temperature weakens metallic bonds and reduces surface tension: the surface tension of liquid zinc is 0.785 N/m at 783 K, dropping to 0.761 N/m when heated to 913 K.
(2) Application value in galvanizing processes: Reducing the surface tension of molten zinc improves its wettability on steel substrates and minimizes missing plating defects. Surface tension can be regulated by temperature control and trace alloy element addition.
## III. Full Set of Key Physicochemical Parameters of Zinc
1. Density: 7.14 g/cm³ for solid zinc at room temperature; 6.83 g/cm³ for liquid zinc at the melting point, and 6.25 g/cm³ for molten zinc at 800°C.
2. Phase transition temperatures: Melting point 692.5 K (419.5°C), boiling point 1180 K (907°C). Phase transition latent heats: heat of fusion 6.7×10³ J/mol, heat of vaporization 1.15×10⁵ J/mol.
3. Thermal conductivity: Solid zinc possesses superior thermal conductivity to liquid zinc, and the thermal conductivity of zinc gradually decreases as temperature increases.
4. Thermal expansion: The linear expansion coefficients along different axes of hexagonal crystals differ drastically, with far greater expansion along the b-axis than the a-axis.
## IV. Practical Summary for Hot-dip Galvanizing Processes
1. Microscopic perspective: The strong polarizing power of zinc ions and the special axial ratio of hexagonal crystals determine the corrosion protection performance of zinc coatings and the internal stress level between substrate and coating.
2. Thermodynamic perspective: Heat capacity and vapor pressure data are adopted to predict heat loss and zinc fume volatilization of high-temperature molten zinc, supporting furnace heating design and flue gas environmental governance.
3. Surface tension acts as a core optimization indicator for processes: Adjusting molten zinc temperature and alloy formulas optimizes coating appearance and reduces missing plating defects. Linear expansion coefficients and density parameters can be used to analyze deformation and coating cracking risks of galvanized workpieces during cooling.