Atomic Theory Foundations and Solidification Principles

Dalton's Atomic Theory

Matter is composed of small, separate, and indivisible particles called atoms (from the Greek term meaning indivisible).

Key postulates include:

• Atoms of the same element are identical in mass and other properties.
• Atoms of different elements are distinguished by their different masses and properties.
• Compounds are formed by atoms of different elements combining in fixed, whole-number ratios.
• Chemical reactions involve the rearrangement, combination, or separation of atoms.
• Dalton referred to the smallest unit of a compound as a "compound atom" (what we now call a molecule). These molecules are formed by the union of atoms from different elements and possess characteristic properties, such as mass, which are consistent for all molecules of that compound and distinct from the molecules of other compounds.

Thomson's Atomic Model

J.J. Thomson demonstrated the existence of tiny, negatively charged particles that originated from within atoms. These particles were named electrons.

According to Thomson's model (often called the "plum pudding" model), atoms were envisioned as solid spheres of positively charged matter with electrons embedded within them, ensuring the atom was electrically neutral overall.

The Process of Solidification

Solidification begins with a substance in the liquid state, where particles move past each other but remain relatively close due to intermolecular forces.

To induce solidification, heat must be removed from the liquid. This causes the particles to lose kinetic energy, move more slowly, and consequently, the temperature of the liquid decreases.

As heat removal continues, the temperature keeps falling until it reaches a specific point. At this temperature, the particles move so slowly (possess very little kinetic energy) that the forces of attraction between them compel them to settle into fixed positions within a structured lattice. In these positions, particles can only vibrate.

Once particles arrange into this fixed structure, the substance is in the solid state.

During the phase transition from liquid to solid, although heat is continuously removed, the temperature remains constant. This is because the energy being removed (latent heat of fusion) is associated with the formation of bonds in the solid structure, not with changing the kinetic energy of the particles remaining in the liquid phase. The average kinetic energy of the particles transitioning into the solid state decreases, while the average kinetic energy of the particles still in the liquid phase remains constant at the freezing point.

Solidification is, therefore, an isothermal process, meaning it occurs at a constant temperature. This specific temperature is known as the freezing point, which is characteristic for each pure substance and is identical to its melting point.

We can define the melting point (or freezing point) of a pure substance as the constant temperature at which the substance undergoes both melting (solid to liquid) and solidification (liquid to solid).