The most stable phase of a substance at any given temperature is the phase with the lowest

free energy

Under different conditions of temperature and pressure, the physical states of a substance changes and these variations can be graphically illustrated through phase diagram. As we cross the lines or curves on the phase diagram, a phase change occurs.  At low temperature, enthalpy dominates and solids has the lowest enthalpy. When temperature is high, entropy dominates and gases has the highest entropy.

Free energy is characterized by both enthalpy and entropy. The balance between enthalpy and entropy is controlled by the temperature and spontaneous changes always lead to a lower overall free energy.

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explanation:

covalent bonds

consist of pairs of electrons shared by two atoms, and bind the atoms to fixed orientation and requre high energies to break them

ionic bonds

when two atoms differ considerably in their electronegativity, then one of the atoms will lose its electron to the other atom. this results in a positively charged ion(cation) and a negatively charged ion (anion)

periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.