Which concept describes why atoms can combine to form molecules?

The concept that describes why atoms can combine to form molecules is chemical bonds. Chemical bonds are the forces that hold atoms together within a molecule. There are different types of chemical bonds, such as covalent bonds, where atoms share electrons, and ionic bonds, where one atom transfers electrons to another, resulting in charged ions that attract each other. These interactions allow atoms of different elements to combine and form stable structures, known as molecules, which are essential for the formation of compounds and ultimately the chemistry of life.

The other options do not pertain to the principle of atomic combination in the same way. Electrolysis refers to a process involving the breakdown of a substance using an electric current, which is not about the combination of atoms. Isotope variations relate to differences in the number of neutrons within atoms of the same element, affecting their stability and properties rather than how they combine. Phase changes describe transitions between different states of matter (solid, liquid, gas), which are unrelated to the chemical bonding process.