The main factor affecting the chemical properties of materials is their atomic structure, particularly the arrangement of electrons in atoms. Electron configuration determines how atoms interact with one another, leading to properties such as reactivity, electronegativity, and bonding behavior. For example, the presence of certain functional groups can influence organic compounds' solubility, acidity, and reactivity. Changes in atomic structure can lead to significant differences in the behavior of materials, making understanding these interactions vital in fields such as chemistry and materials science.