Which particle is primarily responsible for the chemical properties of an atom?

The particle primarily responsible for the chemical properties of an atom is electrons. Electrons occupy electron shells or energy levels around the nucleus of an atom and are involved in chemical bonding. The arrangement and number of electrons in the outermost shell, known as the valence shell, determine how an atom interacts with others, including the types of bonds it can form (ionic, covalent, metallic) and its reactivity.

For instance, atoms with a full valence shell are typically stable and less likely to react, whereas those with unpaired electrons are more reactive. This behavior is crucial in determining an element's placement on the periodic table, where elements in the same group have similar chemical properties due to having the same number of electrons in their outermost shell.

In contrast, neutrons and protons, while essential for forming the atomic nucleus and determining the overall identity and mass of an atom, do not directly influence its chemical behavior. The mass number itself, which is the total number of protons and neutrons in an atom's nucleus, does not determine chemical reactivity or bonding. Thus, electrons are integral to understanding the chemical properties of an atom.