The atomic number is more important than atomic mass in determining chemical properties because it is directly related to the fundamental characteristics of an atom and its behavior. The atomic number represents the number of protons in the nucleus of an atom, which determines its identity as a particular element. In the periodic table, elements are arranged in order of increasing atomic number.
The number of protons in an atom's nucleus determines its electronic structure and the arrangement of its electrons in energy levels or shells. These electrons are responsible for the chemical behavior of an atom because they participate in chemical bonding and interactions with other atoms. The arrangement of electrons determines an atom's valence electrons, which are crucial for forming chemical bonds and determining the reactivity of an element.
Atomic mass, on the other hand, represents the average mass of all the isotopes of an element, taking into account the mass and abundance of each isotope. While atomic mass is important in certain calculations, such as determining molar masses or isotopic abundance, it does not directly dictate an element's chemical behavior. Different isotopes of an element have the same atomic number and identical chemical properties because they have the same number of protons and electrons, even though they may have different masses.
In summary, the atomic number determines an element's identity and the arrangement of its electrons, which directly influences its chemical properties. Atomic mass, while significant in other contexts, does not play a primary role in determining an element's chemical behavior.