Question

The atomic number of an element is more important to the chemist than its relative atomic mass. Why?

Answer 1

The smallest unit of ordinary matter that makes up a chemical element is an atom. Atoms that are neutral or ionised make up every solid, liquid, gas, and plasma. Atoms are very tiny, measuring around 100 picometers in diameter. Due to quantum phenomena, it is impossible to correctly anticipate their behaviour using traditional physics—as if they were tennis balls, for example.

Complete answer:
The number of protons in the nucleus of every atom of a chemical element is known as the atomic number or proton number (symbol Z). A chemical element's atomic number is the only way to identify it. It is the same as the nucleus' charge number. The atomic number is also equivalent to the number of electrons in an uncharged atom.
An atom's mass number A is determined by multiplying its atomic number Z by the number of neutrons N. Because protons and neutrons have nearly the same mass, and the mass defect of nucleon binding is always small in comparison to the nucleon mass, any atom's atomic mass is within 1% of the whole number A when expressed in unified atomic mass units (creating a quantity called "relative isotopic mass").
Because of the amount of electrons present in the neutral atom, which is Z, each element has a unique set of chemical characteristics (the atomic number). The quantum mechanics laws dictate the arrangement of these electrons. The amount of electrons in each element's electron shells, especially the outermost valence shell, determines its chemical bonding behaviour. As a result, the chemical characteristics of an element are determined only by its atomic number; as a result, an element may be described as any mixture of atoms with a particular atomic number.

Note:
Atomic numbers are commonly used to describe the search for new elements. All elements with atomic numbers 1 to 118 have been discovered as of 2021. New elements are produced by hitting target atoms of heavy elements with ions until the sum of the atomic numbers of the target and ion elements matches the atomic number of the new element. In general, as the atomic number grows, the half-life of a nuclide decreases, however undiscovered nuclides with specific "magic" numbers of protons and neutrons may have longer half-lives and form a stable island.