The chemical properties of an element is determined by the atomic number. Atomic number gives the electronic configuration of atoms of an element, so it helps in finding the valency, number of valence electrons.As the chemical properties depend upon arrangement of electrons, hence chemical properties are determined by atomic number.

Introduction:
The chemical properties of an element are determined by the atomic number, not the atomic mass. The atomic number represents the number of protons in the nucleus of an atom, which is a fundamental characteristic of an element. The atomic mass, on the other hand, is the average mass of all the isotopes of an element and does not directly influence its chemical properties. Let's explore this in more detail.

Atomic Number and Chemical Properties:
The atomic number of an element is a unique identifier that determines its position on the periodic table. It represents the number of protons in an atom, which defines the element's chemical identity. The number of protons determines the electron configuration and the arrangement of electrons in an atom's energy levels. These electrons play a crucial role in chemical reactions and bonding.

Electron Configuration and Chemical Reactivity:
The electron configuration of an atom determines its chemical reactivity. It describes the arrangement of electrons in different energy levels or orbitals around the nucleus. The outermost energy level, known as the valence shell, is particularly important in chemical reactions. The number of valence electrons determines how an atom will interact with other atoms to form compounds.

Valence Electrons and Bonding:
The number of valence electrons is directly related to the group number of an element in the periodic table. Elements in the same group have similar chemical properties because they have the same number of valence electrons. For example, all elements in Group 1 (such as hydrogen, lithium, sodium) have one valence electron, which makes them highly reactive and prone to losing that electron to achieve a stable electron configuration.

Chemical Reactivity and Atomic Mass:
The atomic mass of an element is the average mass of all its isotopes, which may vary due to the different numbers of neutrons in the nucleus. Isotopes of an element have the same number of protons (same atomic number) but different numbers of neutrons. However, the number of neutrons does not significantly affect the chemical behavior of an element. Chemical reactions primarily involve the outermost electrons, which are determined by the atomic number.

Conclusion:
In conclusion, the chemical properties of an element are determined by its atomic number, not the atomic mass. The atomic number determines the number and arrangement of electrons, which play a crucial role in chemical reactions and bonding. The atomic mass, representing the average mass of all isotopes, does not directly influence an element's chemical properties. Understanding the relationship between atomic number, electron configuration, and valence electrons is essential for comprehending an element's chemical behavior.