The chemical properties of an element refer to its behavior in chemical reactions and its ability to form compounds. These properties are determined by the arrangement and behavior of the electrons in the atoms of the element.

Option 1: The number of isotopes of an element refers to the different forms of the element with varying numbers of neutrons. While isotopes may have slightly different physical properties, they do not have a significant impact on the chemical properties of the element. Therefore, option 1 is not correct.

Option 2: The mass number of an element represents the total number of protons and neutrons in the atom. While the mass number is important for determining the atomic mass of an element, it does not directly affect the chemical properties of the element. Hence, option 2 is not correct.

Option 3: The total number of neutrons in an element also does not directly determine its chemical properties. Neutrons contribute to the stability of the atom, but the chemical behavior is primarily determined by the arrangement and interaction of the electrons. Therefore, option 3 is not correct.

Option 4: The number of electrons in the outermost shell or valence shell of an atom has a significant influence on the chemical properties. The outermost electrons are involved in bonding