Introduction
Valence electrons are the electrons in the outermost shell of an atom that are involved in chemical bonding. They determine the chemical properties of an element and are responsible for the formation of chemical bonds. Various factors can influence the behavior of valence electrons, which in turn affects the chemistry of an element. However, one of the factors mentioned in the options does not directly affect the valence shell.

Explanation
Let's analyze each option to understand how it influences the valence shell:

a) Valence principal quantum number (n)
The valence principal quantum number (n) represents the energy level or shell in which the valence electrons reside. As the value of n increases, the distance of the valence electrons from the nucleus also increases. This leads to a decrease in the attraction between the valence electrons and the nucleus, resulting in weaker bonding and a larger atomic size. Therefore, the valence principal quantum number (n) directly affects the valence shell.

b) Nuclear charge (Z)
The nuclear charge (Z) refers to the number of protons in the nucleus of an atom. It determines the attractive force experienced by the valence electrons. As the nuclear charge increases, the attraction between the valence electrons and the nucleus becomes stronger, resulting in a smaller atomic size and stronger bonding. Consequently, the nuclear charge (Z) directly affects the valence shell.

c) Nuclear mass
The nuclear mass, or the mass of the nucleus, does not directly influence the valence shell. The valence shell is mainly determined by the arrangement of electrons around the nucleus, while the nuclear mass refers to the total mass of the protons and neutrons in the nucleus. Although the nuclear mass indirectly affects the atom's properties, it does not directly influence the behavior of the valence shell.

d) Number of core electrons
Core electrons are the electrons in the inner energy levels or shells of an atom. They are not involved in chemical bonding and do not participate in reactions. The number of core electrons directly affects the size of the atom but does not directly influence the behavior of the valence shell and its electrons.

Conclusion
Out of the given options, the nuclear mass (option C) does not directly affect the valence shell. The valence principal quantum number (n), nuclear charge (Z), and number of core electrons all play essential roles in determining the behavior and properties of the valence shell and its electrons.