What are the valence electrons of an element responsible for? 

Chemical properties of an element 

The correct answer is option 2. Chemical properties of an element.

Valence electrons are the electrons in the outermost energy level (or shell) of an atom. These electrons are located farthest from the nucleus and are the ones involved in chemical bonding and interactions with other atoms.

The behavior of atoms in chemical reactions is primarily determined by the interactions of their valence electrons. Here's why valence electrons are so important:

1. Chemical Bonding:  Atoms bond with each other to achieve a stable electron configuration, typically by filling or emptying their outermost energy level. Valence electrons participate in the formation of chemical bonds, whether it's through sharing electrons (covalent bonding), transferring electrons (ionic bonding), or a combination of both (polar covalent bonding).

2. Chemical Reactivity:  The number of valence electrons dictates an element's reactivity. Elements with full or nearly full outer shells (e.g., noble gases) tend to be chemically inert because they have little tendency to gain, lose, or share electrons. Conversely, elements with incomplete outer shells (e.g., alkali metals, halogens) are highly reactive as they seek to achieve a stable electron configuration by gaining, losing, or sharing electrons.

3. Chemical Properties: The chemical properties of an element, such as its ability to form compounds, its reactivity with other elements, and its behavior in chemical reactions, are largely determined by the number and arrangement of its valence electrons. Elements with similar numbers of valence electrons tend to exhibit similar chemical properties.

In summary, valence electrons play a crucial role in determining the chemical properties and behavior of an element. They are responsible for chemical bonding, reactivity, and the formation of compounds, making them essential for understanding the chemistry of elements and compounds.