What determines the valence of an atom?

The valence of an atom is determined by the number of electrons in its outermost shell, also known as the valence shell. These outer electrons are crucial in chemical bonding because they are involved in the formation of bonds with other atoms. Atoms tend to seek stability by achieving a full outer shell, which typically contains eight electrons for most elements (known as the octet rule).

When atoms have one or more electrons in their outermost shell, they may readily lose, gain, or share these electrons to bond with other atoms, thereby influencing their reactivity and the types of bonds they can form.

The other aspects mentioned, such as the total number of protons, atomic weight, and the number of neutrons, do not determine the valence. Protons affect the atomic number and identity of the element, atomic weight considers both protons and neutrons, and neutrons contribute to the mass but do not influence the bonding nature of the atom. Hence, the number of electrons in the outermost shell is the key factor in determining an atom's valence.