The electron configuration for arsenic, As, is \([\mathrm{Ar}] 3 d^{10} 4 s^{2} 4 p^{3} .\) How many valence electrons does an As atom have? Write the symbol for the ion it forms to achieve a noble-gas configuration.

Valence electrons are the outermost electrons of an atom and are crucial in determining its chemical properties and reactivity. These electrons occupy the highest energy levels in an atom's electron configuration and are the ones involved in forming bonds with other atoms.

For instance, in the electron configuration of arsenic, \( [Ar] 3d^{10} 4s^{2} 4p^{3} \), the electrons in the 4s and 4p orbitals are considered valence electrons because they are in the highest principal energy level, which is 4 for arsenic. Counting these up, we find that arsenic has a total of 5 valence electrons (2 in the 4s orbital and 3 in the 4p orbital).

It's important to recognize that the valence electrons are the ones that will interact during chemical reactions, determining how the atom will bond and what ions it might form.

Atoms strive to achieve a noble-gas electron configuration, which tends to be more stable. A noble-gas configuration refers to an electron arrangement similar to that of the noble gases, which are the elements found in group 18 of the periodic table. These gases, such as helium, neon, and argon, have complete valence shells and are chemically inert.

To reach this stable configuration, atoms will lose, gain, or share electrons to complete their outermost shell, often resulting in a filled shell with 8 electrons—also known as an 'octet.' In the example of arsenic, the noble-gas immediately preceding it is argon (Ar). Arsenic achieves a noble-gas configuration by adding 3 electrons to its valence shell, mimicking the electron configuration of the next noble gas, krypton (Kr). Therefore, when arsenic forms an ion to attain this stable state, it becomes an arsenide ion with a notation of As^-3.

The octet rule is a chemical rule of thumb that states atoms tend to bond in such a way that they each have eight electrons in their valence shell, achieving a configuration similar to that of the noble gases. The rule is based on the observation that atoms with eight electrons in their outer shell are unusually stable.

Exceptions to this rule include hydrogen, which is stable with two electrons in its valence shell, and some transition metals, which have more complex valence shells. However, for many atoms, following the octet rule guides them to a noble-gas configuration, which is the most energetically favorable arrangement of electrons.

In tasks involving the octet rule, students often have to predict the ion that an element will form to obtain a complete octet. That's what we see with arsenic: to comply with the octet rule, it has to gain 3 electrons, resulting in a -3 charge, achieving the stable electron configuration of a noble gas.