What symbol would represent a chlorine ion with a stable arrangement of eight valence electrons?

Cl⁻

Explanation:

Elements in the periodic table (with the exception of group 0 elements) want to achieve there octet configuration and as such gain or lose electron(s) in order to achieve that. When an atom loses electron(s), it becomes positively charged and when it gains, it becomes negatively charged.

Chlorine (Cl) has 17 electrons.

₁₇Cl  →  1S² 2S² 2P⁶ 3S² 3P⁵

From the above, it can be deduced that chlorine has 7 (5+2) electrons in its outermost shell and hence needs just one electron to achieve it's octet configuration. When chlorine gains this one electron it requires to achieve it's octet configuration, it becomes negatively charged (as explained earlier). Thus, it is represented as Cl⁻.

nacl or sodium chloride is an ionic compound. this is because an ionic compound pertains to a compound that consists of positively and negatively charged ions. in this case, na (sodium) has a tendency to lose one electron when it reacts with with other atoms, so the resulting sodium ion contains 11 protons and 10 electrons, thus making it positively charged (na+), when chlorine reacts, it usually gains one electron to form an ion called chloride, thus making it negatively charged ( since the ions are oppositely charged, they attract.