Question

Why do ions form? What makes an octet special that an atom like chlorine will hold an extra electron when it is electrically neutral with 7 electrons in its valence shell?

Answer 1

A chlorine atom with 7 valence electrons is electronically neutral, but chemically reactive. In other words, even though the 17 protons(+) balance the 17 electrons(--) charge-wise, chlorine atoms are hardly ever observed alone due to the reactivity inherent to an atom with 7 valence electrons. Eight outer electrons (or a complete outer shell) is much preferred arrangement, and a net gain of an electron will make a chloride 1- anion. The original source of this transferred electron will become positively charged, and this new +/- electrostatic force will keep the chloride ion stable.