The equivalence point is when the concentration of H⁺ in solution is equal to the concentration of OH⁻ in solution. Since H⁺ and OH⁻ react with each other to make water (H⁺(aq)+OH⁻(aq)→H₂O(l)) the pH at the equivalence point is 7 due to everything being neutralized. (The equivalence point only has a pH of 7 when a strong acid is being titrated with a strong base).
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A subshell which is 100% full or 50% full is more stable than subshells which are partially filled with a number of electrons less than or greater than half the number of electrons which can be held by the subshell.
