Answer:
Found the answer on quora by Eric Ressner
Explanation:
So a chlorine atom, atomic number 17:
The nucleus has a charge of +17
The electron configuration is 1s2 2s2 2p6 3s2 3p5.
The outer shell is #3. If you count all the other electrons (in shells #1 and #2), you find 10. Ten electrons have a combined charge of -10.
So the effective nuclear charge = +17 + (-10) = +7.
