Rank these elements according to first ionization energy.
Highest to lowest.
Ar, Na, Al, P, Cl, Si, S, Mg
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The first ionization energy shows periodicity within a period (row). It depends on the number of electrons between the outer electron and the nucleus due to the shielding effect. An electron close to the nucleus will be much more strongly attracted than one further away. It means the attraction falls off rapidly with distance. Another factor is that the more protons there are in the nucleus, the more positively charged the nucleus is, and the more strongly electrons are attracted to it. I know it gets confusing. Just remember a general rule. The first ionization energy generally increases from LEFT to RIGHT within a row or period, and that the nobel gases have the highest ionization energies because of their complete octet.
So the order here will be Na, Mg, Al, Si, P, C, Cl and Ar
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The hint given on my Sapling Learning homework when I got it wrong by entering the common trend answer (Ar, Cl, S, P, Si, Al, Mg, Na) was something along the lines of “ns^2 is full and somehow has more ionization energy. ns^2p^3 also has more IE.” Trend base is halogen (highest) and alkaline (lowest) with these exceptions. Mg is the full ns^2 and P is the ns^2p^3. Not sure how to figure these out without an IE chart.
Correct IE order (highest to lowest): Ar, Cl, P, S, Si, Mg, Al, Na
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