Question

Arrange the following atoms in order of decreasing ionization energy? `"Cl"`, `"S"`, `"Se"`

Answer 1

`overbrace("IE"_("Cl"))^("12.968 eV") > overbrace("IE"_("S"))^("10.360 eV") > overbrace("IE"_("Se"))^("9.752 eV")`

Data from NIST.

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You should refer back to the periodic table trends for:

• increasing atomic radii due to new quantum levels
• decreasing atomic radii due to increased effective nuclear charge

The former is a vertical trend and the latter a horizontal trend. [They are gone into more detail here.](https://socratic.org/questions/why-do-the-atomic-radiii-decrease-down-the-period-and-increase-down-the-group) In short:

`color(white)(""^(color(black)"atomic radii increase") color(black)(darr){(stackrel(color(black)("atomic radii decrease"))(overbrace(stackrel(" ")stackrel(" ")color(black)"Li"" "color(black)"Be"" "color(black)(cdots)" "" "color(black)"F"" ")^(color(black)(->)))),(color(black)"Na"),(color(black)(vdots)),(color(black)"Fr") :})`

Knowing that, we refer to the periodic table:

![http://ptable.com/]()

Since `"Cl"` is to the upper-most-right, and `"Se"` is to the lower-most-left, `"Cl"` is smallest and `"Se"` is the largest in atomic radius. `"S"` is intermediate in atomic radius. That is,

`r_("Se") > r_("S") > r_("Cl")`

The smallest atom holds onto its valence electrons most tightly.

Therefore, `"Cl"` is hardest to ionize, and `"Se"` is easiest to ionize. As ionization energy is large for atoms that are difficult to ionize, we have:

`color(blue)(barul(|stackrel(" ")(" ""IE"_("Cl") > "IE"_("S") > "IE"_("Se")" ")|))`