# How would you arrange the following elements in order of increasing ionization energy: Te, Pb, Cl, S, Sn?

Oct 31, 2015

Pb, Sn, Te, S, Cl

#### Explanation:

The ionization energy increases across a period but decreases down a group.

This means that the elements with the lowest ionization energies would be in the bottom left-hand corner of the periodic table. The change in ionization energies is also bigger going down the periodic table (by change within a group) than going across the periodic table (by change within a period).

So let's start from the bottom of the periodic table:
$P b$ is the element that is in the lowest period at 6 (and lowest group at 14) in the periodic table; it's the smallest ionization energy.

The period above (5) has two of the elements: Sn and Te. Well, since ionization energy increases across a period, Sn will have a smaller ionization energy than Te.
$P b , S n , T e$

Now, let's go to the third period, where $S$ and $C l$ are. Since $S$ is before $C l ,$ $S$ has a lower ionization energy than $C l$.
$P b , S n , T e , S , C l$

Nov 15, 2016

The order is $\text{Sn < Pb < Te < S < Cl}$.

#### Explanation:

You have learned that ionization energy increases from top to bottom and from left to right in the Periodic Table.

You probably saw a diagram something like this.

Here's the portion of the Periodic Table that includes the elements in this question.

You would naturally predict the order to be

$\text{Pb < Sn < Te < S< Cl}$

This is almost correct, but the correct order is $\text{Sn < Pb}$, as shown in the image below.

Why is this so?

The electron configuration of $\text{Sn}$ is ${\text{[Kr] 5s"^2 "4d"^10 "5p}}^{2}$.

The electron configuration of $\text{Pb}$ is ${\text{[Xe] 6s"^2 "4f"^14 "5d"^10 "6p}}^{2}$.

The $\text{4f}$ electrons in $\text{Pb}$ are poor at shielding the outermost electrons.

Thus the outer electrons experience a greater effective nuclear charge, and it is more difficult to remove them.

Hence $\text{Pb}$ has a higher ionization than $\text{Sn}$, and the correct order is $\text{Sn < Pb < Te < S < Cl}$.