# Compare and contrast the **first** ionization energies of K and Ca? Compare and contrast the **second** ionization energies of K and Ca?

Jan 22, 2018

$\text{Ca}$ has a higher first ionisation energy than $\text{K}$.

${\text{K}}^{+}$ has a higher ionisation energy than ${\text{Ca}}^{+}$, so $\text{K}$ has a higher second ionisation energy than $\text{Ca}$.

#### Explanation:

There are three factors which decide how easily an electron can be removed, in this order of priority:

1. Number of shells (distance from the nucleus in effect)
2. Effect of shielding
3. Nuclear attractive force from protons

First ionisation

Both $\text{K}$ (potassium) and $\text{Ca}$ (calcium) are in Period 4.
To ionise once, we must remove one electron from these two metals.

$\text{K}$ is in Group 1. It, therefore, has one electron in its outer shell (in the $s$ orbital). $\text{Ca}$ is in Group 2, and has two electrons in its outer shell (again both in the $s$ orbital).

$\text{Ca}$ has the same number of shells as $\text{K}$, a similar amount of shielding, but more protons in the nucleus. This means there is a stronger attraction between the nucleus and the electron to be removed, meaning more energy is required to remove it (moving it to a potential of $\text{0 eV}$).

$\text{Ca}$ has a higher first ionisation energy than $\text{K}$.

In the case of $\text{K}$, this leaves it with a full third shell, as a ${\text{K}}^{+}$ ion. For $\text{Ca}$, it becomes a ${\text{Ca}}^{+}$ ion, with one electron still remaining in the 4th shell.

Second ionisation

In this case, we are ionising the ${\text{K}}^{+}$ ion and the ${\text{Ca}}^{+}$ ion. These are formed after the first ionisation.

${\text{K}}^{+}$ has the electron configuration $\left[\text{Ar}\right]$. This means it has three full shells. ${\text{Ca}}^{+}$ has the same electron configuration as an atom of $\text{K}$, i.e. $\left[\text{Ar}\right] 4 {s}^{1}$.

${\text{Ca}}^{+}$ has more shells and shielding than ${\text{K}}^{+}$. This outweighs the fact that ${\text{Ca}}^{+}$ has more protons in the nucleus, meaning means there is a weaker attraction between the nucleus and the electron to be removed, meaning less energy is required to remove it.

${\text{K}}^{+}$ has a higher ionisation energy than ${\text{Ca}}^{+}$, so $\text{K}$ has a higher second ionisation energy than $\text{Ca}$.

Hope this helped :)