Ionization Energy

Ions are charged atoms. Ionization energy is how much energy is involved in creating an ion specifically a positive one. At a more basic level ionization energy measures how easily an atom will give up one or two of its electrons.

Coulomb’s Law Yet Again

You are probably noticing a theme here and are getting tired of seeing Coulomb’s Law. Electrostatics really govern all of the electron-based properties and I promise that I will stop referencing it when talking about further periodic trends. We will shift instead towards thinking about ionization energy and the remaining trends via our analogy for Z_eff.

Weightlifting Revisited

As a quick review. Imagine that the protons represent the physical strength of the nucleus. Elements with more protons are like competitive weightlifters while atoms with fewer protons are like me, fairly wimpy when it comes to lifting heavy things.

Regardless of your strength the further you have to reach the harder it is to pull on that object. So electrons that are further away from the nucleus are harder to hold onto as well.

Lastly, anything that gets in the way of grabbing an object makes it more difficult to pull on as well. Imagine if you had to reach through a vat of honey to grab an object versus grabbing it out of the air. Other electrons inside of the valence electrons act like honey diminishing the attractive force that the nucleus can exert on them.

Holding On

Atoms that have a tight grip on their electrons are going to have a tough time giving them up. This means that bigger nuclei have higher ionization energies because it takes more energy to wrench an electron out of their iron grip. Therefore as we progress from left to right on the periodic table ionization energy increases.

Atoms that have to reach further have a harder time holding on to their electrons. More shells means more distance and more shielding. Therefore elements near the bottom of the periodic table have a harder time holding onto their electrons and lower ionization energies. While elements at the top have an easier time holding onto their electrons since they don’t have to reach as far nor through a giant vat of honey.

It’s About Stability Too

If ionization energy were purely based on our analogy we would predict that like Zeff, ionization energy increases as we go up and to the right. Generally speaking, this is true, however, this isn’t the whole story. Atoms want to be their best, most stable selves. For electrons, this means having fully filled shells like the noble gases (group 8), fully filled subshells like the alkaline earth metals (group 2), or half-filled subshells as nitrogen does.

As a result, we are going to see some dips and peaks in our trend. If you are in one of the stable configurations as listed above you will have a higher ionization energy. If you are an element to the right of one of these stable configurations you are going to have a lower ionization energy. Since all they have to do is lose one electron to get to a more stable place.

Even though we see these small fluctuations we should treat the general trend ionization energy increasing up and to the right as correct.

First Ionization Energy

Up to now, I have been talking about first ionization energy or how much energy it takes to lose the first valence electron. The elements with the lowest first ionization energy include those in the first group. As a result, they tend to easily form cations and they exist as such all the time. Examples include Na⁺, K⁺, Li⁺, and H⁺.

This means that if a question asks us to determine the element with the lowest first ionization energy we can look for group 1 elements first. Then we can use our trend if more than one shows up. Conversely, if a question asks for an element with the highest ionization energy we can immediately start looking for nobles gases or group 8.

Second Ionization Energy

Second ionization energy, as the name suggests, refers to how much energy it takes to lose a second valence electron. Here the trend is of lesser importance. What we need to understand is whether or not losing a second valence electron makes you more or less stable. For example, a Na⁺ ion has an electron configuration of a noble gas. It is its best, most stable self. As a result, it really doesn’t want to lose another electron. Thus Na⁺ has a really high second ionization energy.

Ca⁺ on the other hand is really close to having a noble gas configuration it just needs to lose one more electron. By comparison, it’s second ionization energy is quite low. This is why we see the alkaline earth metals (group 2) frequently carrying 2+ charges. Think Ca²⁺, Mg²⁺, Be²⁺, and Sr²⁺.

This means that if a question asks us to determine the element with the lowest second ionization energy we can look for group 2 elements first. Then we can use our trend if more than one shows up. Conversely, if a question asks for an element with the highest ionization energy we can immediately start looking for nobles gases (group 8) or other elements with identical configurations (Na⁺).