Which group of elements on the periodic table contains elements with the lowest ionization energy for their period? A) Transition metals, B) alkali metals, C) alkaline earth metals, D) halogens, or E) noble gases.
Now, some of these elements are what we call groups on the periodic table, but others aren’t. So, we can think about the word group more generally as simply collection of elements. The ionization energy of an element refers to the energy required to remove the outermost electron from gaseous single atoms of that element. We start off with a single gaseous atom. Of course, most elements aren’t actually stable as single atoms in the gas state at room temperature. So, ionization energies have to be calculated in other ways and then adjusted so that we get the right value for this process.
So, theoretically, we’re putting in just enough energy into the single gaseous atoms to form the singly positive ion. And the electron being removed is the outermost least stable electron. Now, let’s have a look at where our groups of elements sit on the periodic table. There are many schools of thought about exactly what constitutes a transition metal. But the definition I’m going to use is an element with one or more possible ions with unfilled d orbitals. So, roughly speaking, these are the boundaries of the transition metals. Although we could include the lanthanides and actinides as they are sometimes referred to as the inner transition metals. However, as we’ll come to see, the precise boundaries of the transition metals don’t change the outcome of the question.
Next, we have the alkali metals, which is the other name for the metals in group one of the periodic table. Hydrogen commonly placed at the top of group one is not a metal, so it’s not an alkali metal. Next, we have the alkaline earth metals in group two, which contains beryllium, magnesium, calcium, and a few others. For our next group, we travel all the way over to the right of the periodic table, group 17, the halogens, fluorine, chlorine, bromine, and so on. And lastly, we have the noble gases, group 18 containing helium, neon, and so on. Now that we know where these groups of elements are, we can start to think about what factors influence the ionization energy of an element.
We’re looking for the group of elements where if we took each individual member of the group, it would have the lowest ionization energy for its period. Without too much thinking about it, we can easily dismiss the transition metals. The transition metals are the only group of elements that we’ve been given that span across a period. We can see the top of the transition metal group is eight elements wide. It simply isn’t possible for all eight elements to have the lowest ionization energy for their period.
However, all the other groups only have one element per period. So, for instance, if we take period four, we’d be looking at one alkali metal, one alkaline earth metal, one halogen, and one noble gas. To figure out the answer, we need to think about the trends that might affect the ionization energy as we move across a period. As we move left to right across a period, the atomic number of the elements increases. So, the number of protons in atoms of that element will be going up. And the positivity of their nuclei will be increasing.
However, for all the atoms of the elements in the same period, the outer electron will be in the same shell. For example, in period four, the alkali and alkaline earth metal have their electrons removed from the 4s subshell. And the halogen and the noble gas have their electron removed from the 4p subshell. The eagle-eyed among you might have remembered that we actually populate the 3d subshell with the first row transition metals. But when we remove electrons, we take them from the 4s subshell. So, by the end, what it all boils down to is that as we move left to right across a period, the ionization energy increases.
The trend isn’t perfect, but it is pretty reliable at either end of the periodic table. So, to the far right, the noble gases have the highest ionization energies for their periods. And all the way over to the left, the alkali metals have the lowest ionization energies for their period. Therefore, out of these five groups of elements on the periodic table, the only one that contains elements with the lowest ionization energy for their period is the alkali metals.