(ATPT) Trends in Atomic Radius Lesson

Trends in Atomic Radius

Have you ever lost your phone? Ever wonder why something you cooked did not taste quite right? Do you try to win at video games? These are some questions you may come across in your day to day life. Did you know you can use the scientific method to help you with these questions and more?

Group Trend

First, let's look at the size of the atom from the top of a group to the bottom of a group. For example, in Group 1 metals each of the elements have the same number of valence electrons but as you go down the table each row (period) represents an additional energy level. Look at the picture below to see the drawings of the atoms in Group 1.  

As you can see from the picture, that with each additional energy level the size of the atom is larger. Therefore, the atomic radius of atoms generally increases from top to bottom within a group.

Period Trend

In order to determine the trend for the periods, we need to look at a couple of different factors. Let's look at period 2 to determine and explain the periodic trend of atomic radii. As we go across the periodic table from left to right, the atomic number increases. However, the number of energy levels remain the same. All elements in period 2 have 2 energy levels. Therefore, with each additional proton there is an additional electron on that same energy level. As more electrons are added to the energy level, there is an increase in the pull between protons and the outer level of electrons Since the force of attraction between nuclei and electrons increases, the size of the atoms decreases. The trend as you go across the periodic table from left to right is a decrease in atomic radii.

Considering all the information about atomic size, you will recognize that the largest atom on the periodic table is all the way to the left and all the way to the bottom, francium, #87, and the smallest atom is all the way to the right and all the way to the top, helium, #2.

The fact that the atoms get larger as you move downward in a family is probably exactly what you expected before you even read this section, but the fact that the atoms get smaller as you move to the right across a period is most likely a big surprise. Make sure you understand this trend and the reasons for it.

Trends in Ionization Energy

Ionization energy is the energy required to remove an electron from a specific atom.  The lost electron is always a valence electron because the electrons in the outermost principal energy level are farthest from the nucleus. The ionization energies of various elements are influenced by the size of the atom, the nuclear charge, and the electron energy levels.

Group Trend

The ionization energies of the representative elements generally decrease from top to bottom within a group. This trend is explained by the increase in size of the atoms within a group. The valence electron that is being removed is farther from the nucleus in the case of a larger atom. The attractive force between the valence electron and the nucleus weakens as the distance between them increases, resulting in a lower ionization energy for the larger atoms within a group.

Period Trend

Moving from left to right across the periodic table, the ionization energy for an atom increases.  We can explain this by considering the nuclear charge of the atom. The more protons in the nucleus, the stronger the attraction of the nucleus to electrons. This stronger attraction makes it more difficult to remove electrons. 

Trends in Electronegativity

Electronegativity is how much an atom "wants" an electron. This is very related to the valence electrons of any given atom. For example, Fr (Francium) has one valence electron in seventh period; it is very far away from the nucleus. Since it is so far away, its Ionization Energy is already so low and the atom will be most stable with eight valence electrons. Since it is so much easier to lose the electron (to go down one orbital) rather than gain seven more, it does lose it willingly. Because of this, its electronegativity (the measure of how much the atom wants to gain an electron) is one of the lowest.  

Since metals have few valence electrons, they tend to increase their stability by losing electrons to become cations. Consequently, the electronegativities of metals are generally low. Nonmetals have more valence electrons and increase their stability by gaining electrons to become anions. The electronegativities of nonmetals are generally high.

Group Trend

The electronegativity of atoms decreases as you move from top to bottom down a group in the periodic table. This is because as you go from top to bottom down a group, the atoms of each element have an increasing number of energy levels.    

Period Trend

Electronegativity increase going left to right. This is because as you go from left to right across a period, the atoms of each element have the same number of energy levels. However, the nucleus charge increases, so the attraction that the atoms have for the valence electrons increases.

Summary

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