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Periodic Trends: Atomic Radius

Practice Periodic Trends: Atomic Radius
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Periodic Trends


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Note: This image does not have all the trends. However, it does contain many of them to get an idea of the trends. [Figure1]

The wonderful thing about the Periodic Table is that it's periodic! There are many patterns that can be found within the periodic table that can help understand the chemistry and interactions of different elements at a glance. These trends reduce the need of memorization of these characteristics; all that a student needs to do is understand how these trends work. A lot of them have to do with how the electrons are acting.

Atomic Radius

Atomic Radii are important because it tells us the sizes of the atoms in question. The definition of an Atomic Radii is the distance from the nucleus to the outer orbital; unfortunately the outer orbital's boundaries are varying. How is the Atomic Radius then found or determined?

The Trends

The Atomic Radius decreases left to right, (there are some exceptions).Why do you think the trend works that way? Answer before you go on.

This is because the electrons on the outer most orbital are pulled in tighter since there are more protons added (having a larger net positive charge) while the electrons are in one shell even though they are increasing in number as well.

The Atomic Radius increases going down. This is because the electrons are now added into a whole new orbital. New orbitals are farther and farther away from the nucleus.

Consider the image above. Why is there a large jump in Atomic Radius every now and then? What does it signify? 

The answers can be found here.

Ionization Energy

When talking about Ionization Energy (IE) only the first IE is usually talked about unless otherwise stated. IE is the energy it takes to remove one electron from the atom. What is the unit of measurement for IE? 

The Trends

IE increases from left to right. This is also related to the reason the Atomic Radius decreases going left to right. While protons are added, the effective proton pull becomes stronger, thus more energy is needed to overcome this attractive force. Knowing this, what is the trend going down a group and why it is like that?

Answers can be found here.

Electron Shielding

The Ionization energy is highly dependent on Electron Shielding. The electrons closer to the center block the nucleus's positive attraction on the outer most electrons, allowing the election to be removed easily. What is Electron Shielding? Give an example of how this affects an IE of an atom. 

The answers can be found here.

Electron Affinity

Electron Affinity is basically the amount of energy put out when an electron is added to an atom. Electron Affinity is written as a negative value. Why is Electron Affinity written as a negative number?

An important concept to remember here (and in electronegativity) is the Octet Rule. This is important because the atom is most stable with eight valence electrons. That is why the noble gasses are so stable. 

An example to connect the two concepts: Fluorine (F) has seven electrons so it will gladly except an extra electron, thus it will have a high (more negative) Electron Affinity. On the other hand, Sodium (Na) has one valence electron, it will more easily loose that extra electron than accept seven new ones, thus it has a low (less negative) Electron affinity

The trends

This does not have a steady fast rule, but there is a pattern. Electron Affinity becomes more negative going left to right (it becomes easier to add an electron). They become less negative going down. There are many exceptions.

Give a few examples of these exceptions
Answers may be found here.

Ionic Radii

Just like there is an Atomic Radius there is an Ionic Radius. What is an ion? Does a positive ion loose or gain an electron(s), what about a negative ion? What is a positive ion called? A negative ion? (click on the blue "ion" to check your answers). As one would guess, the ionic radii are the radii of ions. 


When an ion loses an electron it becomes smaller. This is because when there are less electrons the proton pull is less divided and each electron can be pulled in tighter. The ion becomes even smaller if it an orbital completely disappears after losing an electron. 


When an ion gains an electron it becomes larger. This is because when there are more electrons the proton pull is now less powerful on the electrons (electron shielding is in play here) and thus the radius becomes larger. It becomes even larger if the gained electron is in a whole new orbital. 

License: CC BY-NC 3.0

A graphical table of some of the ions and its parent. [Figure2]

Look at this image. Are you able to explain why the ion is larger or smaller than the parent and if it is a cation or an anion?

Answers may be found here.


Electronegativity is how much an atom "wants" an electron. This is very related to the valence electrons of any given atom. 

Note: Do not confuse this with electron affinity. Electron affinity is the actual amount of energy released while electronegativity is based on a relative scale. 

Which is the most electronegative element on the periodic table?

As mentioned before, the octet rule is important here too. 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 IE 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 loose it willingly. Because of this, its electronegativity (the measure of how much the atom wants to gain an electron) is one of the lowest. Explain why the element you said was the most electronegative is how it is. Why is it so electronegative?

The Trends

Electronegativities increase going left to right. This means the atom more strongly attracts extra electrons.
Electronegativites decrease going down. This means the atom does not attract extra electrons as strongly. 

Can you describe why the trends are the way they are?
Answers can be found here.

Metallic and Nonmetallic Character

Everyone is familiar with metals, but some elements are more metallic than others. Metallic character is the measure of how reactive a metal is. Metals tend to lose elections very easily and do not attract elections that much. So elements that have high electronegativities and IEs are the most nonmetallic. 

Using the knowledge of the periodic trends above, what is the basic trend of metallic and nonmetallic characteristics?
Which elements are the most metallic, the least?

Answers can be found here.

More reading and information can be found here.

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