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Animation Activity: Atomic & Ionic Radii Mark as Favorite (0 Favorites)

ACTIVITY in Periodic Table, Atoms, Subatomic Particles, Atomic Radius, Ionic Radius. Last updated December 12, 2023.


In this activity, students will view an animation that explores atomic and ionic radii. They will look at the different sizes of atoms in the third period and the atoms in the sixth group to see trends across periods and down groups. They will also look at an atom and its corresponding cation as well as an atom and its corresponding anion.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

  • HS-PS1-1: Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
  • HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
  • Scientific and Engineering Practices:
    • Developing and Using Models


By the end of this activity, students should be able to:

  • Identify trends in atomic radius across a period and down a group on the periodic table.
  • Identify patterns in cation and anion formation.

Chemistry Topics

This activity supports students’ understanding of:

  • Atomic radius
  • Ionic radius
  • Periodic trends


Teacher Preparation: minimal
Lesson: 10-30 minutes



  • No specific safety precautions need to be observed for this activity.

Teacher Notes

  • All of the animations that make up the AACT Animation collection are designed for teachers to incorporate into their classroom lessons. Intentionally, these animations do not have any spoken explanations so that a teacher can speak while the animation is playing and stop the animation as needed to instruct.
  • If you assign this to students outside of class time, you can create a Student Pass that will allow students to view the animation (or any other video or ChemMatters article on the AACT website).
  • We suggest that a teacher pause this animation at several points or watch it more than once to give students the opportunity to make notes, ask questions, and test their understanding of the concepts presented. The animation is about two and a half minutes long and moves quickly, so students will likely require pausing or multiple viewings to successfully complete the student activity sheet if you choose to use it. Here are some of the points at which you might want to pause the video to allow students to answer the questions on the activity sheet:

  • Question 1 – 0:15-0:53 (you may want to replay this section a couple of times to allow students time to fully respond to this question)
  • Question 2 – 0:57
  • Question 3 – 1:08-1:46 (see note for question 1)
  • Question 4 – 1:46
  • Question 5 – 1:55
  • Question 6 – 1:58
  • Question 7 – 2:11
  • Question 8 – 2:14

  • This animation can be used to introduce the concept of periodic trends in a unit on the periodic table and/or atomic structure. It only addresses atomic radius, but this can serve as a foundation for discussion of other periodic trends, such as ionization energy and electron affinity. These can be explored further in the two Periodic Trends simulations (see the last bullet point in the teacher notes for links to these and more resources on this topic).
  • The size of the atomic radii in this animation are only meant to show relative sizes – for example, the image presented at 0:57 is intended to show that the size of atoms decreases across a period, but they do not correspond to specific numerical values for atomic radii. The changes in size from one atom to the next may be somewhat exaggerated to emphasize the overall trend.
  • The number of neutrons provided for the atoms in this animation generally correspond to the number of neutrons in an atom of the most abundant isotope of that element (or the one with the longest half-life, in the case of Po). There are a few exceptions – in this animation, phosphorus is shown to have 15 n, but the most common isotope has 16 n; and tellurium is shown with 66 n, but the most common isotope has 78 n. Additionally, selenium’s most abundant isotope contains 46 n, as is displayed in the animation, which would give it a mass number of 80. However, students may notice that the atomic mass on the periodic table rounded to the nearest whole number would be 79. If students notice this, it would be a good opportunity to show them that rounding the atomic mass to the nearest whole number does not guarantee that the isotope with that mass number is the most abundant if that element has more than two or three isotopes that occur in significant quantities.
  • The conclusion questions require students to think “big picture” about the interactions between subatomic particles. These questions can be used as discussion prompts (maybe in a think-pair-share activity) to help students practice deep thinking about the interactions between subatomic particles. Students may or may not draw accurate conclusions initially about why the periodic trends are what they are, but the value is not so much in making sure they write down the correct answer; rather, these discussions can give them an opportunity to practice making and evaluating scientific arguments and supporting their claims with evidence and reasoning based on what they know about subatomic particles.
  • Related classroom resources from the AACT Library that may be used to further teach this topic:

For the Student

Atomic and Ionic Radii

As you view the animation, answer the questions below.

  1. Complete the table below with the number of subatomic particles in the atoms of the third period (Na to Ar).
  1. As you progress from left to right across the third period (Na to Ar), do the numbers of each subatomic particle always change? Give specific examples to support your claim.
  2. Does the number of protons change by the same amount from one atom to the next, or can the change in protons differ? What about electrons? Neutrons? Give specific examples to support your claim for each subatomic particle.
  1. What happens to the atomic radius as you progress from left to right across the third period?
  2. Complete the table below with the number of subatomic particles in the atoms of the sixth group (O to Po).
  1. What happens to the number of neutrons relative to the number of protons and electrons as you move down the sixth group (O to Po)?
  1. What happens to the atomic radius as you progress from top to bottom down the sixth group?
  2. How are positive ions (cations) formed?
  3. How does the radius of the cation compare the radius of the neutral atom?
  4. How are negative ions (anions) formed?
  5. How does the radius of the anion compare to the radius of the neutral atom?


  1. Look at your answer to question 3 about the number of neutrons in an atom compared to its electrons and protons and how that changes as you go down a group. Suggest a reason for the pattern you saw. (Think about interactions between the particles in the nucleus!)
  2. Summarize the pattern you see in the change in atomic radius across a period and down a group (questions 2 and 4). What do you think accounts for each of those patterns?