Atomic Radii Information Page
Broad definition of what we are considering
We consider videos focused on conceptual understanding related to atomic or ionic radii and what causes the atomic radius to increase down a column and increase left to right across a row.
We did NOT consider videos that just presented the trends and then used the trends to answer questions/problems like which atom of X, Y, Z has the largest radius.
Johnstone's Triangle Elements
Atomic Radii Specific Criteria
| Element | Examples |
|---|---|
| Macroscopic | General Definition for Macro: things we can see (observations) OR specific measurements. Reported measurements of atomic radii [Must give actual radii values for this to be macroscopic.] Reactivity can be a macroscopic representation, but there must be some specific reaction (e.g. alkali metals reacting with water). |
| Symbolic | General Definition for Symbolic: standardized, discipline-specific short-form representations that efficiently communicate significant information. It is a universal shorthand that bridges the macroscopic and particulate levels. Periodic table (PT) is a symbolic representation IF it bridges connected to the concept (i.e. including the arrows to show trends across a row and/or down a column). Zeff calculations, nucleus with circles around it indicating shells without electrons depicted. Graphs that show trends in atomic radii. Element or ion abbreviations are considered symbols if used directly with the topic (e.g. comparing Li, Na, K… or Na and Na+) |
| Particulate | General Definition for Particulate: visual model that helps “visualize” arrangement, movement, interaction, or composition of matter as it relates to the concept of interest. E.g. Particle depictions showing examples of atoms and their relative sizes in periodic table (need to show trend - just two atoms like Li and Ne is not enough). Bohr model of the atom showing # protons in the nucleus and electrons in orbits around the nucleus (Note: if electrons are not present - just nucleus and rings, that is symbolic); particle level depictions of how radii (covalent or vanderWaals) radii are measured |
| Connection between levels |
Videos should make explicit connections between two or more levels for this to be
coded as present. For example, showing a table with relative sizes of atoms and giving values for atomic radii. Graph of atomic radii if explicitly connected to the PT - Just saying you can see the trend in radii going across the PT is not good enough. Here is an example that makes connections well: |
Core Ideas
| Core Ideas Elements | |
| Electrostatic Bonding and Interactions | Atomic/Molecular Structure & Properties |
|---|---|
| Attractive and repulsive electrostatic forces govern noncovalent and bonding (covalent and ionic) interactions between atoms and molecules. The strength of these forces depends on the magnitude of the charges involved and the distances between them. | The macroscopic physical and chemical properties of a substance are determined by the three-dimensional structure, the distribution of electron density, and the nature and extent of noncovalent interactions between the particles. |
Video needs to include:
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Video needs to include:
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Casual Mechanistic Reasoning
Must have both How and Why?
Option 1: What causes atoms to get smaller going across the periodic table?
How: As we go across the periodic table, atoms gain an additional proton and electron in the same electron shell (must explicitly address this). This will increase the attractive force (Z or Zeff) between the electrons and the proton.
Why: Increased attractive forces between the electron and the nucleus will pull the electrons in more strongly. Electrons at the same shell do not effectively screen each other from the nucleus (this would be good to include but not required). The answer should address the concept of Coulomb’s Law, though specifically naming it is not required.
Option 2: What causes atoms to get larger going down the periodic table?
How: As we go down the periodic table, atoms gain an additional occupied shells of electrons and size of atom is determined by electron cloud (this can be implicit – e.g., showing larger size with more “rings”).
Why: Increased shells will increase the distance of the electrons from the nucleus. Although the nucleus also has more protons, the Zeff will remain relatively constant meaning that the outermost electrons do not experience a greater pull OR force of attraction is inversely proportional to the distance so electrons in shells further from the nucleus experience a weaker force of attraction.
Option 3: What causes cations to be smaller than their neutral atoms?
How: Cations have fewer electrons than their related neutral atom.
Why: When electrons are lost, still holding the same number of protons, there’s a stronger attraction for the remaining electrons because there are fewer. OR If removing electrons results in fewer electron shells being filled, then can use the rationale that the remaining shells are closer to the nucleus making is smaller.
Option 4: What causes anions to be bigger than their neutral atoms?
How: Anions have more electrons than their neutral atom counterpart.
Why: When electrons are gained, electron-electron repulsion forces increase, while Zeff remains the same and the overall size of the atom increases.
Option 5: What effect does atomic radius have on chemical reactivity within a group?
How: Chemical reactivity depends on valence electrons and atomic radius affects the strength of the attraction for valence electrons.
Why: Electrons that are further from the nucleus will be attracted to the nucleus with a lesser force (higher energy) so are easier to remove and/or electrons closer to the nucleus will be attracted to the nucleus with greater force so exert stronger attraction on valence electrons from other atoms (e.g., F is more reactive than I as it is smaller in size and thus attracts electrons more effectively). The answer should address the concept of Coulomb’s Law (strength of attraction decreases exponentially with distance), though specifically naming it is not required.
Scientific Practices
| Most Frequent Science Practices |
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| Note: A trend in atomic radii was considered to be a phenomena/observation. So as these videos are focused on trends, all should have phenomena/observations, so so what we focused on was whether a complete explanation is given. If videos had CMR, they by default also had the SP of Constructing Explanations, though wher another practice was present (using models or interpreting data) we identified that practice preferentially. |
Developing and Using Models
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Analyzing and Interpreting Data
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Constructing Explanations and Engaging in Argument from Evidence
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Multimedia Principles
Multimedia learning as knowledge construction: The goal of multimedia presentations is not only to present information but also to provide guidance for how to process the presented information – that is, for determining what to pay attention to, how to mentally organize it, and how to relate it to prior knowledge. We will categorize the videos along each of the following criteria (High and Low should be mutually exclusive.)
| High | Low |
|---|---|
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Misleading vs. Inaccuracies
- Several minor spelling issues is misleading, not inaccurate
Example High Quality Videos
Examplar Videos
These are examples of videos that were found to provide good illustrations of some or many of the framework components for this concept. Develop a deep understanding of chemistry concepts requires multiple experiences with the material and these videos may provide one such experience.
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Exemplar Video 1 |
Exemplar Video 2 |
Exemplar Video 3 by FreeSciencelessons Must be watched on YouTube. Time: 3:32 |
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Description This video begins with a quick review of relative atomic sizes and their relationship to the periodic table, then provides a deeper explanation of why those trends occur. It compares nitrogen and oxygen, then lithium and sodium, using Bohr models and effective nuclear charge. It also compares sodium with sodium ions and oxygen with oxide ions using electron repulsion to explain differences in size. |
Description This video discusses atomic and ionic radii. It explains why atoms do not have a clear boundary and defines atomic radius as half the distance between two bonded nuclei. It then describes trends down a group and across a period using particulate-level diagrams, shielding, nuclear charge, and electron attraction. The video also discusses ionic radii, including why cations are smaller and anions are larger than their neutral atoms. |
Description This video describes trends in atomic radius across and down the periodic table. It reviews the periodic table, discusses how atomic radius is measured, uses a graph to identify trends, and explains period 2 using nuclear charge and electron shells. It also discusses shielding by inner electrons and uses Bohr diagrams for lithium, sodium, and potassium. |
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Strengths This video includes strong core ideas, science practices, and causal mechanistic reasoning because the instructor explains why the trends happen rather than only giving memorization rules. |
Strengths This video makes strong use of Bohr models to show how and why atomic radius trends occur across and down the periodic table. It also provides causal mechanistic reasoning for both periodic trends and the size differences between cations and their neutral atoms. |
Strengths This short video packs in a lot of quality content. It explains how atomic radius is determined and uses causal mechanistic reasoning to justify trends both across and down the periodic table. It also uses shielding to help explain the observed trends. |
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Weaknesses This video does not include a self-assessment for students to practice the concept they just learned. It also lacks an analogy and macroscopic representations. |
Weaknesses The images are hand-drawn, so they are not as crisp as some other representations. The video is also lacking macroscopic representations. |
Weaknesses The video scores low on multimedia criteria because the narrator is on screen the full time and the captions cannot be removed. It also misses an opportunity to make a stronger connection to macroscopic representations, since specific atom sizes appear in the graphs but are not explicitly connected in the narration. |
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