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Periodic Trends: Atomic Radius

The size of an atom. Measured from the center of the nucleus to the outside of the electron cloud.

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Making Layered Cookies

Atomic Radius Trends Across and Down the Periodic Table

License: CC BY-NC 3.0

Chocolate chip and M&Ms as layers surrounding the centers of the cookies. [Figure1]

In the periodic table, atomic radius varies in two different directions. As you move horizontally (column to column), the atomic radius decreases. As you move vertically (row to row), the atomic radius increases.

The atomic radius decreases as you move horizontally because the nucleus becomes more compact as a result of the increasing number of protons, creating a higher effective nuclear charge. With a higher effective nuclear charge, electrons are pulled inward, decreasing its distance with the nucleus.

The atomic radius increases as you move vertically because there is an increase in energy levels, meaning that atoms have more shells. Each shell layers over the other, increasing the distance between itself and the nucleus.

Whenever you bake a batch of cookies, you can think of atomic radius as the amount of “free space” on each cookie. Some cookies have only a layer of chocolate chip. Others have layers of chocolate chip and M&M’s. 

Creative Applications

1. What do M&M’s and chocolate chip represent in terms of the periodic table?

2. When you increase one chocolate chip or M&M within a layer, what direction are moving along periodic table?

3. When you maintain the same area of dough but fill it with more chocolate chip, what happens to the amount of “free space?” What periodic trend does this illustrate?

4. When you fill a full layer, you would increase the area of dough. What happens to the amount of “free space?” What periodic trend does this illustrate?



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  1. [1]^ License: CC BY-NC 3.0

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