Atomic radius is measured from the center of an atom's nucleus to its outermost electron energy level.
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The radius of an atom is extremely small, on the order of tens or hundreds of trillionths of a meter. These measurements are reported in angstroms (1 angstrom (
Å) = 0.0000000001 meter) or picometers (1 picometer (pm) = 0.000000000001 meter).

A good analogy for an atom having an atomic radius of 100 pm is to say that if a typical marble is the size of its atomic nucleus, then its outermost electrons are on average 1000 meters away!

To give you [[#|an idea]] as to how small these atoms are, consider a copper atom, which has an atomic radius of 135 pm. A penny is coated in copper, so imagine if you lined up the copper atoms across the face of a penny. That would require 74 million copper atoms!

Atomic Radius Trend

Atomic radius is influenced by one property - electronegativity. As the pull on electrons gets stronger, their orbitals get smaller, and the atoms are thus smaller in size.

Within a vertical group, atoms get larger going down. There are more layers of electrons, which are further from the nucleus.

Within a horizontal period, the atoms get smaller from left to right because the number of protons in the nucleus increases.

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Here is a video explaining atomic radius trends.

This chart shows some radius values in angstroms.

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