Atomic radius refers to the size of an atom (the distance from the nucleus to the valence electrons). The further that the valence electrons are from the nucleus, the larger the atom.
Moving down a group (column) increases atomic radius. As you move down a group, there are more principal energy levels (more electron shells). This majorly increases the size of the nucleus because you are adding another “ring”. Furthermore, moving down a group increases the number of core electrons, so the screening effect is also greater. The valence electrons are further from the nucleus, so atomic radius increases. Moving up a group does the opposite.
Moving left to right across a period (row) decreases atomic radius. As you move left to right, the number of protons increases. This increases the effective nuclear charge, so electrons are pulled closer to the nucleus. There is more electron-electron repulsion as the number of electrons also increases (which does slightly increase the radius, but keep reading). However, that barely affects the radius. The effect of having more protons outweighs the increased electron-electron repulsion, pulling the electrons closer and decreasing atomic radius. The number of core electrons does not change, so the screening effect remains the same.
Note that this is the only trend that increases down and to the left. All other trends increase up and to the right.
Example Problem #1
Which atom has a larger atomic radius?
Na vs. K
Answer
Potassium has a larger atomic radius because it has more principal energy levels. Its valence electrons are further from the nucleus.
Example Problem #2
Which atom has a larger atomic radius?
P vs. Cl
Answer
Phosphorus has a larger atomic radius. These atoms have the same amount of principal energy levels. However, chlorine has more protons, and therefore a greater effective nuclear charge. Chlorine pulls its valence electrons in closer, so phosphorus is larger.
Example Problem #3
Which atom has a larger atomic radius?
F vs. S
Answer
Sulfur has a larger atomic radius. Sulfur has more principal energy levels and a weaker effective nuclear charge, so its valence electrons are further from the nucleus.
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