Multiple Choice
Which trend is observed in the atomic radii of main-group elements as you move down a group in the periodic table?
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10. Periodic Properties of the Elements
Periodic Trend: Atomic Radius
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Which of the following elements has the largest atomic radius?
A
K
B
Cl
C
Mg
D
Na
Verified step by step guidance1
Recall that atomic radius generally increases as you move down a group (column) in the periodic table because additional electron shells are added, making the atom larger.
Also remember that atomic radius generally decreases as you move from left to right across a period (row) due to increasing nuclear charge pulling electrons closer to the nucleus.
Identify the positions of the elements in the periodic table: K (potassium) and Na (sodium) are in Group 1 (alkali metals), Mg (magnesium) is in Group 2, and Cl (chlorine) is in Group 17.
Compare the periods of these elements: K is in period 4, Na is in period 3, Mg is in period 3, and Cl is in period 3. Since K is in a lower period, it has more electron shells than the others.
Conclude that because K is in the same group as Na but in a lower period, it has the largest atomic radius among the given elements.
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