The atomic radii of the elements of the main groups of the periodic table increase

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Atomic radii change with position on the periodic table. Down a group, radii increase because each step adds a new electron shell, which enlarges the size, and inner electrons shield outer electrons from the nucleus, so the outer electrons experience a weaker pull. Across a period, electrons are added to the same shell while the nuclear charge increases, pulling outer electrons closer and shrinking the radius. Therefore, the statement that atomic radii increase applies when you move down a group, not across a period, up a group, or left to right.