The variation of atomic radii in group 3A,4A,5A and 6A elements is caused by increase in size of the nuclear charge.
Periodic trends are the properties that change down the group and across the period in the periodic table. These properties include; atomic radii, ionic size, electron affinity, electronegativity, ionization energy, etc.
The atomic radii of elements is directly related to the size of the nuclear charge. Across the period, the size of the nuclear charge increases whereas screening remains constant (no new shell is added).
As a result of increased electrostatic pull of the nucleus on the outer electrons, the atomic radii in group 3A,4A,5A and 6A elements decreases progressively across the period.
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