Which element possesses the biggest atomic radii?
A. P
B. Si
C. Al
D. Mg

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Hint: We know that the horizontal rows present in a periodic table are called periods. Phosphorus, silicon, aluminium and magnesium belong to the third period of the periodic table.

Complete Step by Step Solution:
Periodic table was created based on the periodic law. Periodic law states that the properties of the elements are the function of their atomic weights. In the periodic table, the horizontal rows are the periods. The vertical sections are known as groups.

As we move along a period or left-hand side to the right-hand side of a period, the atomic radii of the elements decrease. The atomic radius of an element is defined as the distance between the centre of the nucleus and the outermost valence electron.

Along the period the atomic number increases. Due to this the number of protons increases which makes the nucleus more positively charged. Electrons are put into the same shell. The impact of an increase in protons is more effective than the increase in electrons.

Effective nuclear charge increases which means that the nuclear attraction between the nucleus and electrons increases. The nucleus holds the electrons more tightly and near to itself. This decreases the atomic radius. So, along a period atomic radii decrease. Hence, Mg has the biggest atomic radii.
So, option D is correct.

Note: On moving from the top to the bottom of a group, atomic radii increases.
The valence electrons are filled up in the higher energy levels i.e., n. So, these electrons become distant from the nucleus as the energy level rises. Electrons are not close to the nucleus and are no more under the influence of the nucleus. Hence, atomic radii increase.