Answer
The decreasing order of the atomic radius is,
Na$\gt$ Mg$\gt$ Al$\gt$ P$\gt$ Cl.
Work Step by Step
The atomic radius of elements decreases from left to right in a period. This is because from left to right along a period, the nuclear charge increases, and the electrons are added in the same shell.
The given elements are Na, Al, P, Cl, Mg with atomic numbers 11, 13, 15, 17, 12 respectively. These elements belong to the third period of the periodic table. As the atomic number increases nuclear charge also increases. Here the electrons are added to the same shell, which is the third shell, and hence atomic radius decreases as the atomic number increases.
The decreasing order of atomic radius is Na$\gt$ Mg$\gt$ Al$\gt$ P$\gt$ Cl.
