Three elements have atomic number \[{\mathbf{7}},{\text{ }}{\mathbf{8}},{\text{ }}{\mathbf{and}}{\text{ }}{\mathbf{9}}\], respectively. Arrange these elements in the decreasing order of their atomic radii.

Hint: In the given question random element and their atomic number had been given, we have to recognize the elements from their atomic number and also state the order of atomic radius. From the given atomic number, we can find the atomic radii by using formula.

Complete Step by step answer: We can define the atomic radius as one-half the distance between the nuclei of equal atoms that are bonded together. According to periodic trends the atomic radius of atoms usually decreases from moving left to right across a period.
We can find the mass of an element by looking up the atomic mass on the periodic table or by calculating the sum of protons and neutrons for a single atom lastly by calculating the subjective average for all atoms of an element.
Hence, the Three elements \['X',{\text{ }}'Y'{\text{ }} \;and\;{\text{ }}'Z'\]having atomic numbers \[7,{\text{ }}8{\text{ }}and{\text{ }}9\] are nitrogen, oxygen and fluorine respectively. We know that the atomic size decreases from left side to right side in a period in the modern periodic table. Hence, the decreasing order of their atomic radii is\[N > O > F\].

Note: We all know that Atomic mass is the total amount of the masses of the electrons, protons, and neutrons, into an atom, or known as the usual mass, in a group of atoms. Also, the Atomic radius is resolute as the distance between the nuclei of two alike atoms bonded together in it.