Question

Is radius of atom directly proportional to its mass number or atomic number?

Answer 1

The radius of an atom depends on its atomic number. This is because as we move across a period in a periodic table, the atomic number increases ( with an addition of an electron as we move ). This increases the effective nuclear charge experienced on the electrons of the outer shell.

Hence the nucleus pulls electrons closer to the nucleus resulting in the decrease of the atomic radius.

As we move down a group, the opposite occurs.

Even though there is an increase in the nuclear charge ,as additional shells are being added the distance increases ( i.e. The atomic radius)