Which of the following statements is INCORRECT?

Atomic size increases from top to bottom. - As , the number of electrons increases , its size also increases.

All the elements of a group have same valency. - For ex - alakali metals have one valence electron.

Isotopes of the same elements are not placed in the same group. - The isotopes of a same element have same atomic number though their mass number is different.Hence they are placed at one place in the same group of the modern periodic table. e.g All the three isotopes of hydrogen have same number of protons ,so they are placed at one place in the same group of the periodic table .

Atomic radius generally decreases from left to right. - This is caused by the increase in the number of protons and electrons across a period. One proton has a greater effect than one electron; thus, electrons are pulled towards the nucleus, resulting in a smaller radius.

So , the answer would be option c)Isotopes of the same elements are not placed in the same group.