The order of increasing sizes of atomic radii among the elements O, S, Se and As is :As <S<O<SeSe<S<As<OO < S < As <SeO<S<Se<As

The order of increasing atomic radii among the elements O, S, Se, and As is: O < S < Se < As.

Here's why:

1. Atomic radius generally increases as you move down a group (vertical column) in the periodic table. This is because each row adds a new electron shell.

2. Oxygen (O), Sulfur (S), and Selenium (Se) are all in the same group (Group 16, the Oxygen family), but Oxygen is in the second period (row), Sulfur is in the third, and Selenium is in the fourth. Therefore, O < S < Se.

3. Arsenic (As) is in the same period as Selenium (Se), but it's in Group 15, one group to the left. Elements generally have larger atomic radii as you move to the left in a period, because the increase in positive charge in the nucleus as you move to the right pulls the electron cloud closer and decreases the atomic radius. However, the difference in atomic radius between elements in adjacent groups is usually less than the difference between elements in different periods. Therefore, As is larger than Se, and the final order is O < S < Se < As.