To solve the question regarding the trend common to both group 2 and group 16 elements in the periodic table as atomic number increases, we can follow these steps:
### Step 1: Identify the Groups
- **Group 2 Elements**: These are the alkaline earth metals, which include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and so on.
- **Group 16 Elements**: These are the chalcogens, which include oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po).
### Step 2: Analyze the Trend
- As we move down a group in the periodic table, the atomic number increases. This means we are adding more protons and neutrons to the nucleus and more electrons to the electron shells.
- For both groups, as we go from top to bottom, the number of electron shells increases.
### Step 3: Determine the Effect on Atomic Radius
- The increase in the number of electron shells leads to an increase in the distance between the nucleus and the outermost electrons. This results in an increase in atomic radius.
- Therefore, the common trend for both group 2 and group 16 elements is that the atomic radius increases as the atomic number increases.
### Step 4: Evaluate Other Options
- **Maximum Valency**: The maximum valency does not increase for these groups as you go down. Group 2 elements have a valency of 2, and group 16 elements have a valency of 6.
- **Reactivity**: Reactivity does not remain constant; it can vary as you move down the groups.
- **Basic Strength of Hydroxides**: For group 2, the basic strength of hydroxides actually increases down the group, not decreases.
### Conclusion
The only correct statement regarding the trend common to both group 2 and group 16 elements as atomic number increases is that the **atomic radius will increase**.
### Final Answer
The trend common to both group 2 and 16 elements in the periodic table is that with increases in atomic number, **the atomic radius will increase**.
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