A trend common to both alkali metals and halogens is that as the atomic number increases

Options

A. atomic radius increases
B. boiling point increases
C. electronegativity increases
D. reactivity with water increases

Explanation

To solve the question regarding the trend common to both alkali metals and halogens as the atomic number increases, we can analyze each option provided:

### Step 1: Analyze Atomic Radius
- **Alkali Metals:** As we move down the group (from lithium to cesium), the atomic radius increases. This is due to the addition of electron shells, which outweighs the increase in nuclear charge.
- **Halogens:** Similarly, as we move down the halogen group (from fluorine to astatine), the atomic radius also increases for the same reason—more electron shells are added.

### Conclusion for Step 1:
Both alkali metals and halogens exhibit an increase in atomic radius as the atomic number increases.

### Step 2: Analyze Boiling Point
- **Alkali Metals:** The boiling point of alkali metals decreases down the group. This is because the metallic bonds weaken as the size of the atoms increases.
- **Halogens:** In contrast, the boiling point of halogens increases down the group due to the increasing van der Waals forces as the size of the molecules increases.

### Conclusion for Step 2:
The trend in boiling points is not common to both groups, as they behave oppositely.

### Step 3: Analyze Electronegativity
- **Alkali Metals:** These elements have low electronegativity because they tend to lose their outermost electron easily.
- **Halogens:** They have high electronegativity because they readily gain electrons to achieve a stable electron configuration.

### Conclusion for Step 3:
Electronegativity trends are also not common, as alkali metals have low electronegativity while halogens have high electronegativity.

### Step 4: Analyze Reactivity with Water
- **Alkali Metals:** They react vigorously with water, producing hydroxides and hydrogen gas.
- **Halogens:** They react with water to a much lesser extent, forming acidic solutions.

### Conclusion for Step 4:
The reactivity with water is not a common trend, as alkali metals react vigorously while halogens react only slightly.

### Final Conclusion:
The only common trend identified is that **the atomic radius increases** as the atomic number increases for both alkali metals and halogens. Therefore, the correct answer is **Option A: atomic radius increases.**