Which one of the following properties of alkali metals increases in magnitude as the atomic number rises ?

Options

A. ionic radius
B. Melting point
C. Electronegativity
D. First ionisation energy

Explanation

To solve the question regarding which property of alkali metals increases in magnitude as the atomic number rises, we can follow these steps:

### Step 1: Identify the Alkali Metals
The alkali metals are located in Group 1 of the periodic table. They include:
- Lithium (Li)
- Sodium (Na)
- Potassium (K)
- Rubidium (Rb)
- Cesium (Cs)

### Step 2: Understand Trends in Alkali Metals
As we move down the group from lithium to cesium, the atomic number increases. We need to analyze the properties of these metals to determine which one increases with atomic number.

### Step 3: Analyze Ionic Radius
One of the properties of alkali metals is their ionic radius. As we move down the group, the ionic radius increases due to the addition of electron shells. Therefore, the ionic radius increases with increasing atomic number.

### Step 4: Analyze Melting Point
The melting point of alkali metals generally decreases as we move down the group. Thus, this property does not increase with atomic number.

### Step 5: Analyze Electronegativity
Electronegativity is the tendency of an atom to attract electrons in a bond. For alkali metals, electronegativity decreases as we move down the group due to the increase in atomic size. Hence, this property does not increase with atomic number.

### Step 6: Analyze Ionization Energy
Ionization energy, the energy required to remove an electron from an atom, also decreases as we move down the group. This is because the outermost electron is further from the nucleus and is less tightly held. Therefore, this property does not increase with atomic number.

### Conclusion
The only property that increases in magnitude as the atomic number of alkali metals rises is the ionic radius. Thus, the correct answer is:

**Ionic Radius increases with increasing atomic number.**

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