Which of the following properties of alkali metals increases as the atomic number rises ?
(a) Metallic character
(b) Ionic radius
(c) Melting point
(d) Density
(e) Ionization enthalpy

Question

Which of the following properties of alkali metals increases as the atomic number rises ? 
(a) Metallic character 
(b) Ionic radius 
(c) Melting point 
(d) Density 
(e) Ionization enthalpy

Accepted Answer

Option 2 is the correct answer because it includes the properties of alkali metals that increase as the atomic number rises: metallic character, ionic radius, and density. Let's explore each of these properties and why they increase with the atomic number:

1. **Metallic Character (a):** As we move down the group in the periodic table, the atomic number of alkali metals increases. This leads to an increase in the number of electron shells, which results in the outermost electron being further away from the nucleus. Consequently, the attraction between the nucleus and the outermost electron decreases, making it easier for the atom to lose this electron and exhibit metallic character. Therefore, the metallic character increases with an increase in atomic number.

2. **Ionic Radius (b):** The ionic radius of alkali metals increases as the atomic number increases. This is because each successive element in the group has an additional electron shell compared to the previous one. As a result, the size of the ion increases due to the increased distance between the nucleus and the outermost electron shell.

3. **Density (d):** Generally, the density of alkali metals increases with an increase in atomic number. This is because, although the atomic size increases, the increase in atomic mass is more significant, leading to a higher density. However, it is important to note that there are exceptions in the trend due to the unique structure of some alkali metals, but the general trend is an increase in density.

Now, let's explain why the other options are incorrect:

- **Melting Point (c):** The melting point of alkali metals actually decreases as the atomic number increases. This is because the metallic bonds become weaker due to the increased atomic size and the decreased attraction between the nucleus and the outermost electrons. Therefore, option 1 and option 3, which include melting point, are incorrect.

- **Ionization Enthalpy (e):** Ionization enthalpy decreases as the atomic number increases. This is because the outermost electron is further from the nucleus and is more easily removed due to the weaker attraction. Therefore, option 3, which includes ionization enthalpy, is incorrect.

- **Option 4 (All):** This option is incorrect because not all the properties listed increase with the atomic number. As explained, melting point and ionization enthalpy decrease with an increase in atomic number.

In conclusion, Option 2 is the correct answer as it accurately identifies the properties of alkali metals that increase with rising atomic number: metallic character, ionic radius, and density.