Question

Which is wrong regarding the density of alkali metals? (1) \(\mathrm{Li}<\mathrm{Na}\) (2) \(\mathrm{Na}<\mathrm{K}\) (3) \(\mathrm{K}<\mathrm{Rb}\) (4) \(\mathrm{Rb}<\mathrm{Cs}\)

Short answer

Statement (2) is incorrect: \(\mathrm{Na} > \mathrm{K} \) in density.

Step-by-step solution

- Understand the Density Trend in Alkali Metals

The density of alkali metals generally increases as you move down the group in the periodic table. This is due to the increase in atomic mass and relatively less increase in atomic volume.

- Evaluate Statement (1)

Check if the density of Lithium (Li) is less than that of Sodium (Na). Li has a lower density than Na, so this statement is correct.

- Evaluate Statement (2)

Check if the density of Sodium (Na) is less than that of Potassium (K). Na actually has a higher density than K, so this statement is incorrect.

- Evaluate Statement (3)

Check if the density of Potassium (K) is less than that of Rubidium (Rb). K has a lower density than Rb, so this statement is correct.

- Evaluate Statement (4)

Check if the density of Rubidium (Rb) is less than that of Cesium (Cs). Rb has a lower density than Cs, so this statement is correct.

- Conclusion

Based on the evaluation of each statement, the incorrect one is statement (2).

Key concepts

periodic table trends

Understanding periodic table trends is crucial in chemistry. These trends refer to patterns in properties of elements across periods (rows) or down groups (columns). For alkali metals, which are found in Group 1, several trends can be observed as you move down the group:

• **Atomic radius:** This increases as you move down, meaning atoms get larger.
  
• **Ionization energy:** This decreases, making it easier to remove an electron.
  
• **Electronegativity:** This also decreases, indicating a lower tendency to attract electrons.
  

Importantly for this exercise, the density of alkali metals generally increases as you move down the group. This increase is because the atomic masses grow faster than the atomic volumes due to additional protons and neutrons in the nucleus.

For students, remembering these trends can simplify understanding the properties of unfamiliar elements. Always cross-reference these patterns when encountering questions about periodic properties.

atomic mass

Atomic mass is another essential concept in chemistry. It represents the mass of an atom, usually measured in atomic mass units (amu). For alkali metals, atomic mass increases as you move down the group due to the addition of protons and neutrons in the nucleus.

Consider Lithium (Li) with an atomic mass of about 7 amu, Sodium (Na) with approximately 23 amu, and so forth until Cesium (Cs) with around 133 amu. Simple calculations and comparisons using atomic mass can help predict various properties, including density.

These increasing atomic masses contribute significantly to their increasing density trends. Larger atomic masses mean heavier atoms, which, when packed in the solid state, create denser materials. This explains why heavier alkali metals like Rb and Cs have higher densities than their lighter counterparts like Li and Na.

atomic volume

Atomic volume is a measure of the space an atom occupies. It usually increases down a group on the periodic table, including for alkali metals. This increase happens because each successive element has an additional electron shell.

For example:

• **Li:** Small atomic volume with only two electron shells
  
• **Na:** Larger atomic volume with three electron shells
  
• **Cs:** Even larger atomic volume due to having more electron shells
  

However, the increase in atomic volume is not as steep as the increase in atomic mass. Therefore, even though atoms get larger, their mass increases more significantly, leading to increased density.

Understanding atomic volume helps students grasp why specific density trends occur. With larger, but relatively lighter atoms higher in the group, densities are lower. As we add atomic mass more quickly than volume, densities increase.