Question

The correct order of density is

• A. $Be > Mg > Ca > Sr$
• B. $Sr > Ca > Mg > Be$
• C. $Sr > Be > Mg > Ca$
• D. $Be > Sr > Mg > Ca$

Answer 1

The Correct Option is C

To determine the correct order of density among the given elements (Be, Mg, Ca, and Sr), we need to understand the general trend of densities in the alkaline earth metals group (Group 2 of the periodic table). Density is defined as mass per unit volume, and it is influenced by both the atomic mass and the atomic volume of elements. 

1. The elements under consideration are all alkaline earth metals. Typically, as we move down the group in the periodic table, the atomic mass increases, but so does the atomic radius or volume. The increase in mass generally outpaces the increase in volume, leading to higher densities.
2. However, there are exceptions due to variations in atomic packing in the crystal lattice structure of these metals, which leads to non-linear trends in density. Therefore, empirical data or known orders of densities need referencing.
3. We consider the actual densities of these elements:
   • Beryllium (Be) - Approximately 1.85 g/cm³
   • Magnesium (Mg) - Approximately 1.74 g/cm³
   • Calcium (Ca) - Approximately 1.54 g/cm³
   • Strontium (Sr) - Approximately 2.64 g/cm³
4. From this empirical data, we find the order of densities to be Sr > Be > Mg > Ca.
5. The correct option that matches this order is B. $Sr > Be > Mg > Ca$.
6. This order is consistent with expectations that while atomic mass increases significantly from Ca to Sr, leading to increased density, Beryllium's density is unusually high due to its compact hexagonal packing and low atomic radius, which compensates for its low atomic mass compared to Sr.

The reasoning above explains why option B is correct, and rules out others. This analysis depends both on the conceptual understanding of periodic trends and empirical knowledge of densities.