Problem 110 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is arranged in order of increasing melting point? (a) \(\mathrm{Be}<\mathrm{Ca}<\mathrm{Sr}<\mathrm{Mg}\) (b) \(\mathrm{Sr}<\mathrm{Mg}<\mathrm{Be}<\mathrm{Ca}\) (c) \(\mathrm{Ca}<\mathrm{Be}<\mathrm{Mg}<\mathrm{Sr}\) (d) \(\mathrm{Mg}<\mathrm{Sr}<\mathrm{Ca}<\mathrm{Be}\)

Step-by-Step Solution

Answer

Option (d) is correct: Mg < Sr < Ca < Be.

Step 1: Understand the Elements and Melting Points

First, recognize the elements involved: Beryllium (Be), Calcium (Ca), Strontium (Sr), and Magnesium (Mg). The melting points of these elements are key to solving the problem. Beryllium has the highest melting point, followed by Calcium, then Strontium, and finally Magnesium.

Step 2: List Actual Order of Melting Points

Recall or research the melting points: - Be: 1287 °C - Ca: 842 °C - Sr: 777 °C - Mg: 650 °C So, the order from lowest to highest melting point is Mg < Sr < Ca < Be.

Step 3: Compare with Provided Options

Compare the order from Step 2 with the options given: - (a) - Be < Ca < Sr < Mg is incorrect since Be has the highest and Mg the lowest. - (b) - Sr < Mg < Be < Ca is incorrect since it places Mg after Sr. - (c) - Ca < Be < Mg < Sr is incorrect placement of Mg before Sr. - (d) - Mg < Sr < Ca < Be is correct as it matches the order found in Step 2.

Key Concepts

Periodic Table Trends

The periodic table of elements showcases an incredible amount of information about the elements and how they compare to each other. One of the fascinating trends observed in the periodic table is the change in melting points across different groups and periods. The melting point is the temperature at which a solid becomes a liquid. It is influenced by the strength of bonds within a substance. Generally, as you move from left to right across a period, the melting points might increase or decrease based on the type of element being considered: metals typically have higher melting points than nonmetals.

For elements in Group 2, such as those in this exercise, the melting points tend to generally decrease as you move down the group. This is because the atomic size increases and the bonding electrons are further away from the nucleus, making the metallic bonds weaker. However, this trend is not absolute, and anomalies can occur based on how electrons fill orbitals and the type of crystal structure formed.

Comparative Analysis in Chemistry

Comparative analysis is a valuable approach in chemistry that involves comparing different elements or compounds to understand their properties and behaviors. In the context of melting points, comparing the elements within a group or across a period helps us predict their physical properties. Such analysis requires a good grasp of the periodic table and understanding the underlying reasons for these properties.

For instance, when comparing the melting points of Beryllium, Magnesium, Calcium, and Strontium (all Group 2 elements), students need to analyze factors like atomic size and metallic bonding. Beryllium (Be) has the highest melting point at 1287 °C due to its smaller atomic size and stronger metallic bonds, while Magnesium (Mg) has the lowest at 650 °C because of its weaker metallic bonds.

• Beryllium, being at the top of the group, has very strong bonding due to its smaller atomic radius.
• As you move to Magnesium, the bonds weaken with an increase in atomic size.
• Calcium and Strontium, found further down, exhibit even weaker bonds, reflected in their lower melting points compared to Beryllium.

Group 2 Elements Melting Points

Group 2 elements, also known as the alkaline earth metals, include Beryllium, Magnesium, Calcium, Strontium, Barium, and Radium. These elements exhibit unique properties including those related to melting points.

The melting points of these metals tend to decrease down the group, primarily due to the increasing atomic radii. With a larger atomic size, the outer electrons are further from the nucleus, hence having a weaker attraction, leading to lower melting points.

• Beryllium (Be) has the highest melting point among commonly considered Group 2 elements.
• Magnesium (Mg) has a noticeably lower melting point, and Calcium (Ca) and Strontium (Sr) follow the trend with even lower melting points compared to Beryllium.

Understanding this trend helps in predicting and comparing the properties of not just these elements, but also their interactions with other elements. Analyzing these trends can provide insights into the behavior of these elements in reactions and their applications in various industries.