5.1.3 Trends in melting point

In the previous sections, we saw that the atomic radius and reactivity of Group 2 elements increase down the group, while ionization energy decreases. Another important physical property that shows a clear trend is the melting point. Understanding why the melting point changes helps us understand the nature of the metallic bonding in these elements.

Definition

• Melting Point: The temperature at which a solid changes to a liquid. For metals, it is a measure of the strength of the metallic bonds that hold the atoms together in the solid lattice.
• Trend in Group 2: The melting point of the elements decreases going down the group.

Core Concept

The core concept is that the melting point of a metal depends on the strength of its metallic bond. Metallic bonding is the electrostatic attraction between the positively charged metal ions (cations) and the delocalized "sea" of valence electrons. The stronger this attraction, the more energy is required to break the lattice and melt the solid. As you go down Group 2, the atoms get larger, and the valence electrons are further from the nucleus. This increased distance weakens the attraction between the nucleus and the delocalized electrons, leading to a weaker metallic bond and a lower melting point.

Detailed Explanation

The Trend in Melting Point

The textbook states that the melting point of the elements decreases going down the group. The table (Table 5.1) and the bar chart (Figure 5.3) in the textbook clearly show this trend.

(Note: The trend is not perfectly monotonic; calcium has a higher melting point than magnesium. This is due to changes in crystal structure. However, the overall trend from Be to Ba is a decrease.)

Why Does the Melting Point Decrease Down the Group?

The textbook explains that the melting point of the elements decreases going down the group as the outer electrons get further away from the nucleus. This means that the attraction between the nucleus and the delocalized electrons decreases, hence the strength of the metallic bond decreases, causing a decrease in melting point.

Key Factors:

• Increased Atomic Radius: As you go down the group, the atomic radius increases (as discussed in section 5.1.2). The valence electrons are in shells further from the nucleus.
• Weaker Attraction: The electrostatic attraction between the positive nucleus and the delocalized valence electrons is inversely proportional to the square of the distance between them. As the distance increases, the attraction weakens.
• Weaker Metallic Bond: A weaker attraction means a weaker metallic bond. Less energy is required to overcome these forces and melt the solid, resulting in a lower melting point.

The Anomaly of Magnesium and Calcium

While the general trend is a decrease, it is worth noting that the melting point of calcium (838°C) is higher than that of magnesium (650°C). This is not due to a change in the strength of the metallic bond but is due to a change in the crystal structure. The atoms in calcium pack more efficiently in the solid state, requiring more energy to disrupt the lattice. However, the overall trend from beryllium to barium is a decrease, which is consistent with the weakening of the metallic bond as the atomic radius increases.

Relationship to Other Properties

The decrease in melting point down the group is consistent with the other trends:

• Atomic Radius: Increases down the group.
• Ionization Energy: Decreases down the group.
• Reactivity: Increases down the group.
• Melting Point: Decreases down the group.

All these trends are a consequence of the increasing atomic size and the decreasing strength of the metallic bond.

Intermediate Level

The melting point of Group 2 elements decreases down the group because the strength of the metallic bond decreases. The metallic bond is the attraction between the metal cations and the delocalized valence electrons. As the atomic radius increases down the group, the valence electrons are further from the nucleus, and the electrostatic attraction is weaker. This weaker bond requires less energy to break, resulting in a lower melting point. While there is a slight anomaly with calcium having a higher melting point than magnesium due to crystal structure differences, the overall trend is a decrease from beryllium to barium.

Important Points

• - Melting Point: Measure of the strength of metallic bonds.
• - Trend: Melting point decreases down Group 2.
• - Reason: Increasing atomic radius leads to weaker metallic bonds.
• - Consequence: Less energy is required to break the lattice.
• - Order: Be > Mg < Ca > Sr > Ba (overall decrease, with an anomaly at Ca).
• - Relationship: Weaker metallic bond → lower melting point.

Remember

• - Melting point decreases down Group 2.
• - Reason: Weaker metallic bonds due to increasing atomic radius.
• - Be has the highest melting point, Ba has a lower melting point.
• - The trend is due to the valence electrons being further from the nucleus.
• - Weaker metallic bond = lower melting point.

Common Mistakes

• - Thinking that the melting point increases down the group. It decreases because the metallic bond gets weaker.
• - Assuming the trend is perfectly smooth. There is an anomaly with calcium, which has a higher melting point than magnesium due to crystal structure differences.
• - Confusing melting point with boiling point. They are related, but the trends may not be identical.
• - Forgetting the reason for the trend. The decrease in melting point is due to the weakening of the metallic bond as the atomic radius increases.
• - Thinking that a lower melting point means a less stable metal. Stability depends on many factors, not just melting point.

Quick Revision

The melting point of Group 2 elements decreases down the group. This is because the strength of the metallic bond decreases as the atomic radius increases. The valence electrons are further from the nucleus, so the electrostatic attraction between the metal cations and the delocalized electrons is weaker. Less energy is required to break the lattice, resulting in a lower melting point. The overall trend is from the high melting point of beryllium to the lower melting point of barium, with a slight anomaly at calcium due to crystal structure.

60-Second Revision

The melting point of Group 2 elements decreases down the group due to the weakening of the metallic bond as the atomic radius increases. The valence electrons are further from the nucleus, making the attraction weaker. Beryllium has the highest melting point, and barium has one of the lowest. Less energy is needed to break the weaker metallic bonds.

Frequently Asked Questions (FAQs)

Q1. Why does the melting point of Group 2 elements decrease down the group? Answer: The melting point decreases because the strength of the metallic bond decreases down the group. As the atomic radius increases, the valence electrons are further from the nucleus. This increased distance weakens the electrostatic attraction between the metal cations and the delocalized electrons. A weaker metallic bond requires less energy to break, resulting in a lower melting point.

Q2. Which Group 2 element has the highest melting point? Answer: Beryllium (Be) has the highest melting point among the Group 2 elements (1280°C), as shown in Table 5.1. This is because it has the smallest atomic radius and the strongest metallic bond.

Q3. Why does calcium have a higher melting point than magnesium? Answer: While the general trend is a decrease down the group, calcium has a higher melting point than magnesium (838°C vs. 650°C). This is not due to a stronger metallic bond but is due to a difference in crystal structure. The atoms in calcium pack more efficiently in the solid state, requiring more energy to disrupt the lattice.

Q4. What is the relationship between atomic radius and melting point in Group 2? Answer: As the atomic radius increases down the group, the melting point decreases. This is because the increased distance between the nucleus and the delocalized electrons weakens the metallic bond, requiring less energy to melt the solid.

Q5. Which Group 2 element has the lowest melting point? Answer: Based on the table in the textbook, barium (Ba) has the lowest melting point among the Group 2 elements listed (714°C). However, it is worth noting that magnesium has a lower melting point (650°C) than calcium, but the overall trend from Be to Ba is a decrease.

Q6. How does the trend in melting point relate to the trend in reactivity? Answer: The trend in melting point is related to the trend in reactivity through the strength of the metallic bond. A weaker metallic bond means lower melting point and also means the valence electrons are more easily lost, which corresponds to higher reactivity. So, as melting point decreases down the group, reactivity increases.

Q7. Why is the melting point of beryllium so high? Answer: Beryllium has a very high melting point (1280°C) because it has the smallest atomic radius in Group 2. The valence electrons are very close to the nucleus, resulting in a very strong metallic bond that requires a large amount of energy to break.

Q8. What happens to the metallic bond strength as you go down Group 2? Answer: The metallic bond strength decreases down Group 2. This is because the valence electrons are further from the nucleus and are less tightly held, leading to a weaker electrostatic attraction between the metal cations and the delocalized electrons.

Q9. Is the trend in melting point of Group 2 elements perfectly monotonic? Answer: No, the trend is not perfectly monotonic. While the overall trend is a decrease from beryllium to barium, there is an anomaly with calcium, which has a higher melting point than magnesium. This is due to differences in crystal structure.

Multiple Choice Questions (MCQs)

• Q1. What is the trend in melting point down Group 2?
• A. It increases.
• B. It decreases.
• C. It remains constant.
• D. It first increases then decreases.
• Correct Answer: B
• Explanation: The melting point decreases down the group because the metallic bond strength decreases as the atomic radius increases.

• Q2. Why does the melting point decrease down Group 2?
• A. Because the atomic mass increases.
• B. Because the strength of the metallic bond decreases.
• C. Because the density increases.
• D. Because the number of valence electrons decreases.
• Correct Answer: B
• Explanation: The melting point is a measure of the strength of the metallic bond. As the atomic radius increases, the bond strength decreases.

• Q3. Which Group 2 element has the highest melting point?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: A
• Explanation: Beryllium has the smallest atomic radius and the strongest metallic bond, so it has the highest melting point.

• Q4. Which of the following factors is responsible for the decrease in melting point down Group 2?
• A. Increase in atomic radius
• B. Decrease in atomic mass
• C. Increase in nuclear charge
• D. Decrease in the number of electrons
• Correct Answer: A
• Explanation: The increase in atomic radius down the group weakens the metallic bond, leading to a lower melting point.

• Q5. Which Group 2 element has a higher melting point than expected based on the trend?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: C
• Explanation: Calcium has a higher melting point than magnesium, which is an anomaly due to its crystal structure.

• Q6. The melting point of a metal is a measure of:
• A. The strength of the ionic bond.
• B. The strength of the metallic bond.
• C. The strength of the covalent bond.
• D. The strength of the hydrogen bond.
• Correct Answer: B
• Explanation: The melting point of a metal reflects the strength of the metallic bond that holds the atoms together in the solid lattice.

• Q7. As the atomic radius increases down Group 2, the metallic bond strength:
• A. Increases.
• B. Decreases.
• C. Remains the same.
• D. Becomes zero.
• Correct Answer: B
• Explanation: A larger atomic radius means the valence electrons are further from the nucleus, weakening the metallic bond.

• Q8. Which of the following is the correct order of melting points (from highest to lowest)?
• A. Be > Mg > Ca > Sr > Ba
• B. Ba > Sr > Ca > Mg > Be
• C. Be > Ca > Mg > Sr > Ba
• D. Ca > Be > Mg > Sr > Ba
• Correct Answer: C
• Explanation: The overall trend is a decrease, but calcium has a higher melting point than magnesium. The correct order is Be > Ca > Mg > Sr > Ba.

Q9. Why is the melting point of beryllium the highest in Group 2? A. Because it has the largest atomic radius. B. Because it has the strongest metallic bond due to its small size. C. Because it has the lowest ionization energy. D. Because it is the most reactive. Correct Answer: B Explanation: Beryllium has the smallest atomic radius, so its valence electrons are closest to the nucleus, resulting in the strongest metallic bond and highest melting point.

• Q10. What happens to the energy required to melt the metal as you go down Group 2?
• A. It increases.
• B. It decreases.
• C. It remains the same.
• D. It becomes zero.
• Correct Answer: B
• Explanation: As the metallic bond strength decreases down the group, less energy is required to break the lattice and melt the solid.

Multiple Choice Questions (MCQs)

• Q1. What is the trend in melting point down Group 2?
• A. Increases
• B. Decreases
• C. Remains constant
• D. First increases then decreases
• Correct Answer: B
• Explanation: The melting point decreases down the group because the metallic bond strength decreases as the atomic radius increases.

• Q2. Why does the melting point decrease down Group 2?
• A. Because the atomic mass increases.
• B. Because the strength of the metallic bond decreases.
• C. Because the density increases.
• D. Because the number of valence electrons decreases.
• Correct Answer: B
• Explanation: The melting point is a measure of the strength of the metallic bond. As the atomic radius increases, the bond strength decreases.

• Q3. Which Group 2 element has the highest melting point?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: A
• Explanation: Beryllium has the smallest atomic radius and the strongest metallic bond, so it has the highest melting point.

• Q4. Which of the following factors is responsible for the decrease in melting point down Group 2?
• A. Increase in atomic radius
• B. Decrease in atomic mass
• C. Increase in nuclear charge
• D. Decrease in the number of electrons
• Correct Answer: A
• Explanation: The increase in atomic radius down the group weakens the metallic bond, leading to a lower melting point.

• Q5. Which Group 2 element has a higher melting point than expected based on the trend?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: C
• Explanation: Calcium has a higher melting point than magnesium, which is an anomaly due to its crystal structure.

• Q6. The melting point of a metal is a measure of:
• A. The strength of the ionic bond.
• B. The strength of the metallic bond.
• C. The strength of the covalent bond.
• D. The strength of the hydrogen bond.
• Correct Answer: B
• Explanation: The melting point of a metal reflects the strength of the metallic bond that holds the atoms together in the solid lattice.

• Q7. As the atomic radius increases down Group 2, the metallic bond strength:
• A. Increases.
• B. Decreases.
• C. Remains the same.
• D. Becomes zero.
• Correct Answer: B
• Explanation: A larger atomic radius means the valence electrons are further from the nucleus, weakening the metallic bond.

• Q8. Which of the following is the correct order of melting points (from highest to lowest)?
• A. Be > Mg > Ca > Sr > Ba
• B. Ba > Sr > Ca > Mg > Be
• C. Be > Ca > Mg > Sr > Ba
• D. Ca > Be > Mg > Sr > Ba
• Correct Answer: C
• Explanation: The overall trend is a decrease, but calcium has a higher melting point than magnesium. The correct order is Be > Ca > Mg > Sr > Ba.

Q9. Why is the melting point of beryllium the highest in Group 2? A. Because it has the largest atomic radius. B. Because it has the strongest metallic bond due to its small size. C. Because it has the lowest ionization energy. D. Because it is the most reactive. Correct Answer: B Explanation: Beryllium has the smallest atomic radius, so its valence electrons are closest to the nucleus, resulting in the strongest metallic bond and highest melting point.

• Q10. What happens to the energy required to melt the metal as you go down Group 2?
• A. It increases.
• B. It decreases.
• C. It remains the same.
• D. It becomes zero.
• Correct Answer: B
• Explanation: As the metallic bond strength decreases down the group, less energy is required to break the lattice and melt the solid.