Main Group Elements: Boiling & Melting Points: Videos & Practice Problems

Boiling Point (BP) and Melting Point (MP) are key properties indicating the energy required to change states of matter. For main group elements (groups 1A to 4A), BP and MP generally increase from left to right and as you move up a group. In contrast, for groups 5A to 8A, these properties tend to decrease. Notably, hydrogen behaves similarly to halogens regarding BP and MP trends. Understanding these periodic trends is essential for predicting the behavior of elements in various chemical reactions.

Boiling Point and Melting Point

Boiling Point (BP) and Melting Point (MP) are critical concepts in understanding the phase changes of elements. BP refers to the energy required to convert a liquid into a gas, while MP indicates the energy needed to change a solid into a liquid. These properties exhibit distinct trends across the periodic table, particularly among the main group elements, which include groups 1A to 4A and 5A to 8A.

In the first trend, as we move from group 1A to 4A, both boiling and melting points generally increase from left to right across a period and as we ascend a group. For instance, lithium has a boiling point of 1347 °C and a melting point of 180 °C, illustrating this upward trend. The values for boiling points in this range show a consistent increase: 671 °C, 688 °C, 766 °C, 881 °C, and finally 1347 °C. This pattern is also observed in melting points, reinforcing the idea that as atomic size decreases and bonding types shift, the energy required for phase changes increases.

However, group 2A presents a unique challenge, as the boiling and melting points do not follow a clear trend. Variations in bonding types and atomic structures lead to inconsistencies, making it difficult to establish a uniform pattern. Similarly, in group 3A, exceptions arise due to differences in bonding types, particularly as elements transition from metallic to covalent network bonding.

In contrast, the second trend observed from groups 5A to 8A indicates that boiling and melting points tend to decrease as we move from left to right across a period and as we go up a group. Hydrogen, while typically placed in group 1A, can also be associated with group 7A due to its similar bonding characteristics with halogens. This trend is evident as we observe the decrease in boiling and melting points from fluorine to hydrogen.

In summary, the key takeaways regarding boiling and melting points in main group elements are: from groups 1A to 4A, both properties increase as we move right and up, while from groups 5A to 8A, they decrease in the same direction. Understanding these trends is essential for comparing and analyzing the physical properties of elements within the periodic table.

Main Group Elements: Boiling & Melting Points Example

When considering which element is most easily vaporized under standard conditions, we focus on the boiling point, as this is the temperature at which a substance transitions from a liquid to a gas. The element with the lowest boiling point will vaporize most readily.

Among the options provided—carbon, silicon, potassium, rubidium, and cesium—it's important to note that carbon and silicon form covalent network bonds, resulting in higher boiling and melting points. Therefore, they are not candidates for the lowest boiling point.

Focusing on the alkali metals, potassium, rubidium, and cesium, we observe that these elements belong to Group 1A of the periodic table. In this group, as we move down the table, the boiling points decrease. Consequently, cesium, being the heaviest of the three, has the lowest boiling point and will vaporize most easily.

Thus, the element that is most easily vaporized under standard conditions is cesium.

Which of the following Group 5A elements would be expected to have the highest melting point?

• A. Nitrogen, N
• B. Phosphorus, P
• C. Oxygen, O
• D. Antimony, Sb
• E. Bismuth, Bi

Answer: (D)

Here’s what students ask on this topic:

What is the general trend for boiling points and melting points in groups 1A to 4A of the periodic table?

In groups 1A to 4A of the periodic table, both boiling points (BP) and melting points (MP) generally increase as you move from left to right across a period and from bottom to top within a group. This trend is due to the increasing strength of metallic bonds and the decreasing atomic size, which require more energy to break. For example, as you move from lithium (Li) to carbon (C) in group 1A to 4A, the BP and MP values increase. However, group 2A is excluded from this trend due to irregularities and discrepancies in BP and MP values.

Why are group 2A elements excluded from the general trend of boiling and melting points?

Group 2A elements are excluded from the general trend of boiling and melting points because they exhibit irregularities and discrepancies in their BP and MP values. These irregularities arise due to various factors, such as differences in bonding types and atomic sizes. As a result, there is no consistent pattern for BP and MP within group 2A, making it difficult to establish a clear trend. Therefore, group 2A is not considered when discussing the overall trends for BP and MP in the periodic table.

How do boiling points and melting points change in groups 5A to 8A of the periodic table?

In groups 5A to 8A of the periodic table, both boiling points (BP) and melting points (MP) tend to decrease as you move from left to right across a period and from bottom to top within a group. This trend is due to the decreasing strength of intermolecular forces and the increasing atomic size, which require less energy to break. For example, as you move from fluorine (F) to hydrogen (H) in group 7A, the BP and MP values decrease. However, there are exceptions within these groups due to variations in bonding types and atomic sizes.

Why can hydrogen be categorized in group 7A for boiling and melting point trends?

Hydrogen can be categorized in group 7A for boiling and melting point trends because it shares similar bonding characteristics and electron configuration with the halogens. Hydrogen is one electron away from having a complete outer shell, similar to halogens like fluorine and chlorine. Additionally, the boiling point (BP) and melting point (MP) trends of hydrogen align with those of group 7A elements, showing a decrease in BP and MP as you move from fluorine to hydrogen. Therefore, hydrogen's placement in group 7A is justified based on its bonding behavior and BP and MP trends.

What factors contribute to the lack of consistent melting point data for certain elements in group 3A?

The lack of consistent melting point (MP) data for certain elements in group 3A is due to variations in bonding types and atomic sizes. As elements transition from metallic to covalent network bonding, their MP values can vary significantly. Additionally, changes in atomic size can affect the strength of intermolecular forces, leading to inconsistencies in MP data. These factors make it difficult to establish a uniform pattern for MP within group 3A, resulting in the exclusion of certain elements from the overall trend analysis.