Question

Arrange the following metals in increasing order of expected melting point: La, W, Ta, Hf. Explain this trend in melting points.

Short answer

The given metals can be arranged in increasing order of their expected melting points as follows: La < Hf < Ta < W. This trend can be explained by considering factors like atomic size, crystal lattice structure, number of valence electrons, and electronegativity. La has a larger atomic size, fewer valence electrons, and a lower electronegativity value, resulting in weaker metallic bonds and a lower melting point. W, on the other hand, has the smallest atomic size, more valence electrons, and the highest electronegativity value, which leads to stronger metallic bonds and the highest melting point. Hf and Ta have intermediate properties between La and W, leading to their melting points being in the middle of the trend.

Step-by-step solution

Understand the factors affecting melting points

First, let's understand the factors that affect the melting points of elements. Melting point is the temperature at which a solid changes into a liquid. The key factors that affect the melting points are: 1. Atomic size: Larger atoms have weaker metallic bonds which result in lower melting points. 2. Crystal lattice structure: Different metals have different crystal lattice structures, which affect the strength of their metallic bonds and, consequently, their melting points. 3. Number of valence electrons: Metals with more valence electrons tend to form stronger metallic bonds, resulting in higher melting points. 4. Electronegativity: Higher electronegativity values indicate stronger attractive forces between atoms, leading to higher melting points. Now that we understand the factors affecting melting points, let's analyze La, W, Ta, and Hf using these factors.

Analyze each metal with respect to the factors affecting melting points

La (Lanthanum): La is a large atom and its atomic size is the largest among the given metals. It has 3 valence electrons and a relatively low electronegativity. W (Tungsten): W is a smaller atom than La. It has 6 valence electrons and belongs to the d-block of the periodic table. It has a high electronegativity value. Ta (Tantalum): Ta is very similar to W in size, electronegativity, and number of valence electrons (5). Both W and Ta belong to Group 5 and have very similar properties. Ta also has a high electronegativity value. Hf (Hafnium): Hf has a similar size to Ta and W, but less valence electrons (4) and a slightly lower electronegativity value.

Arrange the metals in increasing order of expected melting points

Taking into account the factors of atomic size, crystal lattice structure, number of valence electrons, and electronegativity, we can arrange the metals in increasing order of their expected melting points as follows: La < Hf < Ta < W That is, Lanthanum (La) has the lowest expected melting point and Tungsten (W) has the highest expected melting point.

Explain the trend in melting points

We can explain the trend observed in the melting points of La, W, Ta, and Hf using the factors analyzed in Steps 1 and 2: La has a larger atomic size than the other metals, which leads to weaker metallic bonds and a lower melting point. Additionally, it has fewer valence electrons and a lower electronegativity value, which also contribute to its lower melting point. Hf has a similar atomic size to Ta and W but has fewer valence electrons and a slightly lower electronegativity value, resulting in a higher melting point than La but lower than Ta and W. Ta has a smaller atomic size than La and Hf, and also has more valence electrons than both of them, which leads to a higher melting point. Moreover, it has a high electronegativity value, contributing to its high melting point. W has the smallest atomic size and the highest electronegativity value among the given metals, which results in stronger metallic bonds and the highest melting point. It also has more valence electrons than La and Hf, further contributing to its higher melting point.

Key concepts

Atomic Size

Atomic size refers to the distance between the outer boundary of an atom's valence electron cloud and its nucleus. It's a crucial factor influencing the melting points of metals. Larger atomic sizes typically contribute to weaker metallic bonds.

As atoms increase in size, the distance between the positive nucleus and the outermost electrons rises. This results in a weakened attraction between these electrons and the nucleus, leading to softer bonds between atoms in the metal lattice. Consequently, metals with larger atomic sizes generally have lower melting points since their bonds break more easily when heated.

• La (Lanthanum): It has the largest atomic size among the metals La, W, Ta, and Hf, which results in weaker bonds and a lower melting point.
• W (Tungsten): This metal is comparatively smaller and has stronger metallic bonds, so it possesses a higher melting point.

Understanding atomic size helps predict and compare melting points among different metals.

Crystal Lattice Structure

The crystal lattice structure of a metal dictates how atoms are arranged in a repeating pattern. This arrangement greatly influences the metal's stability and bond strength, both of which determine its melting point.

A tightly packed and highly organized lattice tends to have stronger metallic bonds. These bonds resist thermal energy better, resulting in higher melting points. Conversely, a more loosely packed structure means weaker bonds that break easier under heat.

• Some metals, like tungsten (W), have a tightly packed bcc (body-centered cubic) lattice, offering exceptional bond strength and stability, which contribute to its high melting point.
• Lanthanum (La) with a different and loosely packed hexagonal close-packed structure is not as stable, leading to a lower melting point.

Understanding how the crystal lattice structure affects melting points helps anticipate how metals respond to heat.

Valence Electrons

Valence electrons are the outermost electrons of an atom, important in forming bonds between metallic atoms. The number of valence electrons influences the bond strength in a metal.

The more valence electrons a metal has, the stronger its metallic bonds typically are. This occurs because additional electrons can participate in forming electron clouds that surround and bind atoms together in the metallic lattice. Stronger bonds mean higher melting points as more energy is required to break them.

• W (Tungsten) and Ta (Tantalum) both have numerous valence electrons (6 and 5, respectively), contributing to their robust bonds and thus, higher melting points.
• La (Lanthanum), with fewer valence electrons (3), creates weaker bonds, resulting in a lower melting point.

Valence electrons are pivotal in shaping how rigid or flexible metallic bonds are in response to thermal energy.

Electronegativity

Electronegativity is the measure of an atom's ability to attract shared electrons. In metals, higher electronegativity suggests stronger pulls on its electrons, reinforcing the metal's lattice structure.

High electronegativity typically correlates with stronger metallic bonds, as the atoms exert a greater pull on the shared electron cloud. These stronger bonds lead to higher melting points because more energy is needed to overcome the atomic attractions.

• W (Tungsten) has a high electronegativity, contributing to its strong metallic bonds and the highest melting point among the discussed metals.
• La (Lanthanum) has a low electronegativity, leading to weaker bonds and a lower melting point.

Understanding the role of electronegativity can help explain trends in how metals behave under thermal stresses.