Question

Which of the following is correct for hydrides of lanthanides and actinidcs? (1) They are less denser than the parent metal. (2) They have fairly high heat of formation. (3) Their electronegativity values are fairly close to those of Group I and II elements. (4) All are correct.

Short answer

All are correct.

Step-by-step solution

- Understand the Properties of Hydrides

Lanthanides and actinides form hydrides, which are compounds where hydrogen is bonded to these elements. We need to assess the properties of their hydrides based on the given options.

- Evaluate Density

Consider the first property: hydrides are less dense than the parent metals. Typically, the addition of hydrogen to metals reduces the overall density because hydrogen is much lighter than the metals.

- Assess Heat of Formation

Evaluate the second property: hydrides of lanthanides and actinides have a fairly high heat of formation. Forming hydrides generally involves an exothermic reaction, indicating a significant heat of formation due to strong metal-hydrogen bonds.

- Check Electronegativity

Examine the third property: the electronegativity of hydrides compared to Group I and II elements. Lanthanides and actinides are less electronegative than hydrogen, leading to similar electronegativity to Group I and II elements.

- Conclude based on Evaluation

Since properties (1), (2), and (3) are correct based on the physical and chemical characteristics of lanthanide and actinide hydrides, the correct answer is (4) All are correct.

Key concepts

density comparison

When comparing the density of hydrides to their parent metals, it's important to understand that density is essentially the mass per unit volume. Lanthanides and actinides are heavy metals, which means they generally have high atomic masses. When these elements form hydrides by bonding with hydrogen, the very light hydrogen atoms get incorporated into the crystal structure.

Since hydrogen has an atomic mass of just about 1 amu, adding it to these heavy metals reduces the overall density of the resulting compound.

This reduction in density happens because the volume of the hydride increases due to the larger distance between atoms (caused by the introduction of hydrogen into the lattice), but the mass does not increase proportionally. Therefore, it's correct to say that hydrides of lanthanides and actinides are indeed less dense compared to their parent metals.

heat of formation

The heat of formation is a key concept in understanding the properties of hydrides. It refers to the amount of energy released or absorbed when a compound is formed from its elements. When lanthanides and actinides form hydrides, the reaction typically involves the metal reacting with hydrogen gas.

This reaction is often exothermic, meaning it releases energy in the form of heat.

Exothermic reactions occur because the bonds formed between the metal and hydrogen are quite strong and stable, leading to a release of energy when these bonds are created.

Therefore, the fairly high heat of formation for lanthanide and actinide hydrides indicates that these bonds are quite strong and the reaction is energetically favorable, reinforcing the statement that these hydrides have a significant heat of formation.

electronegativity

Electronegativity is a measure of an atom's ability to attract electrons toward itself in a chemical bond. In the case of lanthanides and actinides, their electronegativity values are not particularly high.

Hydrogen, though, is more electronegative than these metals, meaning it tends to attract electrons when forming bonds with lanthanides and actinides.

When comparing these bonds to those of Group I (the alkali metals) and Group II (the alkaline earth metals) elements, you'll find that the overall electronegativity values are fairly low for all these groups.

This similarity occurs because both lanthanides/actinides and Group I/II elements form bonds where the metal is less electronegative than hydrogen, leading to similar chemical behavior in terms of how these atoms share electrons within their respective hydrides.

Thus, the assertion that the electronegativity values of lanthanide and actinide hydrides are close to those of Group I and II elements holds true.