The following pictures represent structures of the hydrides of four second-row elements:
(1)
(2)
(3)
(4)
(b) Which compound has the lowest boiling point?

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Identify the second-row elements in the periodic table, which are lithium (Li), beryllium (Be), boron (B), carbon (C), nitrogen (N), oxygen (O), and fluorine (F).

Recognize that hydrides are compounds formed between hydrogen and another element. For second-row elements, these hydrides are: LiH, BeH2, BH3, CH4, NH3, H2O, and HF.

Understand that boiling points are influenced by intermolecular forces such as hydrogen bonding, dipole-dipole interactions, and London dispersion forces.

Consider that CH4 (methane) is a nonpolar molecule with only London dispersion forces, which are weaker compared to hydrogen bonding present in NH3, H2O, and HF.

Conclude that CH4, having only weak London dispersion forces and no hydrogen bonding, will have the lowest boiling point among the hydrides of second-row elements.

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Hydride Structures

Hydrides are compounds formed between hydrogen and other elements. The structure of a hydride can significantly influence its physical properties, including boiling point. For second-row elements, the type of bonding (ionic, covalent, or metallic) and molecular geometry play crucial roles in determining the boiling point of the hydride.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between molecules. These forces, including hydrogen bonding, dipole-dipole interactions, and London dispersion forces, directly affect boiling points. Compounds with stronger intermolecular forces typically have higher boiling points, while those with weaker forces have lower boiling points.

Trends in Boiling Points

Boiling points of hydrides generally exhibit trends based on molecular weight and electronegativity of the constituent elements. As you move down a group in the periodic table, boiling points tend to increase due to greater molecular weight and stronger dispersion forces. Conversely, for similar molecular weights, electronegativity differences can lead to variations in boiling points due to the strength of intermolecular forces.

The following pictures represent structures of the hydrides of four second-row elements:(1) (2) (3) (4) (b) Which compound has the lowest boiling point?