Question

Write formulas for and name the binary hydrogen compounds of the second-period elements (Li to F). Describe how the physical and chemical properties of these compounds change from left to right across the period.

Short answer

Binary hydrogen compounds vary from ionic to covalent across the period. From $\text{LiH}$ to $\text{HF}$, compounds become more acidic.

Step-by-step solution

Identify Second-Period Elements

The second-period elements are Li, Be, B, C, N, O, and F. We will be creating binary hydrogen compounds with these elements.

Write Formulas for Binary Hydrogen Compounds

Create binary compounds of each second-period element with hydrogen: $\text{LiH},{\text{BeH}}_2,{\text{BH}}_3,{\text{CH}}_4,{\text{NH}}_3,{\text{H}}_2\text{O},\text{HF}$.

Name the Binary Hydrogen Compounds

The names of these binary compounds are: lithium hydride ($\text{LiH}$), beryllium hydride (${\text{BeH}}_2$), borane (${\text{BH}}_3$), methane (${\text{CH}}_4$), ammonia (${\text{NH}}_3$), water (${\text{H}}_2\text{O}$), and hydrogen fluoride ($\text{HF}$).

Describe Physical Properties

As we move from $\text{LiH}$ to $\text{HF}$, the compounds generally change from ionic to covalent, resulting in a shift from solid, high melting-point compounds (e.g., $\text{LiH}$, ${\text{BeH}}_2$) to gaseous or liquid compounds (e.g., ${\text{CH}}_4$, ${\text{NH}}_3$, ${\text{H}}_2\text{O}$, $\text{HF}$).

Describe Chemical Properties

Chemically, these compounds tend to be more acidic from left to right across the period. $\text{HF}$ is a strong acid, whereas $\text{LiH}$ is a hydride ion donor, and ${\text{BeH}}_2$ forms polymeric structures.

Key concepts

Second-Period Elements

In chemistry, second-period elements are those elements in the second row of the periodic table. These elements are: Lithium (Li), Beryllium (Be), Boron (B), Carbon (C), Nitrogen (N), Oxygen (O), and Fluorine (F). Each of these elements forms compounds with hydrogen known as binary hydrogen compounds. - Lithium forms lithium hydride ($\text{LiH}$), a compound characterized by its ionic nature.- Beryllium combines with hydrogen to create beryllium hydride (${\text{BeH}}_2$).- Boron interacts with hydrogen forming borane (${\text{BH}}_3$).- Carbon forms methane (${\text{CH}}_4$), a major component of natural gas.- Nitrogen and hydrogen form ammonia (${\text{NH}}_3$), widely used as a fertilizer.- Oxygen and hydrogen create water (${\text{H}}_2\text{O}$), vital for life.- Finally, hydrogen fluoride ($\text{HF}$) is formed by fluorine and hydrogen.As you can see, each of these second-period elements reacts with hydrogen to produce a unique binary compound.

Ionic to Covalent Transition

The nature of the bond in binary hydrogen compounds can vary significantly as we move from left to right across the second-period elements. This transition is primarily from ionic to covalent bonding, reflecting changes in electronegativity of the elements.

The bond between lithium and hydrogen in $\text{LiH}$ is predominantly ionic. This is because lithium is a metal, and it tends to lose electrons to form positive ions.

Beryllium hydride ${\text{BeH}}_2$ has less ionic character, and as we move to ${\text{BH}}_3$, borane exhibits a substantial covalent character.

In methane ${\text{CH}}_4$, the carbon-hydrogen bonds are strongly covalent.

Ammonia ${\text{NH}}_3$ and water ${\text{H}}_2\text{O}$ continue this trend with polar covalent bonds.

Finally, hydrogen fluoride $\text{HF}$ exhibits the highest polar covalent character, due to the high electronegativity of fluorine.

This transition from ionic to covalent bonds greatly influences the physical properties of these compounds.

Chemical Properties across the Period

Chemical properties of the binary hydrogen compounds vary markedly across the second period. The key chemical characteristics that evolve include reactivity, acidity, and polymer formation.

$\text{LiH}$ is reactive with water and can release hydrogen gas upon reaction, useful in fuel cells.

${\text{BeH}}_2$ can form polymeric structures, indicating complex bonding compared to other simple hydrides.

${\text{BH}}_3$ is an electron-deficient compound and acts as a Lewis acid, important in organic chemistry reactions.

${\text{CH}}_4$ is stable and less reactive, utilized extensively as fuel.

${\text{NH}}_3$, or ammonia, serves as a base and a useful cleaning agent.

${\text{H}}_2\text{O}$, being water, is a universal solvent, essential in countless chemical processes.

$\text{HF}$ is recognized for its acidic properties, capable of dissolving glass, highlighting its reactive nature.

Thus, moving from left to right across the period, the hydrogen compounds transition from being predominantly basic or neutral to acidic.

Acidic Properties

The acidic behavior of binary hydrogen compounds largely depends on their position in the periodic table. As we move to the right from lithium to fluorine, these compounds generally become more acidic. - Lithium hydride $\text{LiH}$ acts as a strong base in reactions, donating hydride ions. - In contrast, hydrogen fluoride $\text{HF}$ is a well-known acid, able to dissolve substances like glass due to its ability to donate protons.The progression in acidity is shaped by the electronegativity and bond strength between hydrogen and the other element:

Elements like oxygen and nitrogen in ${\text{H}}_2\text{O}$ and ${\text{NH}}_3$ show intermediate acidic qualities.

This quality increases significantly as we move towards $\text{HF}$, due to the increased electronegativity of fluorine pulling away shared electrons more effectively.

Understanding these trends is crucial for predicting chemical interactions and compound behaviors in various environments.

Physical Properties of Hydrides

The physical properties of these binary hydrogen compounds vary significantly from lithium hydride $\text{LiH}$ to hydrogen fluoride $\text{HF}$, often correlating their ionic or covalent nature.

Early in the period, $\text{LiH}$ is a solid with a high melting point, typical for ionic compounds, offering stability under high temperatures.

As we advance to ${\text{CH}}_4$ and ${\text{NH}}_3$, these hydrides become gases at room temperature, highlighting their covalent nature.

${\text{H}}_2\text{O}$ exists as a liquid due to extensive hydrogen bonding, a unique characteristic among hydrides.

Lastly, hydrogen fluoride $\text{HF}$ may exist in gaseous, liquid, or solid state under standard conditions, with its physical properties heavily influenced by intermolecular hydrogen bonding.

These varying physical states illustrate the impact that different bonding types have on the melting and boiling points of hydrides. This understanding aids in both academic studies and practical applications of these compounds.