Question

In the following compounds, is hydrogen present as \(\mathrm{H}^{+}, \mathrm{H}\), or a covalently bound \(\mathrm{H}\) atom? (a) \(\mathrm{MgH}_{2}\) (b) \(\mathrm{PH}_{3}\) (c) \(\mathrm{KH}\) (d) \(\mathrm{HBr}\)

Short answer

(a) \(\mathrm{H}^{-}\); (b) Covalent; (c) \(\mathrm{H}^{-}\); (d) Covalent.

Step-by-step solution

Analyze Compound MgH2

In \( \mathrm{MgH}_{2} \), magnesium (Mg) is a group 2 element and typically forms a +2 oxidation state. Hydrogen (H) in this case is bonded to the metal. Since metals form ionic bonds with hydrogen in such compounds, hydrogen is present as hydride ion \( \mathrm{H}^{-} \).

Analyze Compound PH3

Phosphorus (P) is a non-metal and forms covalent bonds with hydrogen (H) in \( \mathrm{PH}_{3} \), making hydrogen covalently bound. Thus, hydrogen in \( \mathrm{PH}_{3} \) is present as a covalently bonded atom.

Analyze Compound KH

Potassium (K) is an alkali metal, which makes its standard oxidation state +1. Thus, in \( \mathrm{KH} \), hydrogen is present in its hydride form \( \mathrm{H}^{-} \), as it forms an ionic bond with the metal.

Analyze Compound HBr

Bromine (Br) is a non-metal and forms a covalent bond with hydrogen (H) in \( \mathrm{HBr} \). Hence, the hydrogen in \( \mathrm{HBr} \) is present as a covalently bound hydrogen atom.

Key concepts

Covalent Bonds

In chemistry, covalent bonds are one of the primary types of chemical bonds. When two non-metals, such as hydrogen and phosphorus in \( \text{PH}_3 \), share electrons, they form a covalent bond.
These bonds occur because atoms want to achieve a stable electron configuration, often resembling the nearest noble gas. By sharing electrons, each atom can fill its outer shell.

• Covalent bonds typically occur between non-metals.
• They involve the sharing of electron pairs.
• Covalent bonds can be single, double, or triple, depending on the number of electron pairs shared.

In the context of \( \text{HBr} \) and \( \text{PH}_3 \), hydrogen is sharing electrons with either bromine or phosphorus, indicating that hydrogen is covalently bound. This shared arrangement is often what gives covalently bonded compounds their distinct characteristics, such as low melting and boiling points compared to ionic compounds. Remember, covalent bonds are about sharing electrons to achieve stability.

Hydride Ion

The hydride ion (\( \text{H}^- \)) is a form of hydrogen that has gained an extra electron, giving it a negative charge. This occurs when hydrogen bonds with a metal, such as in compounds like \( \text{MgH}_2 \) and \( \text{KH} \).
In these instances, hydrogen accepts an electron from the metal, forming an ionic bond where hydrogen acts more like a metal itself by carrying a negative charge.

• Hydride ions are found in metal hydrides.
• They form when hydrogen gains an electron.
• Common in compounds where hydrogen is bonded to metals.

The presence of the hydride ion is indicative of hydrogen's versatility, as it can act differently depending on its bonding partner. In metal hydrides, the hydride ion often exhibits strong reducing properties due to its ability to donate electrons readily.

Oxidation States

Oxidation states represent the number of electrons an atom gains, loses, or appears to use when forming compounds. They are crucial for understanding reactions and bonding, providing insights into electron transfer and the types of bonds present.
In \( \text{MgH}_2 \) and \( \text{KH} \), hydrogen is in a -1 oxidation state, known as the hydride form. This is because it gains an electron from metals, such as magnesium or potassium, contrary to its usual +1 state when bonded with non-metals. Metals like magnesium and potassium exhibit their standard oxidation states of +2 and +1, respectively. This contrast in oxidation states helps identify the type of bond present in the compound.
On the other hand, in covalent compounds like \( \text{PH}_3 \) and \( \text{HBr} \), hydrogen takes its more common +1 oxidation state. These differences highlight how oxidation states can provide a deeper understanding of the nature of the bond, whether it’s ionic or covalent in nature.

• Indicate the electron gain or loss of an atom in a compound.
• Helps in identifying the type of bond (ionic or covalent).
• Varies based on the atoms involved and their bonding.

Recognizing the oxidation state allows chemists to infer the electronic interactions occurring during chemical reactions, making it a fundamental concept in understanding chemical behavior.