Question

Which of the following best describes the character of the bonds in a molecule of ammonium? A. Three polar covalent bonds B. Four polar covalent bonds C. Two polar covalent bonds, two coordinate covalent bonds D. Three polar covalent bonds, one coordinate covalent bond

Short answer

D. Three polar covalent bonds, one coordinate covalent bond

Step-by-step solution

Understand the Structure of Ammonium (NH₄⁺)

Ammonium is a common polyatomic ion with the formula NH₄⁺. This ion is derived from ammonia (NH₃) when it forms a bond with an additional hydrogen ion (H⁺).

Review Ammonia (NH₃) Bonds

In ammonia (NH₃), nitrogen forms three polar covalent bonds with hydrogen atoms. In these bonds, nitrogen shares an electron pair with each hydrogen.

Forming the Fourth Bond with a Hydrogen Ion (H⁺)

When ammonia accepts a hydrogen ion (H⁺), it uses its lone pair of electrons to form a bond with the H⁺. This type of bond, where both electrons come from the same atom, is known as a coordinate covalent bond or dative bond.

Analyze the Bond Types in Ammonium (NH₄⁺)

In the ammonium ion (NH₄⁺), nitrogen has three polar covalent bonds with hydrogen and one coordinate covalent bond with the additional hydrogen ion.

Choose the Correct Answer

Given the bond descriptions, the best answer is the one that states the molecule has three polar covalent bonds and one coordinate covalent bond. Therefore, the correct answer is D.

Key concepts

Polar Covalent Bonds

A polar covalent bond occurs when two atoms share electrons, but the electrons are not shared equally. This happens because one atom attracts the electrons more strongly than the other. As a result, one end of the bond becomes slightly negative, while the other end becomes slightly positive.
In the case of ammonia (NH₃), nitrogen and hydrogen form polar covalent bonds. Nitrogen is more electronegative than hydrogen, so it pulls the shared electrons closer. This creates a partial negative charge on the nitrogen and partial positive charges on the hydrogen atoms.

• Electronegativity Difference: Polar covalent bonds typically have an electronegativity difference between 0.5 and 1.7.
• Dipole Moment: This occurs due to unequal sharing of electrons, leading to a dipole moment where one end of the molecule is more positive and the other end is more negative.

Understanding this concept helps explain the partial charges in molecules like ammonia, which affect how they interact with other substances.

Coordinate Covalent Bonds

A coordinate covalent bond, also known as a dative bond, forms when one atom provides both electrons for a shared pair. This is different from a regular covalent bond where each atom provides one electron.
In the ammonium ion (NH₄⁺), the nitrogen atom uses its lone pair of electrons to create a bond with an extra hydrogen ion (H⁺). Since the hydrogen ion does not bring any electrons, nitrogen supplies both.

• Electron Donor: The atom that provides the electron pair is known as the donor. In NH₄⁺, nitrogen is the donor.
• Stable Complex: Coordinate covalent bonds often form stable complexes. This is important in many biological and chemical systems.

Knowing about coordinate covalent bonds helps explain how atoms can fit together in complex structures like polyatomic ions.

Polyatomic Ions

Polyatomic ions are ions that consist of more than one atom. These atoms are bonded together and carry a net charge due to the loss or gain of electrons.
The ammonium ion (NH₄⁺) is a prime example of a polyatomic ion. It contains one nitrogen atom and four hydrogen atoms, giving it a positive charge overall.

• Overall Charge: The charge on a polyatomic ion is the sum of the charges on its individual atoms.
• Common Examples: Other examples include sulfate (SO₄²⁻) and phosphate (PO₄³⁻).

Polyatomic ions are present in many chemical reactions, and understanding them helps in predicting how different substances react with each other.

Ammonium Ion

The ammonium ion (NH₄⁺) is formed when an ammonia molecule (NH₃) accepts an extra hydrogen ion (H⁺). The nitrogen atom in ammonia has a lone pair of electrons which it uses to bond with the additional hydrogen ion, resulting in a coordinate covalent bond.
In the ammonium ion, there are a total of four bonds between nitrogen and hydrogen. Three of these are polar covalent bonds, and one is a coordinate covalent bond. This mix of bond types makes the ammonium ion an interesting case study.

• Formation: NH₄⁺ is created from NH₃ and H⁺.
• Structure: The ion has a tetrahedral shape.

Understanding the formation and structure of the ammonium ion helps clarify the nature of the bonds and molecular geometry in chemistry.