Definition

1. Noun:
   • A positively charged atom of hydrogen: Specifically, a hydrogen ion is a hydrogen atom that has lost its single electron, resulting in a particle with a net positive charge.
   • A normal hydrogen atomic nucleus: In its most common form, this is simply a single proton, as the most abundant isotope of hydrogen has no neutrons.

Usage

• The term "hydrogen ion" is used primarily in chemistry and physics to describe this fundamental cation.
• It is central to the concept of acidity in the Brønsted–Lowry theory, where an acid is defined as a substance that donates a hydrogen ion.
• In aqueous solutions, hydrogen ions do not exist freely; they are bound to water molecules, forming hydronium ions (H₃O⁺). However, the notation H⁺ is still conventionally used to represent them.

Examples

• Noun:
  • The concentration of hydrogen ions in a solution determines its pH.
  • In the reaction, the acid donates a hydrogen ion to the base.

Advanced Usage

• "Hydrogen ion concentration": A key measurement in chemistry, often expressed as pH, where pH = -log₁₀[H⁺].
  • Scientists carefully monitored the hydrogen ion concentration in the lake water.
• "Hydrogen ion pump": A biological mechanism, such as in cellular respiration, that moves hydrogen ions across a membrane.
  • ATP synthase is driven by the flow of hydrogen ions back into the mitochondrial matrix.

Variants and Related Words

• Hydronium ion (H₃O⁺) (n): The ion formed when a hydrogen ion is attached to a water molecule; the actual species present in acidic aqueous solutions.
  • In water, a free proton immediately reacts to form a hydronium ion.
• Proton (n): A common synonym for a hydrogen ion, especially in the context of physics and nuclear chemistry.
  • The exchange of protons is crucial for many chemical reactions.

Synonyms

• Proton: Especially when referring to the nucleus of hydrogen-1.
• Cation: A general term for any positively charged ion; a hydrogen ion is a specific type of cation.

Related Phrases

• Hydrogen ion activity: A thermodynamic concept related to the effective concentration of hydrogen ions in a solution, which influences chemical potential and reaction rates.
  • The calculation requires knowledge of the hydrogen ion activity, not just its concentration.

Noun

1. a positively charged atom of hydrogen; that is to say, a normal hydrogen atomic nucleus