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Hydronium Ion: Definition & Formula

Hydronium Ion: Definition & Formula
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  • 0:04 Water & Hydronium Ions
  • 1:15 Origins of an Ion
  • 2:05 Hydronium & Hydroxide Ions
  • 4:32 Formulas
  • 5:37 Lesson Summary
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Lesson Transcript
Instructor: Emily Lockhart

Emily has taught science and has a master's degree in education.

In this lesson, you'll learn how hydronium ions are made and why they form in solution. You'll also learn the formulas for calculating hydronium ions by finding the pH.

Water & Hydronium Ions

Leonardo Da Vinci is credited with saying, 'Water is the driving force of all nature.' Water composes most of your body, and the planet earth. For something as prevalent and necessary to life as water is, it must be a remarkable substance!

A single water molecule is made up of two hydrogen atoms that are bonded to one oxygen atom. It's chemically written as H2O. By themselves atoms are small units, but when combined together to make molecules, they compose everything in the universe. Atoms can be likened to letters of the alphabet, and the infinite amounts of words that can be made from only a few letters. Such is the diversity of molecules.

Two atoms, such as hydrogen and oxygen for example, are found in almost every substance on earth. It is the interactions between these two atoms in water that yield a special type of molecule called a hydronium ion.

Hydronium ions are water molecules that have gained an extra positive hydrogen ion. In this lesson, we'll explore how hydrogen can become a positive ion, how water can become a hydronium ion, and how to calculate the concentration of hydronium ions in solution.

Origins of an Ion

All atoms are composed of three subunits:

  1. Electrons, which have a negative charge.
  2. Protons, which have a positive charge.
  3. Neutrons, which as their name implies, are neutral.

The ratio of these three subunits contributes to the property of the atom. Atoms bond together to form molecules, but atoms can break away from the molecules as well. When the atoms break from their bonds, electrons can shift away and stay behind.

This creates an ion, which is an atom or molecule that has lost or gained an electron. Hydrogen is only made of one electron and one proton. Sometimes, when hydrogen breaks its bond with a molecule, its electron stays with the molecule. When the hydrogen loses its electron it becomes an ion, although it's just a single proton. Because of the proton, it has an overall positive charge, written as H+.

Hydronium & Hydroxide Ions

A water molecule, also written as H-O-H, can become unstable over time, and when it does, one hydrogen atom separates from H-O-H and becomes H+. Sometimes, if another molecule like hydrogen chloride (HCl) is dissolved in water, the hydrogen will dissociate as H+.

In an aqueous solution, the added H+ becomes attracted to the negative poles on another water molecule. This leaves an H2O molecule with an extra hydrogen atom, written as H3O, called a hydronium ion.

If a solution has a ton of hydronium ions it becomes acidic. The concentration of hydronium ions directly relates to pH, which we'll discuss in the next part of the lesson.

In the mean time, what happened to the original water molecule abandoned by the hydrogen? Hydroxide ions are what's left of water when a hydrogen ion breaks away. Hydroxide ions are written as OH- and have an overall negative charge, because they steal other atoms' electrons. Hydroxide ions can also enter solutions also when they disassociate from other molecules. When dissolved in water, NaOH, or sodium hydroxide, becomes a sodium ion and a hydroxide ion.

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