Valency Of Hydroxide: Explained

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Hello! I'm here to help you understand the valency of hydroxide! You asked a great question, and I'll provide a clear, detailed, and correct answer to help you learn. Let's dive in!

Correct Answer

The valency of the hydroxide ion (OH⁻) is -1.

Detailed Explanation

Let's break down the concept of valency and then focus on the hydroxide ion.

Key Concepts: Valency

Valency is a fundamental concept in chemistry. It refers to the combining power of an element or a group of atoms. In simpler terms, it tells us how many chemical bonds an atom or a group of atoms can form with other atoms.

  • Think of it like this: Imagine atoms as having "arms" that they use to hold onto other atoms. The number of "arms" an atom has is its valency. If an atom has one "arm," it can form one bond; if it has two "arms," it can form two bonds, and so on.
  • Valency is determined by the number of electrons in the outermost shell (also called valence electrons) of an atom or ion. Atoms strive to achieve a stable electron configuration, usually by having a full outermost shell (like the noble gases).
  • Atoms achieve this stability by:
    • Gaining electrons: If an atom needs to gain electrons to fill its outermost shell.
    • Losing electrons: If an atom has only a few electrons in its outermost shell.
    • Sharing electrons: If atoms share electrons to achieve a full outermost shell.

The Hydroxide Ion (OH⁻)

The hydroxide ion is a polyatomic ion, meaning it's a group of atoms (one oxygen atom and one hydrogen atom) that act together as a single unit and carry an electrical charge. The formula for the hydroxide ion is OH⁻, where the superscript "-" indicates a charge of -1. Let's explore its structure and how its valency comes about.

  • Composition: The hydroxide ion is made up of one oxygen atom (O) and one hydrogen atom (H).
  • Oxygen's Role: Oxygen is in Group 16 (also known as Group VI) of the periodic table and needs two more electrons to complete its outermost shell. It readily forms bonds because of this need.
  • Hydrogen's Role: Hydrogen, with only one electron, can either lose that electron or share it. In the hydroxide ion, it shares its electron with oxygen.
  • Formation: The hydroxide ion forms when an oxygen atom bonds with a hydrogen atom. In the process of formation, they share electrons to achieve a more stable arrangement.

Why the Hydroxide Ion Has a Valency of -1

The hydroxide ion (OH⁻) carries a negative charge. This is because it has gained an extra electron.

  • Oxygen's Electronegativity: Oxygen is more electronegative than hydrogen. Electronegativity is the measure of the tendency of an atom to attract a bonding pair of electrons. Oxygen pulls the shared electrons towards itself.
  • Electron Distribution: The shared electron pair is closer to the oxygen atom, giving it a slightly negative charge (δ-), and the hydrogen atom a slightly positive charge (δ+).
  • Extra Electron: The hydroxide ion gains an extra electron, not from the hydrogen atom, but from another source (for instance, from an alkali metal like sodium or potassium). This extra electron gives the ion its overall -1 charge and thus the valency of -1.
  • Bonding: Because of this charge, the hydroxide ion readily forms ionic bonds with positively charged ions (cations). Examples include:
    • Sodium hydroxide (NaOH): Sodium (Na⁺) bonds with hydroxide (OH⁻).
    • Potassium hydroxide (KOH): Potassium (K⁺) bonds with hydroxide (OH⁻).

Example: Sodium Hydroxide (NaOH) Formation

Let's consider the formation of sodium hydroxide as an illustration:

  1. Sodium (Na) is a highly reactive alkali metal. It readily loses its single valence electron to achieve a stable electron configuration.
  2. When sodium reacts with water (H₂O), the sodium atom loses its electron, forming a sodium ion (Na⁺).
  3. Water molecules break apart, and the released electron combines with a water molecule (H₂O) to form a hydroxide ion (OH⁻).
  4. The sodium ion (Na⁺) and the hydroxide ion (OH⁻) are attracted to each other due to their opposite charges, forming an ionic bond. This results in the formation of sodium hydroxide (NaOH).

Understanding Chemical Formulas with Hydroxide

The valency of hydroxide is crucial in writing correct chemical formulas. When writing the formula for an ionic compound involving hydroxide, you need to consider the charges and use the criss-cross method (or the charge balancing method) to determine the ratio of ions.

  • For instance, Calcium Hydroxide: Calcium (Ca) has a valency of +2 (it forms Ca²⁺ ions), and hydroxide (OH⁻) has a valency of -1. To balance the charges, you need two hydroxide ions for every calcium ion. The correct formula is Ca(OH)₂.
  • The parentheses around the hydroxide ion indicate that the entire group (OH) is taken twice.

Role in Chemical Reactions

Hydroxide ions play significant roles in various chemical reactions:

  • Neutralization: Hydroxide ions are essential in neutralization reactions. They react with acids to neutralize the acidic properties, forming water and a salt. For example: HCl (acid) + NaOH (base) -> NaCl (salt) + H₂O (water).
  • Base Properties: Hydroxide ions are the defining characteristic of bases. They can accept protons (H⁺ ions) from acids. Bases are substances that increase the concentration of hydroxide ions in a solution.
  • Precipitation: Hydroxide ions can cause the precipitation of metal ions from solutions, forming insoluble metal hydroxides. This is often used in qualitative analysis to identify the presence of certain metal ions.

Key Takeaways

  • The valency of hydroxide (OH⁻) is -1, indicating it can form one bond with a positive ion.
  • Hydroxide is a polyatomic ion composed of oxygen and hydrogen, carrying a negative charge due to an extra electron.
  • Hydroxide ions are essential for understanding chemical reactions, especially neutralization, and the properties of bases.
  • Understanding valency is vital for writing correct chemical formulas, such as Ca(OH)₂.