Problem 138 - Introductory Chemistry Atoms First

Question

A student writes the formula for magnesium hydroxide as \(\mathrm{MgOH}_{2}\). What is wrong with that formula? How should the student fix it?

Step-by-Step Solution

Answer

The formula \( \mathrm{MgOH}_{2} \) is incorrect because the oxygen and hydrogen are actually part of the hydroxide ion (OH\(^-\)), which has a charge of -1. The charge of the magnesium ion is +2. To balance these charges, we need two hydroxide ions for each magnesium ion. Hence, the correct formula should be \(\mathrm{Mg(OH)}_{2}\).

Step 1: Identify the elements involved in magnesium hydroxide

Magnesium hydroxide consists of magnesium (Mg), oxygen (O), and hydrogen (H) elements.

Step 2: Determine the valency of each element

The valency of an element is its combining capacity with other elements. The valencies of magnesium, oxygen and hydrogen are as follows: - Magnesium (Mg): +2 (it's in Group 2 of the periodic table) - Oxygen (O): -2 (it's in Group 16 of the periodic table) - Hydrogen (H): +1 (it's in Group 1 of the periodic table)

Step 3: Determine the correct formula based on the valencies of the elements

To find the correct formula, we need to make sure that the positive and negative charges balance out. Since magnesium has a valency of +2 and hydroxide (OH) has a combined valency of -1, we will need two hydroxide (OH) ions to balance the charge with one magnesium ion (Mg). The correct formula for magnesium hydroxide should be: Mg(OH)\(_2\)

Key Concepts

Valency

Valency is a fundamental concept in chemistry that explains how atoms of different elements interact with each other. It describes an atom's ability to combine with other atoms. An element's valency can be understood as the number of electrons an atom needs to lose, gain, or share to fill its outer shell and achieve a stable electronic configuration.

• Magnesium: This element has a valency of +2. It means magnesium tends to donate two electrons to stabilize itself.
• Oxygen: With a valency of -2, oxygen usually gains two electrons to achieve stability.
• Hydrogen: It has a valency of +1, indicating it usually donates one electron.

Understanding valency helps chemists determine how elements will interact and what combinations can form stable compounds. It is a vital concept when deducing the formulas of chemical compounds, especially as seen with magnesium hydroxide.

Magnesium Hydroxide

Magnesium hydroxide is an important compound used in various applications, including medical and industrial fields. It is primarily known for its role as an antacid and a laxative. The chemical formula of magnesium hydroxide is Mg(OH)\(_2\), representing its composition.

• Magnesium (Mg): Provides the central atom for this compound.
• Hydroxide (OH): This is a polyatomic ion consisting of one oxygen and one hydrogen atom. In magnesium hydroxide, there are two hydroxide ions to balance the charge of a magnesium ion.

To understand why magnesium hydroxide takes this form, it helps to remember that the formula must reflect the balance of charges between the ions involved. In this way, the compound remains electrically neutral. Magnesium contributes a +2 charge, and each hydroxide ion provides a -1 charge, so two hydroxide ions will balance one magnesium ion, hence the formula Mg(OH)\(_2\).

Balancing Chemical Equations

Balancing chemical equations ensures that we respect the law of conservation of mass, which states matter is neither created nor destroyed in a chemical reaction. Every chemical equation must therefore start with equal numbers of each type of atom on either side of the equation.

• Element Count: Confirm that every element's atoms are balanced. Element counts shouldn’t change; only coefficients in front can.
• Charge Balance: Charges should also be balanced on both sides. Neutral compounds on the reactant side should have neutral products.
• Simplifying: Sometimes equations require adjustment of all coefficients to reach the simplest form.

For magnesium hydroxide, understanding the valencies and balancing the charges led to realizing that Mg(OH)\(_2\) is the balanced formula. Two negative hydroxide ions balance one positive magnesium ion, making the compound neutral, which is crucial for writing correct chemical formulas. This reflects the principle of charge balance in balanced equations.