Problem 36 - Introductory Chemistry Atoms First

Question

Write the formula for magnesium bicarbonate.

Step-by-Step Solution

Answer

The formula for magnesium bicarbonate is \(Mg(HCO_3)_2\).

Step 1: Identify the Ions

First, we identify the ions in the compound. Magnesium (#Mg#) is a Group 2 element in the periodic table, which means it forms a +2 cation (Mg²⁺). Bicarbonate (#HCO3#) is a polyatomic anion with a -1 charge (HCO₃⁻).

Step 2: Use the Crisscross Method

Next, we use the crisscross method to balance the charges between the cation and the anion. This is done by using the absolute value of the charge from one ion as the subscript for the other ion, and vice versa: Mg²⁺ --> the charge number 2 becomes the subscript for HCO₃ HCO₃⁻ --> the charge number 1 becomes the subscript for Mg

Step 3: Write the Formula

The resulting formula becomes: \(Mg(HCO_3)_2\)

Step 4: Finalize the Formula

Now we have the chemical compound formula for magnesium bicarbonate: \(Mg(HCO_3)_2\)

Key Concepts

Magnesium Cations

Magnesium is an essential element found in the periodic table. It is located in Group 2, which is also known as the alkaline earth metals. This group of elements is characterized by each atom having two electrons in its outermost shell. For magnesium, the electron configuration is such that it tends to lose these two electrons to achieve a stable electron configuration like the nearest noble gas.
Once magnesium loses these two electrons, it forms a cation with a +2 charge, represented as \( \text{Mg}^{2+} \). This charge indicates that the magnesium ion has two fewer electrons than protons.
Magnesium cations are common in various chemical compounds, especially in salts. Due to their positive charge, they tend to pair up with negatively charged anions, like bicarbonate, to form stable ionic compounds.

Bicarbonate Anions

Bicarbonate is a polyatomic ion, which means it is composed of more than one atom. Its formula is \( \text{HCO}_3^- \), indicating that it carries a -1 charge. This negative charge results from the gain of an electron which makes the total number of electrons larger than protons in the compound.
Bicarbonate ions are often found in nature dissolved in water and are an important component of the buffering system in our blood. Their chemical structure allows them to effectively pair with cations, such as magnesium, to create more complex compounds.

• "Bi" in bicarbonate signifies the single hydrogen atom in the ion.
• "Carbonate" refers to the part of the structure with a carbon atom bonded to three oxygen atoms.

The interaction between positively charged cations and negatively charged bicarbonate anions is crucial for forming ionic compounds through attraction between opposite charges.

Crisscross Method

The crisscross method is a simple yet effective technique for deriving chemical formulas from the charges on ions. This method ensures that the overall charge of the compound is neutral by balancing the positive and negative charges of the ions involved.

• Identify the cation and anion from the given elements.
• Write down their respective charges. For instance, magnesium as \( \text{Mg}^{2+} \) and bicarbonate as \( \text{HCO}_3^- \).
• "Crisscross" the magnitude of each ion's charge to become the subscript of the other ion. This helps in achieving a charge balance.

For magnesium bicarbonate, the charge of +2 on magnesium becomes the subscript for bicarbonate, leading to \( (\text{HCO}_3)_2 \), and the charge of -1 on bicarbonate suggests a subscript of 1 for magnesium, simplifying to \( \text{Mg} \).
The final formula of the compound, \( \text{Mg(HCO}_3\text{)}_2 \), is neutral as the two bicarbonate anions balance the charge of one magnesium cation.