Question

Watches The quartz crystals used in watches are made of silicon dioxide. Explain how you use the name to determine the formula for silicon dioxide.

Short answer

The formula for silicon dioxide can be determined by identifying the elements in the compound (silicon and oxygen), determining their valencies (4 for silicon and 2 for oxygen), applying the criss-cross method (resulting in Si_2O_4), and simplifying the formula if possible (dividing the subscripts by their GCD, resulting in SiO_2). Therefore, the formula for silicon dioxide is SiO_2.

Step-by-step solution

Identify the elements in the compound

The two elements in silicon dioxide are silicon (Si) and oxygen (O).

Determine the valencies of the elements in the compound

Silicon belongs to Group 14 of the periodic table, so it has a valency of 4 (4 electrons in its outer shell that can be shared). Oxygen belongs to Group 16 of the periodic table, so it has a valency of 2 (6 electrons in its outer shell with a capacity to share 2 more electrons).

Apply the criss-cross method to determine the formula for silicon dioxide

Using the criss-cross method, take the valency of silicon (4) and oxygen (2) and cross them over to be the subscript of the other element in the compound. So we have Si_2O_4.

Simplify the formula if possible

We can simplify the formula by dividing the subscripts of both elements by their greatest common divisor (GCD). The GCD of 2 and 4 is 2. So, the simplified formula for silicon dioxide is SiO_2. Hence, the formula for silicon dioxide is SiO_2.

Key concepts

Chemical Formula

A chemical formula serves as the shorthand representation of a compound, revealing the type and number of each atom present in the molecule. In the case of silicon dioxide, the formula is expressed as \( ext{SiO}_2 \). This concise representation tells us that in each molecule of silicon dioxide, there is one silicon atom bonded to two oxygen atoms. Determining the chemical formula from a compound's name, such as silicon dioxide, requires understanding the elements involved and their specific interactions based on their valencies.

• Components of a Chemical Formula: It consists of element symbols and numeric subscripts.
• Element Symbols: Each symbol represents a specific atom (e.g., Si for silicon, O for oxygen).
• Numeric Subscripts: These numbers indicate the quantity of each type of atom in the compound.

Criss-Cross Method

The criss-cross method is a straightforward technique used to find the chemical formula of ionic compounds. It involves using the valencies of the involved elements to determine the ratio of atoms in the compound. Here's how you apply the criss-cross method:Consider silicon (Si) with a valency of 4, and oxygen (O) with a valency of 2. By criss-crossing their valencies, you create the ratio in which these atoms combine to form a stable compound. So, you take the valency of silicon (4) and make it the subscript for oxygen, and the valency of oxygen (2) becomes the subscript for silicon. This gives you \( ext{Si}_2 ext{O}_4 \). It's essential to simplify the subscripts to their lowest whole number ratio. In our case, dividing by their greatest common divisor (2), simplifies \( ext{Si}_2 ext{O}_4 \) to \( ext{SiO}_2 \). The criss-cross method provides a quick, error-free way to determine formulas.

Valency of Elements

Valency is a critical concept in chemistry, defining an element's ability to bond with other atoms. It indicates the number of electrons an atom can gain, lose, or share to fill its outer shell, achieving a stable electronic configuration, often resembling the nearest noble gas.

• Silicon: It belongs to Group 14 of the periodic table, with a valency of 4. This indicates that silicon can form four bonds by sharing its four valence electrons.
• Oxygen: Oxygen belongs to Group 16, with a valency of 2, meaning it needs two additional electrons to fill its outer shell, usually via sharing.

Knowing the valency is essential for determining the formula of compounds like silicon dioxide. It ensures that the resulting molecule is neutral, with shared electrons effectively filling valence shells, maintaining a balance between charges.