To determine the value of n in the molecular formula Be_nAl₂Si₆O₁₈, we need to ensure the compound is electrically neutral. This means the total positive charge from cations must equal the total negative charge from anions.

Step 1: Identify the charges of the ions.
Beryllium (Be) forms a +2 cation: Be²⁺.
Aluminum (Al) forms a +3 cation: Al³⁺.
Silicon (Si) in silicates is typically part of the silicate anion, but here we consider the oxygen atoms which carry a -2 charge: O^2-.
The silicate group Si₆O₁₈ is a common ring silicate anion with a charge of -12. This is a known structural unit in minerals like beryl.

Step 2: Write the charge balance equation.
Let the number of Be²⁺ ions be n.
Number of Al³⁺ ions is 2.
Charge from Be ions: $n\times 2$
Charge from Al ions: $2\times 3=6$
Total positive charge: $2n+6$
The silicate anion Si₆O₁₈ has a charge of -12.
For electrical neutrality: Total positive charge + Total negative charge = 0
$2n+6+(-12)=0$

Step 3: Solve the equation for n.
$2n+6-12=0$
$2n-6=0$
$2n=6$
$n=3$

Final Answer: The value of n is 3. The formula is Be₃Al₂Si₆O₁₈, which is the mineral beryl.

Related Topics and Formulae

Charge Neutrality Principle: In any ionic compound, the sum of the positive charges must equal the sum of the negative charges. This is a fundamental rule for determining formulas of ionic compounds.

Common Silicate Structures: Silicates often form anions with specific charges. The Si₆O₁₈^12- anion is a cyclosilicate (ring silicate) found in minerals like beryl and tourmaline.

Ionic Charges: Remembering common ionic charges is crucial: Be²⁺, Al³⁺, O^2-.