Chemical formulas

Writing Chemical Formulas for Ionic Compounds

Understanding Ion Charges

• Group 1 (Alkali Metals): Elements like lithium, sodium, and potassium have one valence electron, forming +1 charge cations (e.g., Na+).

• Group 2 (Alkaline Earth Metals): Elements such as calcium and magnesium form +2 charge cations (e.g., Ca2+).

• Group 13 (3A): Aluminum has a +3 charge as an ion (e.g., Al3+).

• Group 4A: Rarely forms ionic compounds, includes carbon, silicon, and germanium.

• Group 5A: Elements like nitrogen and phosphorus form -3 charge anions (e.g., P3-).

• Group 6A (Chalcogens): Oxygen, sulfur, and selenium form -2 charge anions (e.g., O2-).

• Group 7A (Halogens): Fluoride, chloride, bromide, and iodide form -1 charge anions (e.g., Cl-).

Writing Chemical Formulas: Examples

Example 1: Sodium Bromide (NaBr)

• Ions Involved: Sodium ion (Na+, charge = +1) and bromide ion (Br-, charge = -1).

• Combining Charges: Since both charges are equal in magnitude, combine in a 1:1 ratio.

• Chemical Formula: NaBr.

Example 2: Calcium Sulfide (CaS)

• Ions Involved: Calcium ion (Ca2+, charge = +2) and sulfide ion (S2-, charge = -2).

• Combining Charges: Charges are the same (+2 and -2), meaning a 1:1 ratio.

• Chemical Formula: CaS.

Example 3: Aluminum Phosphide (AlP)

• Ions Involved: Aluminum ion (Al3+, charge = +3) and phosphide ion (P3-, charge = -3).

• Combining Charges: Charges are equal; thus, combine in a 1:1 ratio.

• Chemical Formula: AlP.

Different Magnitudes of Charges

Example 4: Aluminum Chloride (AlCl3)

• Ions Involved: Aluminum (Al3+, charge = +3) and chloride (Cl-, charge = -1).

• Charge Swapping Method: Swap charges to get subscripts.

  • Subscript of Al becomes 1 (omit) and Cl becomes 3.

• Chemical Formula: AlCl3.

• Charge Balance: One Al (3+) balances with three Cl (3-). Total charge = 0.

Example 5: Sodium Oxide (Na2O)

• Ions Involved: Sodium (Na+, charge = +1) and oxide (O2-, charge = -2).

• Using Charge Swap: Swap subscripts, resulting in Na2O.

• Chemical Formula: Na2O.

• Charge Balance: Two Na (2+) balances with one O (2-).

Example 6: Barium Phosphate (Ba3(PO4)2)

• Ions Involved: Barium (Ba2+, charge = +2) and phosphate (PO4 3-, charge = -3).

• Charge Swap Method: Ba3(PO4)2; parentheses used for the polyatomic ion.

• Importance: Always enclose multiple polyatomic ions in parentheses.

Example 7: Iron (III) Sulfate (Fe2(SO4)3)

• Ions Involved: Iron (Fe3+, charge = +3) and sulfate (SO4 2-, charge = -2).

• Using Charge Swap: Leads to the chemical formula Fe2(SO4)3 to balance the charge.

Polyatomic Ions

• Sulfate (SO4 2-): A common polyatomic ion that consists of one sulfur atom bonded to four oxygen atoms.

• Nitrate (NO3 -): Contains one nitrogen atom bonded to three oxygen atoms, commonly found in fertilizers.

• Phosphate (PO4 3-): Composed of one phosphorus atom bonded to four oxygen atoms, essential for biological processes.

• Carbonate (CO3 2-): Consists of one carbon atom bonded to three oxygen atoms, often found in minerals.

Conclusion

• Understanding ion charges is crucial for accurately writing chemical formulas of ionic compounds.

• Using methods like combining charges or swapping for subscripts helps in determining the correct formulas.