Chemistry Notes: Criss Cross Method and Nomenclature

Chemistry Notes

Criss Cross Method

  • General Rule: When forming compounds, the numerical value of the ion charges are criss-crossed to become the subscripts for the other element.

  • Important Note: Negatives do not matter when criss-crossing charges.

  • Never use the number 1 as a subscript.

Example 1

  • Lithium and Nitrogen combine to form Lithium Nitride.

  • Lithium (Li) has a +1 charge (Li⁺¹).

  • Nitrogen (N) has a -3 charge (N⁻³).

  • Criss-cross the numbers (without the signs) to get Li₃N. The correct formula is Li3NLi_3N.

Common Mistake

  • Do not include the '1' as a subscript; it's redundant.

Ion Charge Cancellation

  • If the numbers of ion charges are equal, they cancel out, and no criss-crossing is needed.

Example 2

  • Magnesium (Mg) has a +2 charge (Mg⁺²).

  • Sulfur (S) has a -2 charge (S⁻²).

  • The charges are equal, so they cancel out. The correct formula is MgSMgS.

Simplification

  • After criss-crossing, simplify the subscripts if possible by dividing by their greatest common factor.

Example 3

  • Zirconium (Zr) has a +4 charge (Zr⁺⁴).

  • Oxygen (O) has a -2 charge (O⁻²).

  • Criss-cross to get Zr<em>2O</em>4Zr<em>2O</em>4.

  • Simplify by dividing both subscripts by 2 to get ZrO2ZrO_2.

Compounds with Oxygen

  • If a compound contains oxygen (O), and the criss-cross method results in subscripts that can be simplified, always simplify.

Noble Gases

  • Noble gases like Neon (Ne) do not typically form compounds.

  • If asked for the formula of a compound involving a noble gas, the answer is "nothing".

Naming Compounds

  • For binary compounds (two elements), the non-metal element typically has the suffix "-ide" added to its name.

Example

  • Strontium (Sr) and Nitrogen (N) form Strontium Nitride.

Implied Subscripts

  • If an element has no visible number (subscript), it is implied to be 1, regardless of the charge (+ or -).