Chemical Symbols, Subscripts, and Counting Atoms

Understanding Chemical Symbols and Subscripts

• Element Symbol: Represents one atom of the element.
  • Example:
  • $\text{Ba} = \text{Barium}$
• Subscript: A number written at the lower right corner of a chemical symbol, indicating the number of atoms of that element in a molecule.
  • Example:
  • In $\text{H}<em>2\text{SO}</em>4$, the subscript of H is 2, indicating there are 2 hydrogen atoms.
• Brackets: A subscript outside the brackets indicates the number of molecules of the entire compound inside the brackets.
  • Example:
  • $\text{Ca}<em>3(\text{PO}</em>4)_2$ means there are 3 calcium atoms and 8 oxygen atoms from the 2 phosphates.

Coefficients and Molecular Count

• Coefficient: A number placed in front of a molecular formula that indicates the number of molecules present.
  • Example:
  • In $2\text{Na}<em>2\text{SO}</em>4$, the coefficient 2 indicates there are 2 molecules of sodium sulfate, which contains a total of 8 sodium atoms and 4 sulfate groups.
• Counting Atoms in Compounds:
  • When analyzing compounds, one can identify the total number of each type of atom by accounting for subscripts and coefficients.

Practice Counting Atoms in Compounds

1. K₂CO₃:
   • K: 2
   • C: 1
   • O: 3
2. Na₂CrO₄:
   • Na: 2
   • Cr: 1
   • O: 4
3. NH₄C₂H₃O₂:
   • N: 1
   • H: 7
   • C: 2
   • O: 2
4. Pb(NO₃)₂:
   • Pb: 1
   • N: 2
   • O: 6
5. Ba₃(PO₄)₂:
   • Ba: 3
   • P: 2
   • O: 8
6. CaCl₂:
   • Ca: 1
   • Cl: 2
7. Al₂(CO₃)₃:
   • Al: 2
   • C: 6
   • O: 18
8. Yb(OH)₂:
   • Yb: 1
   • O: 2
   • H: 2

Summary of Counting

• Always consider subscripts for atom counts and coefficients for total molecular counts.
• Use parenthesis for groups of atoms or molecules when assessing complex compounds.