Physical Science / Unit 4: Chemical Bonds and Compounds
4.4 Chemical Formulas
Notes
A chemical formula shows the elements in a compound and how many atoms of each.
- Subscripts give the number of atoms: $\text{H}_2\text{O}$ has 2 hydrogen and 1 oxygen.
- A number in front of a formula is a coefficient: $3\text{H}_2\text{O}$ means 3 molecules.
- Parentheses: $\text{Ca(OH)}_2$ has 1 Ca, 2 O and 2 H, so 5 atoms.
- In ionic compounds the charges must add to zero. $\text{Mg}^{2+}$ and $\text{Cl}^-$ need two chlorides: $\text{MgCl}_2$. $\text{Al}^{3+}$ and $\text{O}^{2-}$ give $\text{Al}_2\text{O}_3$ ($+6$ and $-6$).
Practice
How many atoms are in one molecule of $\text{C}_6\text{H}_{12}\text{O}_6$?
$6 + 12 + 6 = 24$.
What is the formula for the ionic compound formed from $\text{Na}^+$ and $\text{O}^{2-}$?
Two $+1$ charges balance one $-2$ charge.
How many oxygen atoms are in $\text{Al}_2(\text{SO}_4)_3$?
$4 \times 3 = 12$.
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