Build a Compound
Combine elements to form simple compounds like water.
What this lesson covers
Why it matters
Imagine snapping together LEGO bricks of different colors to build a specific castle. In chemistry, atoms snap together in fixed ratios to form compounds, not random piles.
Predict first
If you have 10 Hydrogen atoms and 10 Oxygen atoms, how many water molecules (H₂O) can you build?
Water needs exactly 2 Hydrogens for every 1 Oxygen. With 10 Oxygens, you need 20 Hydrogens. You only have 10 Hydrogens, so you can only make 5 pairs.
- 10 molecules
- 5 molecules — correct
- 20 molecules
What you do
Cross the valencies to snap each formula together — H2O, then NaCl, then CaCl2. The pieces only lock when the valencies (combining capacities) balance.
Check yourself
What does the formula CO₂ tell us about a single molecule of carbon dioxide?
The subscript '2' after O means there are two oxygen atoms bonded to one carbon atom.
Why can't we just mix any amount of Hydrogen and Oxygen to make water?
Compounds have definite proportions. Water is always H₂O, never H₃O or HO.
- It contains 1 Carbon and 2 Oxygen atoms — correct
- It contains 1 Carbon and 1 Oxygen atom
- It contains 2 Carbon and 1 Oxygen atom
- Because atoms bond in fixed, specific ratios — correct
- Because Oxygen is heavier than Hydrogen
- Because they are both gases