Build Compounds: Fixed Ratios
Combine elements in fixed ratios to form compounds.
What this lesson covers
Why it matters
Imagine trying to bake a cake with three eggs and one cup of flour; the result is a mess, not a cake. Chemical compounds are stricter: elements must combine in exact, fixed ratios to form a stable substance.
Predict first
If you have 10 Hydrogen atoms and 10 Oxygen atoms, how many water molecules (H₂O) can you form?
Water requires exactly 2 Hydrogen atoms for every 1 Oxygen atom. With 10 Oxygens, you need 20 Hydrogens. Since you only have 10 Hydrogens, they run out first, limiting you to 5 pairs.
- 10 molecules
- 5 molecules — correct
- 20 molecules
What you do
Cross the valencies to build H2O and MgCl2. Oxygen needs two bonds and chlorine one — the subscripts fall straight out of the charges.
Check yourself
Why can't we form 'H₃O' as a standard stable water molecule?
Compounds form based on valency (combining capacity), not physical size. Oxygen needs 2 electrons to complete its shell, so it bonds with exactly 2 Hydrogens.
If you mix 2g of Hydrogen with 16g of Oxygen, you get 18g of Water. What happens if you add more Hydrogen?
Compounds form in fixed mass ratios. Once the Oxygen is used up, no more water can form, leaving the extra Hydrogen as a separate element.
- Hydrogen atoms are too small to fit three around one Oxygen
- Oxygen has a valency of 2, meaning it can only bond with 2 Hydrogen atoms — correct
- Water is a liquid, so it changes shape
- More water is formed immediately
- The extra Hydrogen remains unreacted — correct
- The water turns into hydrogen gas