Fixed ratio
Elements join in the same proportion every time.
Counting atoms
A water molecule has hydrogen and oxygen atoms. Common salt has sodium and chlorine.
Do the elements in a compound join in any amount?
What this lesson covers
The idea
In a compound the elements are combined in a fixed ratio: water has hydrogen and oxygen atoms in the ratio 2:1, and sodium chloride has sodium and chlorine in the ratio 1:1.
Counting atoms
A water molecule has hydrogen and oxygen atoms. Common salt has sodium and chlorine.
Do the elements in a compound join in any amount?
- No, in a fixed ratio
- Yes, in any amount
- Only on hot days
Count the atoms
Open both tabs.
Fixed ratios
The ratio is the same for a given compound.
In a compound, elements are combined in a fixed ratio. Water: hydrogen : oxygen = 2 : 1. Sodium chloride: sodium : chlorine = 1 : 1.
- Compound | Elements | Ratio of atoms
- Water | Hydrogen : Oxygen | 2 : 1
- Sodium chloride | Sodium : Chlorine | 1 : 1
Notes
The elements of a compound are joined in a fixed ratio: water 2:1, sodium chloride 1:1.
Check yourself
In water, how many hydrogen atoms are there for every 1 oxygen atom?
Answer: 2
The ratio of hydrogen to oxygen atoms is 2:1.
In sodium chloride, how many sodium atoms are there for every 1 chlorine atom?
Answer: 1
Sodium and chlorine are in the ratio 1:1.
In a compound, the elements are combined in …
Common salt is made of sodium and chlorine in the ratio …
- any ratio we like. The ratio is fixed.
- a ratio that changes every day. The ratio is fixed for a compound.
- a fixed ratio — correct. Yes!
- 1:1 — correct. Yes!
- 2:1. 2:1 is the ratio of hydrogen to oxygen in water.
- 1:2. Sodium and chlorine are in the ratio 1:1.