One of the important ideas from Dalton’s atomic theory is atomic mass. Dalton believed that each element has its own special atomic mass. This idea helped explain the law of constant proportions. It encouraged scientists to measure atomic mass carefully.
In the past, scientists used oxygen as a
In 1961, scientists chose carbon-12 as the new standard. They defined one atomic mass unit (u) as of the mass of one atom of carbon-12. This means that the atomic masses of all elements are measured compared to carbon-12.
Fun Idea: Imagine a fruit seller cutting a watermelon into 12 equal pieces. He can use one piece to weigh other fruits. Similarly, scientists use the mass of carbon-12 to compare and define atomic mass.
Here are some atomic masses of common elements:
| Element | Atomic Mass (u) |
|---|---|
| Hydrogen | 1 |
| Carbon | 12 |
| Nitrogen | 14 |
| Oxygen | 16 |
| Sodium | 23 |
| Magnesium | 24 |
| Sulphur | 32 |
| Chlorine | 35.5 |
| Calcium | 40 |
Most elements' atoms do not exist alone. They usually form molecules or ions. These molecules or ions come together in large numbers. Together, they create matter, which we can see, feel, or touch.
Here are some questions to help you think:
Define the atomic mass unit.
Why is it not possible to see an atom with the naked eye?
Scenario: You have a scale that can only weigh fruits. How would you compare a small apple's weight to the watermelon pieces?
Scenario: A scientist is measuring the atomic mass of sodium. Why does she compare it to carbon-12 instead of oxygen?
Scenario: A group of students want to see atoms through a regular microscope. What will happen?
Scenario: If you had one atom of hydrogen, how would you express its mass relative to carbon-12?
Scenario: A chemistry class is experimenting with different elements. Each element forms compounds. Why does this happen?
