Edexcel A Level (IAL) Physics-5.10 Atomic Mass Unit- Study Notes- New Syllabus

Using the Atomic Mass Unit (u) and Converting to SI Units

The atomic mass unit (u) is a convenient unit for expressing the extremely small masses of atoms, nuclei, and subatomic particles.

 Definition of the Atomic Mass Unit

The atomic mass unit is defined as:

\( 1\ \mathrm{u} = \dfrac{1}{12} \times \text{mass of a carbon-12 atom} \)

In SI units:

\( 1\ \mathrm{u} = 1.66\times10^{-27}\ \mathrm{kg} \)

This small unit makes nuclear and atomic mass values easier to work with.

Why Use the Atomic Mass Unit?

• Masses of atoms and nuclei are extremely small in kilograms.
• Using kilograms leads to inconvenient numbers.
• The unit \( \mathrm{u} \) provides simpler numerical values.

Example: The mass of a proton is approximately \( 1.67\times10^{-27}\ \mathrm{kg} \) or \( \approx 1.01\ \mathrm{u} \).

Converting Between u and kg

(a) From u to kg

Multiply by \( 1.66\times10^{-27} \):

\( m(\mathrm{kg}) = m(\mathrm{u}) \times 1.66\times10^{-27} \)

(b) From kg to u

Divide by \( 1.66\times10^{-27} \):

\( m(\mathrm{u}) = \dfrac{m(\mathrm{kg})}{1.66\times10^{-27}} \)

Typical Atomic and Nuclear Masses

• Proton: \( 1.01\ \mathrm{u} \)
• Neutron: \( 1.01\ \mathrm{u} \)
• Electron: \( 5.49\times10^{-4}\ \mathrm{u} \)
• Carbon-12 nucleus: \( 12.0\ \mathrm{u} \)

Importance in Nuclear Physics

• Used to calculate mass defect
• Used in nuclear binding energy calculations
• Simplifies comparison of nuclear masses
• Essential for using \( \Delta E = c^2 \Delta m \)